B.Sc
GENERAL CHEMISTRY-III
NEW REGULATION
FOR III SEMESTER B.Sc CHEMISTRY
MAJOR STUDENTS
ENGLISH MEDIUM
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CONTENT
1.1. Kinetic molecular model of a gas:
1.2. postulates and derivation from the
kinetic gas equation;
1.3. Maxwell –Boltzmann distribution of
speed of molecules
1.6. Most probable velocity and average
kinetic energy
1.7. law of equipartition of energy
1.12. Deviations
from ideal gas behaviour, (Andrew’s and Amagat’s plots);
1.13. compressibility
factor, Z, and its variation with pressure for different gases.
1.14. Equations
of states for real gases-van der Waal’s equation
1.15. Virial
equation; Boyle temperature
1.16. Numerical
problems based on equations of states for real gases
1.17. critical
phenomena – isotherms of CO2
1.18. Van
der waal’s equation and the critical state
2.2. Surface tension, viscosity and their
applications.
2.3. Crystalline and amorphous
2.4. Differences - geometry, isotropy and
anisotropy, melting point;
2.5. isomorphism, polymorphism.
2.6. Crystals –size and shape;
2.8. symmetry elements – plane, centre and axis; Miller indices
2.9. Unit cells and space lattices
2.10. classification
of crystal systems
2.18. Co-ordination
number in typical structures NaCl, CsCl, ZnS, TiO2
2.19. comparison
of structure and properties of diamond and graphite
2.20. numerical
problems involving core concepts
2.23. Non-stoichiometric
defects.
2.25. classification
and applications.
3.1. Natural radioactivity - α, β and ɣ rays
3.3. Fajan–Soddy group displacement law
3.5. isotopes, isobars, isotones
3.12. nuclear
stability - neutron- proton ratio
3.18. Nuclear
energy; nuclear fission and fusion
3.19. major
nuclear reactors in India
3.20. Radiation
hazards, disposal of radioactive waste and safety measures.
4. Halogen derivatives Aliphatic
halogen derivatives
4.1. Nomenclature and classes of alkyl
halides
4.4. Di, Tri & Tetra Halogen
derivatives:
4.7. Aromatic halogen compounds
4.12. Mechanism
of nucleophilic aromatic substitution – benzyne intermediate.
4.1. Aryl alkyl halides Nomenclature
4.9. Oxidation of diols by periodic acid
4.10. Oxidation
of diols by lead tetraacetate.
5.2. Preparation from diazonium salts
5.7. acidic character and effect of
substitution on acidity.
ELECTROPHILIC
SUBSTITUTION REACTIONS
5.10. Reimer
– Teimen Reaction
5.18. Aromatic
alcohols Nomenclature
5.21. Reduction
of benzaldehyde
UNIT I
1.
Gaseous state
Amongst the three common states of matter, the gaseous
state is simplest. The laws of gaseous behaviour are more uniform and are
better understood. The well known laws of gaseous behaviour are Boyle’s law,
Charle’s law Graham’s law, Dalton’s law and Avogadro’s law. There was no
theoretical background to justify them. In the nineteenth century, however,
Kronig, Clausius, Maxwell and Boltzmann developed a theory known as kinetic
molecular theory of gases, which provided sound theoretical basis for the
various gas laws.
There are two opposite molecular forces, the forces of
attraction and the disruptive forces operating between molecules. If the
thermal energy is much greater than the forces of attraction, then we have
matter in its gaseous state.
In contrast with solids and liquids gases occupy the
same volume as that of the closed vessel, they are characterised by low density
and high compressibility.
The characteristic properties of gases are given
below.
1.
No definite shape and volume: Gases occupy all available space i.e. the
shape and volume of the container in which they are filled.
2.
Expansibility: Gases have limitless expansibility. They expand to fill
the entire vessel they are placed in.
3.
Compressibility: Gases are easily compressed by application of pressure.
4.
Diffusibility: Gases can diffuse rapidly through each other to form a
homogeneous mixture.
5.
Pressure: Gases exert pressure on the walls of the container in all
direction. You can site the example of a gas balloon.
6.
Effect of heat: When a gas confined in a vessel is heated, its pressure
increases. Upon heating in a vessel fitted with a piston, volume of the gas
increases.
1.1. Kinetic molecular model
of a gas:
Suppose a volume of gas enclosed in a cubical vessel
at a fixed temperature.
Suppose that :
the length of each side of cube = l cm
the number of gas molecules = n the mass of one
molecule = m
the velocity of a molecule = υ
Let us consider one single molecule of a gas can be
evaluated by calculating the momentum during collisions.
According to kinetic model the molecules of the gas
are moving in straight lines in all possible directions. They collide with one
another frequently as also with the walls of the container. Since their mutual
collisions are perfectly elastic and do not involve the loss of energy, these
may be neglected. Here we will, therefore, assume that gas molecules move in
all directions but rebound whenever they strike the wall of the container. Now
you proceed to derive kinetic gas equation in the following steps
According to the kinetic theory', a molecule of a gas
can move with velocity in any direction velocity is a vector quantity can be
resolved into components υx, υy, υz along the X, Y and Z
axes. These components are related to velocity υ by the following expression.
Let us consider a molecule moving in ox direction
between opposite faces A and B. It will strike the face A with velocity υx
and rebound with velocity -υx. To hit the same face again the
molecule must travel l cm to
Collide with opposite face B and then again l cm to
return to face A. Therefore, time taken between two collisions can be
calculated as follows
The molecule travels .x cm in 1 sec
·
hence 1 cm in 1/υx sec
·
And 2l cm in 2l/υx sec…………………………. (2)
·
In 2l/υx sec molecule suffers 1 collision
·
In 1 sec no of collisions = υx/2l………………… (3)
Each impact of the molecule on the face A causes a
change of momentum which is mass x velocity.
·
Momentum of the molecule before impact = mυx
·
Momentum of the molecule after impact = - mυx
·
Hence change of momentum = m.x - (-m.x) = 2mυx
·
But the number of collision per second on face A= υx/2l
Therefore total change of momentum per second on face
A caused by one Molecule = 2m/x x υx/2l
= mυx2/l……………….
(4)
As there are two faces along x- direction, total
change of momentum per second considering both the faces along x-direction will
be
2mυx2/l……………….
(5)
This is change of momentum caused by one molecule
along x-direction. The change of momentum caused by one molecule along y-
direction per second will be 2mυ2/l and change of momentum caused by
one molecule along z-direction per second will be 2mυ2 /l
Total change of momentum caused by one molecule
considering along three directions will be
2mυx2/l + 2mυy2/l + 2mυ2/l
= 2m/l (υx2+υy2+υz2)
= 2mυ2/l………………….(6)
Since there are n molecules in the vessel then total
change of momentum due to n molecules will be
2mnυ2/l…………………...(7)
υ2= mean square velocity
Since change of momentum per second is force
Hence force = 2mnυ2/2
Since pressure =
Total force/Total area
Since there are six faces in a cube, area of each cube
is l2. Hence total area is 6l2
Then pressure =
2mnυ2/l x 1/6l2
= mnυ2/3l2
As l2
= volume V
Hence pressure P=
1/3 mnυ2/V…………….(8)
This is known as Kinetic gas equation. This equation
has been derived for a cubical vessel. It is equally valid for vessel of any
shape. The available volume in the vessel may be considered as made up of large
number of infinitesimally small cubes, for each of them the equation is valid.
1.2. postulates and
derivation from the kinetic gas equation;
Boyle’s law
From his observations Boyle’s in 1660 formulated a
generalisation known as Boyle’s law. Boyle’s law states that at constant
temperature, the volume of a given mass of gas is inversely proportional to its
pressure.
According to kinetic theory, kinetic energy is
directly proportional to temperature (in absolute scale).
Charle’s law : for a definite quantity of gas at constant
pressure, its volume is directly proportional to the absolute temperature. It
was established in 1787.
From above discussion V= 2/3 KT/P
At constant pressure V= constant xT Or V α T when P is constant.
This is Charle’s law
Avogadro’s law: It is states that equal volume of gases at
same temperature and pressure contain equal number of molecules.
Suppose there are two gases for first gas mass of one
molecule is m1, velocity is υ1 and number of molecules are n1. And
for the second gas mass of one molecule is m2, velocity is υ2
and number of molecules are n2
1.3. Maxwell –Boltzmann distribution
of speed of molecules
In
a classroom, the air molecules are moving in random directions. The speed of
each molecule is not the same even though macroscopic parameters like
temperature and pressure are fixed. Each
molecule collides with every other molecule and they exchange their speed.
we
calculated the rms speed of each molecule and not the speed of each molecule
which is rather difficult. In this scenario we can find the number of gas
molecules that move with the speed of 5 ms-1 to 10 ms-1
or 10 ms-1 to 15 m s-1 etc. In general our interest is to
find how many gas molecules have the range of speed from 𝛎 to 𝛎 + d𝛎. This is given by Maxwell’s speed distribution function.
The
above expression is graphically shown as follows
From
the Figure it is clear that, for a given temperature the number of molecules
having lower speed increases parabolically
(𝛎2) but decreases exponentially
To
know the number of molecules in the range of speed between 50 m s−1
and 60 m s−1, we need to integrate
But
we can infer the behavior of gas molecules from the graph.
(i)
The area under the graph will give the total number of gas molecules in
the system
(ii)
Figure shows the speed distribution graph for two different temperatures.
As
temperature increases, the peak of the curve is shifted to the right.
It
implies that the average speed of each molecule will increase. But the area
under each graph is same since it represents the total number of gas molecules.
1.4.
Average
It
is defined as the mean (or) average of all the speeds of molecules
If
𝛎 1, 𝛎 2, 𝛎 3………. 𝛎 N are the individual
speeds of molecules then
1.5. Root mean square
Root
mean square speed (𝛎 rms) is defined as the
square root of the mean of the square of speeds of all molecules. It is denoted
by 𝛎 rms
From
the equation we infer the following
(i) rms speed is directly proportional to square
root of the temperature and inversely proportional to square root of mass of
the molecule. At a given temperature the molecules of lighter mass move faster
on an average than the molecules with heavier masses.
Example:
Lighter molecules like hydrogen and helium have high ‘vrms’ than heavier
molecules such as oxygen and nitrogen at the same temperature.
(ii) Increasing the temperature will increase the
r.m.s speed of molecules We can also write the vrms in terms of gas
constant R. Equation can be rewritten as
follows
Since
NAk = R and NAm = M (molar mass) The root
mean square speed or r.m.s speed
The
equation can also be written in terms of rms speed
Impact
of v in nature:
Moon has no atmosphere.
The
escape speed of gases on the surface of Moon is much less than the root mean
square speeds of gases due to low gravity. Due to this all the gases escape
from the surface of the Moon.
No hydrogen in Earth’s atmosphere.
As
the root mean square speed of hydrogen is much greater than that of nitrogen,
it easily escapes from the earths atmosphere.
In
fact, the presence of nonreactive nitrogen instead of highly combustible
hydrogen deters many disastrous consequences.
1.6. Most probable velocity and
average kinetic energy
The average kinetic energy of gas molecules is
directly proportional to absolute temperature. This means that the average
kinetic energy of molecules is the same at a given temperature.
This must be clear to you that all the above
postulates are applicable to ideal gases only i.e. the gas which obey Boyle’s
and Charle’s law under all conditions of temperature and pressure. These are
only approximately valid for real gases.
1.7. law of equipartition of
energy
The average kinetic energy of a molecule
moving in x direction is
According
to kinetic theory, the average kinetic energy of system of molecules in thermal
equilibrium at temperature T is uniformly distributed to all degrees of freedom
(x or y or z directions of motion) so that each degree of freedom will get ½ kT
of energy. This is called law of equipartition of energy.
Average kinetic energy of a monatomic molecule (with f=3)
Average
kinetic energy of linear triatomic molecule (with f = 7)
1.8. Degrees of freedom
The
minimum number of independent coordinates needed to specify the position and
configuration of a thermo-dynamical system in space is called the degree of
freedom of the system.
Example:
A
free particle moving along x-axis needs only one coordinate to specify it
completely. So its degree of freedom is one.
Similarly,
a particle moving over a plane has two degrees of freedom.
A
particle moving in space has three degrees of freedom.
Suppose
if we have N number of gas molecules in the container, then the total number of
degrees of freedom is f = 3N.
But,
if the system has q number of constraints (restrictions in motion) then
the degrees of freedom decreases and it is equal to f = 3N-q where N is
the number of particles.
1.9. Collision frequency
The collision frequency of a gas is defined as: The
number of collisions taking place per second per unit volume (c.c.) of the gas.
Let a gas contain N molecules per cc. From kinetic
considerations it has been established that the number of molecules, n, with
which a simple molecule will collide per second, is given by the relation
where υa = average velocity and
σ =collision diameter.
If the total number of collisions taking place per
second is denoted by z we have
Since each collision involves two molecules, the
number of collision per second per cc, of the gas will be z/2 Hence the
collision frequency
Evidently, the collision frequency of a gas increases
with increase in temperature, molecular size and the number of molecules per
c.c.
1.10.
collision diameter
The kinetic theory of gases treats molecule as point
masses. When two such molecules approach each other, a point is reached at
which they cannot come closer beyond a certain distance.
The closest distance between the centres of the two
molecules taking part in collision is called the collision diameter. It is
denoted by σ.
Whenever the distance between the centres of two molecules
is σ, a collision occurs. The collision diameter can be determined from
viscosity measurements. The collision diameter of hydrogen is 2.74 Å and that of oxygen is 3.61 Å
1.11.
mean free path
Usually
the average speed of gas molecules is several hundred meters per second even at
room temperature (27°C). Odour from an open perfume bottle takes some time to
reach us even if we are closer to the room. The time delay is because the odour
of the molecules cannot travel straight to us as it undergoes a lot of
collisions with the nearby air molecules and moves in a zigzag path. This average
distance travelled by the molecule between two successive collisions is called
mean free path (λ). We can calculate the mean free path based on kinetic
theory.
Expression for mean free path We know from postulates of kinetic theory that
the molecules of a gas are in random motion and they collide with each other.
Between
two successive collisions, a molecule moves along a straight path with uniform
velocity. This path is called mean free path.
Consider
a system of molecules each with diameter d. Let n be the number of molecules
per unit volume.
Assume
that only one molecule is in motion and all others are at rest
If
a molecule moves with average speed v in a time t, the distance
travelled is vt. In this time t, consider the molecule to move in an
imaginary cylinder of volume πd2vt. It collides with any
molecule whose center is within this cylinder. Therefore, the number of
collisions is equal to the number of molecules in the volume of the imaginary
cylinder. It is equal to πd2 vtn. The total path length
divided by the number of collisions in time t is the mean free path.
Though
we have assumed that only one molecule is moving at a time and other molecules
are at rest, in actual practice all the molecules are in random motion. So the
average relative speed of one molecule with respect to other molecules has to
be taken into account.
After
some detailed calculations (you will learn in higher classes) the correct
expression for mean free path
The
equation implies that the mean free path is inversely proportional to number
density. When the number density increases the molecular collisions increases
so it decreases the distance travelled by the molecule before collisions.
Case1:
Rearranging the equation using ‘m’ (mass of the molecule)
But
mn=mass per unit volume = ρ (density of the gas)
Also
we know that PV = NkT
The
equation implies the following
1.
Mean free path increases with increasing temperature. As the temperature
increases, the average speed of each molecule will increase. It is the reason
why the smell of hot sizzling food reaches several meter away than smell of
cold food.
2.
Mean free path increases with decreasing pressure of the gas and diameter of
the gas molecules.
1.12.
Deviations from ideal gas behaviour, (Andrew’s and Amagat’s plots);
An ideal gas is one which obeys the gas laws for the
equation PV = RT at all pressures and temperatures. However, no gas is ideal.
They approach perfection as the temperature gets farther from their boiling
points. Thus the gases H2, N2 and CO2 which
fail to obey the ideal-gas equation are termed as non ideal or real gases
The extent to which a real gas depart from ideal
behaviour may be depicted in terms of a function called compressibility factor,
denoted by Z.
It is defined
Z = PV/RT
The deviation from ideality may be shown by a plot of
compressibility factor, Z against P.
For an ideal gas Z =1. For real gases the deviation
from ideal behaviour will be determined by the value of Z being greater or less
than unity.
Andrews in 1869 determined the isotherm of carbon
dioxide at different temperatures.
The isotherms of carbon dioxide determined by him at
different temperature. Consider the first Isotherm at 13.10C.
The point A represents carbon di-oxide in the gaseous
state occupying a certain volume under a certain pressure. On increasing the
pressure its volume diminishes as is indicated by the curve AB. At B
liquification of gas commences and there after a rapid decrease in volume takes
place at the same pressure as more and more of gas is converted into the liquid
state. At C, the gas has been completely liquified. Now, as the liquid is only
slightly compressible further increase of pressure produces only a very small
decrease in volume. This is shown by a steep line CD which is almost vertical.
Thus along AB, carbon dioxide exists as gas; along BC,
it exists partly as gas and partly as liquid while along CD, it exists entirely
as liquid.
The curve EFGH at 21.50C shows a similar
behaviour except that now the liquification commences at higher pressure and
the horizontal portion FG, representing decrease in volume, becomes smaller. At
still higher temperature, the horizontal portion of the curve becomes shorter
and shorter until at 31.10C it reduces just to a point represented
by X.
The curve passing through this point X marks the
boundary between gaseous carbon dioxide and on the right and liquid carbon
dioxide on the left.
Andrews noted that above 31.10C there was
no possibility of liquefaction of carbon dioxide how great the pressure is
applied. At this temperature the gas is in critical state. The point X is then
called the critical point. The isotherm passing through this point is
called the critical isotherm and the temperature corresponding to this
isotherm (31.10C) is called critical temperature.
The critical phenomenon observed by Andrews in
connection with carbon dioxide may be observed with any other gas. The pressure
required to liquefy the gas at critical temperature is called the critical
pressure and the volume occupied by one mole of the gas under these
conditions is called critical volume.
It is possible to convert liquid carbondioxide into
gas and vice-versa, without any discontinuity that is without having at any
time more than one phase present, on joining the end of the horizontal portion
of the various isotherm, a bonding curve CGXFB represented by the dotted line
is obtained. At the top lies the critical point X, with in the area of the
boundary curve, both liquid and gaseous state can coexist but outside this area
either liquid or gaseous state alone can exist. Because of this coexistence
curve, it is possible to distinguish between the two states of matter, namely,
gas and liquid. However, in practice this is not always true because it is
possible to convert matter from one state into another without any sharp
discontinuity.
(i) Increase the temperature of the gas keeping volume
constant. The pressure rises along xy.
(ii) Having reached y, the pressure is kept constant
and the gas is cooled; this decrease the volume along the line yz.
Thus we have passed from x to z without the gradual
change as it occurs along the line BC, ie condensation in the usual sense of
the term did not occur. Point 2 could be said to represent a highly compressed
gaseous state of the substance. Whether we refer to the state in the region of
point z as liquid state or as highly compressed gaseous state depends purely
upon which of the two view points happens to be convenient at the moment.
Thus, in the absence of the surface of discontinuity,
there is no way of distinguishing between liquid and gas. Von der Waals
equation and critical state Thomas in 1871 studied the isotherms of carbon
dioxide drawn by Andrews. He suggested that there should be no sharp points in
the isotherms below the critical temperature. These isotherms should really
exhibit a complete continuity of state from gas to liquid. This he showed by a
theoretical wavy curve.
The curve MLB represents a gas compressed in a way
that would remain stable. The curve MNC represents a superheated liquid. This
type of discontinuity of state is predicted by von der Waals cubic equation.
According to it, for any given values of P and T there
should be three values of v. These values are indicated by B, M and C of the
curve. The three values of v become closer as the horizontal part of the
isotherm. At the critical point, these values become identical.
This enables the calculation of critical temperature, critical
pressure and critical volume in terms of von der Waals constants.
The actual determination of critical constants is
often a task of considerable difficulty of these critical pressure and critical
temperature can be measured relatively easily with the help of Cagniard de la
Tour’s apparatus. The most accurate method for determining critical volume in
due to Amagat.
1.13.
compressibility factor, Z, and its variation with pressure for different
gases.
An ideal gas is one which obeys the gas laws for the equation
PV = RT at all pressures and temperatures. However no gas is ideal. They
approach perfection as the temperature gets farther from their boiling points.
Thus the gases H2, N2 and CO2 which fail to
obey the ideal-gas equation are termed as non ideal or real gases
The extent to which a real gas depart from ideal
behaviour may be depicted in terms of a function called compressibility factor,
denoted by Z.
It is defined
Z = PV/RT
The deviation from ideality may be shown by a plot of
compressibility factor, Z against P.
For an ideal gas Z =1. For real gases the deviation
from ideal behaviour will be determined by the value of Z being greater or less
than unity.
Effect of pressure
The compressibility factor Z, plotted against pressure
for H2, N2 and CO2 at constant temperature.
At very low pressure for all these gases Z is
approximately one. This indicates that all real gases exhibit ideal behaviour
(upto 10 atm). For hydrogen curve lies above ideal gas curve at all pressure.
For nitrogen and carbon di-oxide, Z first decreases.
It passes to a minimum then increases continuously with increase of pressure.
For gas like CO2 the dip in the curve is greatest as it is most
easily liquified.
1.14.
Equations of states for real gases-van der Waal’s equation
Von der Waal’s 1873 studied the postulates of kinetic
theory in detail and found that there are two faulty postulates.
(i) The molecules in a gas are point masses and
possess no volume. (ii) There are no intermolecular attractions in a gas.
Von der Waal’s was the first to introduce
systematically the correction terms due to the above two invalid assumptions in
the ideal gas equation PV = nRT.
His corrections are given below.
Volume correction
Volume of the gas in the available space for the
movement of gas molecules. Volume V of an ideal gas is the same as the volume
of the container. The dot molecule of ideal gas has zero-volume and the entire
space in the container is available for their movement.
But von der Waals assume that molecules of real gas
are rigid spherical particles which posses a definite volume. The volume of
real gas is, therefore ideal volume minus the volume occupied by gas molecules.
If b is the effective volume of molecules per mole of the gas then corrected
volume should be V-b = Videal For n moles Videal = V-nb b is also known as
excluded volume.
Now let us consider two molecules of radius r
colliding with each other Obviously they cannot approach each other closer than
a
1.15.
Virial equation; Boyle temperature
Experiments of Andrews, Amagat and others show that no
gas obeys Boyle’3 law except over a very restricted range. Kamerling Onnes
observed that the behaviour of real gases can be expressed by an empirical
equation of the form
pV = A + Bp + Cp2 + Dp3 + ……
where the coefficients A, B,C,D, …. are
functions of temperatures and are called virial coefficients -
A is the first virial coefficient, B the second virial
coefficient etc. For a given temperature, however, they are constants
characteristic of the fluid concerned. The coefficients decrease rapidly for
higher terms so that C, D, …. etc. are small and the terms involving
them become important only at very high pressures.
Plainly, the first virial coefficient A = RT,
since for p → 0, equation should reduce
to the ideal gas equation for one mole of the gas.
The second virial coefficient B is particularly
important. For all gases, it varies in a similar way and at very low
temperatures, it has a large negative value, increases gradually to zero with
rise in temperature and finally becomes positive. At room temperature, B
< 0 for O2, N2 and CO2 and B > 0
for H2 and He.
If at any temperature, B - 0, then neglecting
C, D, etc. which are very small, we get from equation pV = const. = A
= RT
and the Boyle’s law is obeyed over a wide range of
pressure. The temperature at which the value of the second virial
coefficient B vanishes is the Boyle temperature or Boyle point.
One may as well any that the Boyle temperature is the temperature at which B
changes sign.
while ordering Amagat’s how the concept of boyle temperature originated. At moderate and
low pressures, it is sufficient to retain only the first two terms of the
equation
pV = A + Bp
As p →0, pV →A end the gM obeys Boyle’s law more and more
accurately Th, correction to gas scale thermometers is usually made through
equation
1.16.
Numerical problems based on equations of states for real gases
Problem 1
Calculate the partial pressures N2 and H2
in a mixture of two moles of N2 and two moles of H2 at
STP.
Problem 2
If a gas diffuses at the rate of one-half as fast as O2,
find the molecular mass of the gas.
Solution
Applying Graham's law of diffusion.
Problem 3
50ml of gas A effuse through a pin - hole in 146
seconds. The same volume of CO2 under identical conditions effuse in
115 seconds. Calculate the molecular mass of A.
Problem 4
One mole of carbon-dioxide was found to occupy a
volume of 1.32 litre at 48°C and at a pressure of 16.4 atm. Calculate the
pressure of the gas that would have been expected to behave ideally and
non-ideally.
Problem 5
Vanderwaal's constants for hydrogen chloride gas are a
= 3.67 atm lit-2 and b = 40.8 ml mol-1. Find the critical
temperature and critical pressure of the gas.
Problem 6
The critical temperature of hydrogen gas is 33.2°C and
its critical pressure is 12.4 atm. Find out the values of 'a' and 'b' for the
gas.
ISOTHERMS OF REAL GASES
1.17.
critical phenomena – isotherms of CO2
Thomas
Andrew gave the first complete data on pressure-volume- temperature of a
substance in the gaseous and liquid states. He plotted isotherms of carbon
dioxide at different temperatures. From the plots we can infer the following.
At
low temperature isotherms, for example, at 130C as the pressure
increases, the volume decreases along AB and is a gas until the point B is
reached. At B, a liquid separates along the line BC, both the liquid and gas
co-exist and the pressure remains constant. At C, the gas is completely
converted into liquid. If the pressure is higher than at C, only the liquid is
compressed so, there is no significant change in the volume.
The
successive isotherms shows similar trend with the shorter flat region. i.e. The
volume range in which the liquid and gas coexist becomes shorter. At the temperature
of 31.10C the length of the shorter portion is reduced to zero at
point P. In other words, the CO2 gas is liquefied completely at this
point.
This
temperature is known as the liquefaction temperature or critical temperature of
CO2. At this point the pressure is 73 atm. Above this temperature CO2
remains as a gas at all pressure values. It is then proved that many real gases
behave in a similar manner to carbon dioxide.
Though
the nature of isotherm remains similar, the critical temperature, the
corresponding pressure and volume are characteristics of a particular gas.
Now
we can define the critical constants as follows. Critical temperature (Tc)
of a gas is defined as the temperature above which it cannot be liquefied even
at high pressure. Critical pressure (Pc) of a gas is defined as the
minimum pressure required to liquefy 1 mole of a gas at its critical
temperature. Critical volume (Vc) is defined as the volume occupied
by 1 mole of a gas at its critical temperature and critical pressure. The
critical constants of some common gases are given in Table.
|
Name of the Gas |
Critical
Temperature (T) in K |
Critical
Pressure (Pc) in atm |
Critical
Volume (Vc) cm3 mol-1 |
|
Helium (He) |
5.2 |
2.26 |
57.8 |
|
Carbon dioxide(CO2) |
304.2 |
72.9 |
94.0 |
|
Oxygen (O2) |
154.8 |
50.14 |
78.0 |
|
Nitrogen (N2) |
126.3 |
33.54 |
90.1 |
|
Hydrogen (H2) |
33.2 |
12.80 |
65 |
|
Water (H2O) |
647.4 |
218.3 |
55.3 |
|
Ammonia (NH3) |
405.5 |
111.3 |
72.5 |
|
Hydrogen Chloride
(HCl) |
324.7 |
81.5 |
81.0 |
|
Methane (CH4) |
190.6 |
45.6 |
98.7 |
|
Ethylene (C2H4) |
283.1 |
50.50 |
124 |
1.18.
Van der waal’s equation and the critical state
The
van der Waals equation for n moles is
For
1 mole
From
the equation we can derive the values of critical constants Pc, Vc
and Tc in terms of a and b, the van der Waals constants, On
expanding the above equation
Multiply
equation by V2 / P
When
the above equation is rearranged in powers of V
The
equation is a cubic equation in V. On solving this equation,
we
will get three solutions. At the critical point all these three solutions of V
are equal to the critical volume VC. The pressure and temperature
becomes Pc and Tc respectively
we
can equate the coefficients of V2, V and constant terms
The
critical constants can be calculated using the values of van der waals constant
of a gas and vice versa.
1.19.
Liquefaction of gases
For
important commercial operations such as LPG and rocket fuels, we require gases
in their liquid state.
The
liquefication methods are based on the Joule-Thomson effect. He observed
appreciable cooling when the compressed gas is forced through an orifice plug
into a low-pressure region. This phenomenon of lowering of temperature when a
gas is made to expand adiabatically from a region of high pressure into a
region of low pressure is known as Joule- Thomson effect. This effect is
observed only below a certain temperature, which is a characteristic one for
each gas. This temperature below which a gas obeys Joule-Thomson effect is
called inversion temperature (Ti). This value is given using
van der waals constants a and b.
Gases
like O2, He, N2 and H2 have very low T, hence
Joule-Thomson effect can be applied for cooling effectively At the inversion
temperature, no rise or fall in temperature of a gas occurs while expanding.
But above the inversion temperature, the gas gets heated up when allowed to expand
through a hole.
There
are different methods used for liquefaction of gases:
In Linde’s method, Joule-Thomson effect is used
to get liquid air or any other gas.
In Claude’s process, the gas is allowed to
perform mechanical work in addition to Joule-Thomson effect so that more
cooling is produced.
In Adiabatic process, cooling is produced by
removing the magnetic property of magnetic material such as gadolinium
sulphate. By this method, a temperature of 10-4 K i.e. as low as 0 K
can be achieved.
Conditions of liquefaction of gases
Many industrial processes require large quantities of
liquid air, liquid ammonia, liquid carbondioxide etc. The production of liquids
from various gases is therefore an important commercial operation.
There are different methods of liquefaction of gases,
such as (i) based on the concept of critical temperature followed by the
compression (ii) based on Joule-Thomson effect (iii) Adiabatic demagnetisation.
In the case of gases like NH3, Cl2,
SO2 and CO2 whose Tc values are near and below the
ordinary temperatures, they can be liquefied easily by increasing the pressure
alone at their respective Tc values.
Gases like H2, O2, N2
and He have very low Tc values and hence Joule Thomson effect may be applied to
bring in effective cooling.
Helium is cooled by Joule-Thomson effect to a lower
temperature and further cooling for its liquefaction, is carried out by the
method of adiabatic demagnetisation.
1.20.
QUESTIONS
1.
Write the mathematical expression for Boyle's law.
2.
Compare the partial pressures of gases A and B when 3 moles of A and 5
moles of B mixed in constant volume, and 25oC and 1 atm pressure.
3.
Give the correction factors for the volume and pressure deviation for a
Vanderwaal's gas.
4.
A sample of an ideal gas escapes into an evacuated container, there is
no change in the kinetic energy of the gas. Why?
5.
What is the change in temperature when a compressed real gas is allowed
to expand adiabatically through a porous plug.
6.
Define Boyle's law and Charle's law.
7.
What are measurable properties of gases?
8.
What is the molar volume of nitrogen at 500K and 600 atm according to
ideal gas law?
9.
Define Graham's law of diffusion.
10.
Give the values of R-gas constant in calories and Joules.
11.
What are the units of Vanderwaals constants 'a' and 'b' ?
12.
Write the significance of Vanderwaal's constants.
13.
Write the limitations of vanderwaal equation of state.
14. Define Joule-Thomson effect.
UNIT-II
1.
Liquid and Solid State
1.1. Properties of Liquids
As you have studied earlier in this unit that the
properties of liquids arise from (i) The nature and (ii) The magnitude of
intermolecular forces of attraction existing between their molecules. The
important properties of liquids are
1. Vapour pressure 2. Surface tension 3. Viscosity 4.
Refraction
1.2. Surface tension,
viscosity and their applications.
The existence of strong intermolecular forces of
attraction in liquids gives rise to a property known as surface tension. The
phenomenon of surface tension can be described as follows.
A molecule in the interior of a liquid is attracted
equally in all directions by the molecules around it. A molecule in the surface
of a liquid is attracted only sideways and towards the interior. The forces on
the sides being counterbalanced, the surface is pulled only inward the liquid.
These unbalanced attractive forces acting downward tend to draw the surface
molecules into the body of the liquid and, therefore, tend to reduce the
surface to minimum. The liquid then behaves as if it were under a strain or
tension. It is this force which is called surface tension. It may be defined as
“the force in dynes acting on the surface of the liquid at right angles to one
centimetre length of the surface”. It is represented by a symbol ɣ (gama).
In CGS system the unit of surface tension is dynes per
centimetre (dyne cm-1). In SI system, the unit is Newton per metre
(Nm-1). Both these units are related as follows
Effect of temperature on surface tension
When temperature increases, there is increase in
kinetic energy of liquid molecules (KE α T) thereby decreasing intermolecular
forces. It results in decrease in inward pull functioning on the surface of the
liquid. That means you can say surface tension decreases with increase in
temperature. As surface tension arises of the attractional forces operating
between the molecules, Ramsay and Shields gave the following relationship
between the surface tension of a liquid and its temperature.
γ (M/d)2/3 = k(tc-t-6)
where k is constant tc is critical temperature and t
any other temperature γ (M/d)2/3 represents molar surface energy of liquid.
Viscosity
Some liquids flow more rapidly than others. In other
words, liquid molecules pose resistance to the flow of one layer over the
other. This property of liquids which determines their flow is termed
viscosity. The property of the liquid which determines its flow is called
viscosity of the liquid.
The resistance to flow of one layer of liquid
molecules over another depends on the following factors.
1. The intermolecular attractive forces do not permit
a free flow of molecules in a liquid. The strength of intermolecular forces
gives a rough major of the viscosity of the liquids.
2. The molecular weight or mass of the molecules of a
liquid also determines flow of the liquid. Thus heavier the molecule of a given
liquid the greater will be its viscosity.
3. Structure and shape of the molecules of a liquid
place an important role in influencing its viscosity. Liquids with the large
irregularly shaped molecules are generally known to be more viscous than those
with small and symmetrical molecule. Since only hard symmetrical molecules have
perfectly elastic collision, the large and irregular molecules will have less
elastic molecules amongst themselves. Thus collisions between large molecules
involves the loss of kinetic energy and as a consequence the intermolecular
forces dominating the molecules tends to stick together. This increases the
viscosity of the liquid.
4. An increase in temperature decreases the viscosity
of the liquid, the molecular motion increases at the expense of cohesive forces
causing resistance to flow.
5. The increase of pressure goes to strengthen the
cohesive forces between molecules.
Hence with increase of pressure the viscosity of a
given liquid increases somewhat.
The flow is a characteristic property of liquids. Let
us consider flow of a liquid. A liquid may be considered to be consisting of
molecular layers arranged one over the other. When shearing force is applied,
it flows.
However, the force of friction between the layers
offers resistance to this flow. Viscosity of a liquid is a measure of its
frictional resistance.
Let us examine a liquid flowing on a glass surface.
The molecular layer in contact with the stationary surface has zero velocity.
The successive layers above it move with increasingly higher velocities in the
direction of the flow.
Now consider two adjacent moving layers of a liquid.
Let these be separated by a distance dx having velocity difference d . The
force of friction (F) resisting the relative motion of the two layers is
directly proportional to the area A and velocity difference d , while it is
inversely proportional to the distance between the layers dx.
where η (eta) is the proportionality constant. It is known as
coefficient of viscosity or simply viscosity of a liquid. It may be defined by
the above equation as:
the force of resistance per unit area which will
maintain unit different of velocity between two layers which are unit distance
apart.
Unit of viscosity:
n = F/A x dυ/dx
= force/area x distance/velocity
= mass x length x time-2/length2
X length/length/time
= mass x length-1 x time-1
In CGS system the unit of 1'1 is expressed as g cm-1s-1,
it is called poise. In practice smaller units centipoise (10-2
poise) and millipoise (10-3 poise) are used.
A liquid is said to have coefficient of viscosity as
one poise when a force of one dyne maintains a velocity difference of one
centimetre per second between two parallel layers of the liquid one cm apart
and have an area of contact equal to on square cm. The reciprocal of
viscosity is known as fluidity.
Effect of temperature on viscosity
As the temperature increases, the molecular motion
increases at the expense of cohesive forces causing resistance to flow.
Therefore, the viscosity of liquids is found to decrease by 1 to 2 per cent for
each degree rise of temperature.
Determination of viscosity
The apparatus used for determination of viscosity in
the laboratory is knwon as Ostwald’s viscometer. A simple form of Ostwald
viscometer, the left- hand limb is essentially a pipette with two celibration
marks A and B. A length of capillary
tube joins the pipette to the bulb D in the right-hand limb.
A definite volume of liquid (say about 25 ml) is
poured into the bulb D with a pipette. The liquid is sucked up near to the top
of the left-hand limb with the help of rubber tubing attached to it. The liquid
is then released to flow back into the bulb D. the time t1 to flow from A to B
is noted with a stopwatch. Then the apparatus is cleaned and the experiment is
repeated with water taking about the same volume. The time of flow of water t2
from A to B is recorded. The density of the liquid d and that of water dw are
determined with the help of density bottle. The relative viscosity is
calculated from the expression
η/ηw = dt1/dwt2
where is η is coefficient of viscosity of the
experimental liquid and ηw is the coefficient of viscosity of water. Knowing
the value of coefficient of viscosity of water ηw at the temperature of
experiment, the absolute viscosity coefficient η of the given liquid can be
found.
Viscosity and chemical constitution As you know viscosity
is largly due to intermolecular attractions which resist the flow of liquid. Therefore,
some sort of relationship between viscosity and molecular structure should be
there. Viscosity is also dependent on the shape, size and mass of the liquid
molecules. The following general rules have been discovered.
(i) Dunstan Rule: Dunstan in 1909 showed
that coefficient of viscosity η and molecular volume (d/M) were related as d/M x ηx106 = 40 to 60 This expression holds
only for normal (unassociated) liquids for associated liquids the value is much
higher than 60. For example, the value for benzene is 73 and for water it is
559 and for ethanol it is 189. This shows benzene is a normal liquid while
water and ethanol are associated liquids.
(ii) Molar Viscosity: The product of molar
surface and viscosity is termed as molar viscosity. That is molar viscosity =
molar surface x viscosity = (M/d)2/3 x ή Thorpe and Rodger (1894) found that
molar viscosity is an additive property at the boiling point. They worked out
the molar viscosity contributions of several atoms (C, H, O, S, etc) and
groups. From these, they calculated the molar viscosity of liquid from its
proposed structure. By tallying this value with the experimental one, they were
able to ascertain the structure.
(iii) Rheochor: Newton Friend (1943)
showed that if molecular volume (M/d) be multiplied by the eighth root of the
coefficient of viscosity it gives a constant value [R], it is termed as
Rheochor M/d x η1/8 = R Like parachor, rheochor is both additive and
constitutive property.
1.3. Crystalline and
amorphous
Solids can generally be classified into two broad
categories:
(i) Crystalline solids (ii) Amorphous solids A
crystalline solid exists as small crystals, each crystal having a
characteristic geometrical shape. In a crystal, the atoms, molecules or ions
are arranged in a regular, repeating three-dimensionl pattern called the
crystal lattice. examples are sugar, salt etc.
An amorphous solid has atoms, molecules or ions
arranged at random and lacks the ordered crystalline lattice. Examples of
amorphous solids are rubber, plastics and glass.
In their disordered structure, amorphous solids are
regarded as supercooled liquids with high viscosity. The liquid nature of glass
is sometimes apparent in very old window panes that have become slightly
thicker at the bottom due to gradual downward flow.
1.4.
Differences - geometry, isotropy and anisotropy, melting point;
Anisotropy and isotropy: Amorphous substances differ
from crystalline solids and resemble liquids in another important aspect. Their
properties such as electrical conductivity, thermal conductivity mechanical
strength and refractive index are same in all directions.
Amorphous substances are said to isotropic. Liquids
and gases are also isotropic.
Crystalline solids on the other hand are anisotropic,
because their physical properties are different in different directions. For
example the velocity of light through a crystal varies with the direction in
which it is measured. Thus, a ray of light enter such a crystal may split up
into two components each following different velocity. This phenomenon is known
as double refraction.
Anisotropy in crystals is due to different arrangement
of particles in different directions only two different kinds of atoms is
depicted if the properties are measured along the direction indicated by the
slanting line CD, they will be different from those measured in the direction
indicated by the vertical line AB. The reason is that while in the first case,
each row is made up of alternate types of atoms, in the second case; each row
is made up of one type of atoms only. In amorphous solids, atoms or molecules
are arranged at random and in a disorderly manner and, therefore all directions
are identical and all properties are alike in all directions.
1.5. isomorphism,
polymorphism.
In our surroundings, we can find numerous types of
compounds, which are different in their appearances, or we can say that their
morphologies differ. Some chemical properties are determined just by
appearances. The morphology of a compound describes its external structure. The
expressions isomorphism and polymorphism are utilised to describe the external
features of compounds. The major difference between the two is that in
isomorphism, two or more compounds show identical morphologies, whereas in the
case of polymorphism, the same compound shows different morphologies.
- Introduction to Isomorphism
The law of isomorphism was first given by Mitscherlich
in 1819. When identical morphology is found in two or more compounds, they are
called isomorphous compounds. This implies that crystal structure is present in
not only a single compound but may be present in other compounds as well. This
property is called isomorphism.
- Properties of Isomorphic Substances
The ratio of atoms found in isomorphic compounds is
the same, which indicates the same empirical formula but the compounds differ
in accordance with their atomic structure; therefore, they possess different
physical properties. The different physical properties include density, mass,
chemical reactivity, etc.
Conditions of Isomorphism
Polar organic compound:
- Chemical formulas must be comparable.
- There should be the same chemical and geometrical arrangement of
ions in unit cells.
- The size of the ions must not vary as much.
- The polarisation of ions should be the same.
Non-polar organic compound:
- Molecular volume does not contain major differences.
- The similarity in the crystal structure.
- Similar chemical constituents are used.
Examples
The following examples show isomorphic nature:
- Calcium carbonate and sodium nitrate
The shape of both the compounds is trigonal. The
atomic ratio of all the elements present in them is 1:1:2. But it should be
noted that the physical properties, chemical properties, and molar mass are
different in both compounds.
- Another example is potassium chromate and potassium sulphate.
Applications of Isomorphism
It is used for the following purposes:
- Determination of atomic weight
- Identifying the valency
- Correction of atomic weight
Limitations of isomorphism
- Isomorphism is shown by some of the compounds having differences in
accordance with their crystalline structure. Examples of such compounds
are ferrous sulphate and zinc sulphate.
- It is mandatory for isomorphous substances that different
structures of compounds have an equal number of ions in their unit cell.
However, some of the compounds still do not obey this rule, yet they are
isomorphous compounds.
- Compounds with similar chemical formulas and the same number of
atoms disobey the isomorphism condition and are non-isomorphous compounds.
Introduction to Polymorphism
When the same compound shows different morphologies,
it is termed as polymorphism, and that particular substance is called a
polymorphic substance. The shape and structure may differ for the same
compound.
- Properties of Polymorphic Substances
There is a difference in the physical and chemical
properties of polymorphic substances. The physical properties like melting
point, density, hardness, solubility, electrical conductivity differ in various
polymorphic substances. The chemical reactivity of a substance also changes in
different polymorphic substances.
- Factors Affecting Polymorphism
- The polymorphism is affected by temperature and humidity. For
example, solid ice melts down to liquid with an increase in temperature.
- The polymorphism is affected by photostability. On exposure to
visible light, the chemical and physical properties of a substance change.
- The polymorphism is also affected by grinding. On grinding, the
bond between the atom gets weaker. That is the reason why the anhydrous
form is less stable than the dihydrate form.
Types of Polymorphism
- Monotropic polymorph: Monotropic polymorph means that the compound
exhibits several polymorphic forms, but out of them, only one form shows
stability at all temperatures. Examples: Glyceryl stearate,
Chloramphenicol palmitate, Metolazone.
- Enantiotropic polymorph: At a certain temperature and pressure, one
polymorph is stable, while others are stable at different ranges of
pressure and temperature. Example: Sulphur
Examples
- Carbon has two polymorphs, which are graphite and diamond.
- Calcium carbonate has two polymorphs, one is orthorhombic
aragonite, and the other is hexagonal calcite.
- Silicon oxides have six polymorphs.
Application of Polymorphism
Polymorphism has a wide range of applications in the
pharmaceutical industries for the production of various kinds of drugs and
medicines. It depends upon the chemical properties of the polymorphic forms.
Since different polymorphic forms show different chemical properties, the
effectiveness of drugs on the body can be determined accordngly.
Differences in Isomorphism and Polymorphism
|
Isomorphism |
Polymorphism |
|
Two or more compounds
having the same morphologies are referred to as isomorphous substances. |
Different
morphologies are depicted by the same compounds. |
|
Identical shapes |
Different shapes |
|
There must be two or
more different compounds required. |
There are different
forms present for the same compound. |
|
They have the same
atomic ratio which is depicted by empirical formulas. |
The polymorphic
compounds may show similar or different atomic ratios. |
1.6. Crystals –size and
shape;
Crystals are bound by surface which is usually
planner. These surfaces are called faces and where two faces intersect an edge
is formed. The angle between the normals to the two intersecting faces is the
interfacial angle or the angle between any two faces is called interfacial
angle. Although the size of the faces or even shapes of crystals of one and the
same substances may vary widely with the condition of formation or other
factors, yet the interfacial angles between any two corresponding faces of the
crystal remain invariably the same throughout. Now it is clear to you that
although the external shape is different yet the interfacial angles are the
same.
Space lattice
Rather than drawing the entire pattern, it is much
more convenient to represent the unit of pattern by a point. Each point then
represents the position of an atom, ion, molecule or group of ions and
molecules. The regular three-dimensional arrangement of identical points in
space gives rise to what is known as space lattice or crystal lattice (Fig 5.3)
the positions occupied by the particles in the space lattice are called lattice
sites or lattice points.
Unit cell:
It is defined as “the smallest geometrical portion of
the crystal, which when repeated in three dimensional, would generate the
complete crystal”. Each unit cell, in turn, must be constituted of atoms,
molecules or ions, as the case may be and arranged to give the particular
geometrical configuration of the crystal.
Unit cells are of following types;
(a) Simple or primitive unit cell (P): The simplest unit
cell which has the lattice points at the corners is called a simple or
primitive unit cell. It is denoted by P.
(b) Non primitive or multiple unit cell: When unit cell contains more than one lattice
points, it is called non primitive or multiple unit cell. It is further divided
into the following three categories:
(i) Face centred unit cell (F): When a unit cell,
besides the points present at the corners of the unit cell,
There is one point at the centre of each face, it is
called face centred arrangement or face centred unit cell. It is denoted by F.
(ii) Body
centred unit cell (I): When in a unit cell, besides the points at the corners
of the cell, there is one point at the centre with in its body, it is called
body-centred arrangement or body-centred with cell. It is denoted by I.
(iii) Side centre or end face unit cell: When in a unit cell,
besides the points at the corners of the cell, the points are located at the
centre of any two parallel faces of the unit cell, it is called side-centred or
end face unit cell. It is denoted by c.
1.7. laws of
crystallography;
There are three laws of crystallography which deal
with the interfacial angles and the rational indices.
Law of constancy of interfacial angle
The crystal may be smaller or bigger in size and may
be prepared by any method, but the interfacial angles are always the same.
Law of rational indices
Now it will be clear to you that crystal lattice
consists of unit cells arranged in parallel planes. Thus each crystal plane
lies parallel to the crystal face as also to the unit cell face. These planes
cut the three axes along the three crystallographic axes (ox, oy, oz), Hauy
proposed that a given crystal plane could be described in terms of intercepts
along the axes. The reciprocals of these intercepts are small whole numbers,
these numbers h,k and l are called Miller indices after the name of British
Scientist W.H. Miller. Thus Miller indices of a plane may be defined as the
reciprocals of the intercepts which the plane makes will the axes.
For example let us consider a crystal system with the
axes OX, OY and OZ. ABC represents a unit cell surface while LMN depicts
another crystal plane under study
The intercepts of the unit plane are OA, OB and OC
which have the length a,b and c respectively. The intercepts of the plane under
study are OL, OM and ON. These can be expressed as multiples of the intercepts
a,b,c i.e. la, mb and nc. Here l m and n are either integral whole numbers or
fraction of whole numbers. The reciprocal of these numbers are written together
in brackets (h,k.l) to give the Miller indices of the plane under study.
To find the Miller indices proceed as follows.
(i) Write the intercepts as multiples of a,b,c say la,
mb, nc
(ii) Take the reciprocals of l, m and n
(iii)Clear fraction to get whole numbers h,k,l. (iv)
Miller indices to the plane are (h,k,l).
Example:
calculate the Miller indices of crystal planes which cut through the
crystal axes at
1.8. symmetry elements –
plane, centre and axis; Miller indices
Law of symmetry
Besides the interfacial angles, another important
property of crystals is their symmetry. The law of symmetry states that: All
crystals of the same substance possess the same elements of symmetry.
Symmetry in crystals may be with respect to a plane, a
line or a point, accordingly there are three types of symmetry associated with
a crystal.
Plane of symmetry When an imaginary plane
can have divided a crystal into two parts such that one is the exact mirror
image of the other, the crystal is said to have a plane of symmetry.
Axis of symmetry An axis of symmetry is
a line about which the crystal is rotated such that it presents the similar
appearance more than once during complete rotation i.e. rotation through an
angle of 3600. Depending upon its nature, a crystal may have 2-fold, 3-fold,
4-fold or 6-fold axes of rotation.
For example in the case of a cube, an axis passing
perpendicularly through the centre is such that when the cube is rotated it
presents similar appearance in three rotation of 900 each and the
same appearance after the fourth rotation, such an axis is called a four-fold
or tetrad axis. If the same similar appearance is repeated after an angle of
1800, the axis is called two-fold or diad axis. In the same way, if
the same or similar appearance is repeated after an angle of 1200,
the axis is called a three-fold or triad axis . If the same or similar axis is
repeated after an angle of 600, as in the case of a hexagonal
crystal, the axis is called six-fold or hexad axis. In general, if the same or
similar appearance of a crystal is repeated on rotation through an angle of
360/n, around an imaginary axis, the axis is called an n- fold axis.
100, 110 and 111 planes of a crystal
100 plane , 110 plane ,
111 planes
Centre of Symmetry
It is a pound at the centre of the crystal so that any
line drawn through it will meet the surface of the crystal at equal distance on
either side.
It may be pointed out that a crystal may have number
of planes or axis of symmetry but it has only one centre of symmetry.
1.9. Unit cells and space
lattices
Crystalline
solid is characterised by a definite orientation of atoms, ions or molecules,
relative to one another in a three dimensional pattern. The regular arrangement
of these species throughout the crystal is called a crystal lattice. A
basic repeating structural unit of a crystalline solid is called a unit cell.
The following figure illustrates the lattice point and the unit cell.
A
crystal may be considered to consist of large number of unit cells, each one in
direct contact with its nearer neighbour and all similarly oriented in space.
The number of nearest neighbours that surrounding a particle in a crystal is
called the coordination number of that particle.
A
unit cell is characterised by the three edge lengths or lattice constants a, b
and c and the angle between the edges α, β and
ɣ.
1.10.
classification of crystal systems
There
are seven primitive crystal systems; cubic, tetragonal, orthorhombic,
hexagonal, monoclinic, triclinic and rhombohedral. They differ in the
arrangement of their crystallographic axes and angles. Corresponding to the
above seven, Bravais defined 14 possible crystal systems as shown in the
figure.
1.11.
Bravais lattices
1.12.
X – ray diffraction
You know that when x-rays were first investigated,
problem arose of measuring their wave length. It is a well known fact that, if
light is allowed to strike a surface consisting either of a series of edges or
lines spaced closely enough to be of the same order of magnitudes as that of
the wavelength of light, the beam of light is diffracted. And the various
radiations are dispersed into a series of spectra known as, first, second,
third etc. order of spectra. Further, there is definite relation between the angle
of diffraction, the wavelength of radiation and the spacing of the lines on the
ruled grating. Since x-rays are of the same nature as light, it should be
theoretically possible to determine the wavelength of this radiation in the
same way. However, it is impossible by any mechanical means to rule a grating
as fine as that required, namely one with 108 lines per centimeter.
For this purpose, Laue (1912) suggested that crystal can act as grating to
x-rays as wavelength of x-rays is comparable to the interatomic distance. When
a beam of x-rays is allowed to fall on a crystal, a large number of images of
different intensities are formed. If the diffracted waves are in the same
phase, they reinforce each other and a series of bright spots are produced on a
photographic plate placed in their path. On the other hand, if the diffracted
waves are out of phase, dark spots are caused on the photographic plate. From
the overall diffraction pattern produced by a crystal, can arrive at the
detailed information regarding the position of particles in the crystal.
1.13.
Bragg’s equation
Bragg’s
pointed that the scattering of x-rays by crystal could be taken to be
equivalent to reflection from successive planes of atoms in the crystal.
However, the reflection of x-rays can take place only at certain angles which
are dependent on wavelength of the x-rays and the distance between the planes
of the crystal. The fundamental equation which gives a simple relation between
the wave length of x-rays, the interplaner distance in the crystal and the
angle of reflection is known is known as Bragg’s equation. This equation can be
derided as follows.
The
horizontal lines represent parallel planes in the crystal structure separated
from one another by a distance d. suppose a beam of x-rays incident at an angle
falls on the crystal. Some of them will be reflected from uppermost plane at
the same angle, while the other will be absorbed and get reflected from
successive planes, as shown in
X-Ray
diffraction analysis is the most powerful tool for the determination of crystal
structure. The inter planar distance (d) between two successive planes of atoms
can be calculated using the following equation form the X-Ray diffraction data
2dsinθ =
nλ
The
above equation is known as Bragg’s equation.
Where
λ is the wavelength of X-ray used for diffraction. θ is the angle of diffraction n is the order of
diffraction By knowing the values of θ,λ
and n we can calculate the value of d.
d = nλ /
2sinθ
Using
these values, the edge length of the unit cell can be calculated.
Packing in atomic solids
1.14.
simple cubic
In
the simple cubic unit cell, each corner is occupied by an identical atoms or
ions or molecules. And they touch along the edges of the cube, do not touch
diagonally. The coordination number of each atom is 6. Each atom in the corner of the cubic unit cell
is shared by 8 neighboring unit cells and therefore atoms `per unit cell is equal
to NC /8 where Nc is the number of atoms at the corners.
∴ Number of atoms in a SC unit cell
1.15.
body centered cubic
In
a body centered cubic unit cell, each corner is occupied by an identical particle
and in addition to that one atom occupies the body centre. Those atoms which
occupy the corners do not touch each other, however they all touch the one that
occupies the body centre. Hence, each atom is surrounded by eight nearest
neighbours and coordination number is 8. An atom presents at the body
centrebelongs to only to a particular unit cell i.e unshared by other unit
cell.
∴ Number of atoms in a bcc unit cell
1.16.
face centered cubic
In
a face centered cubic unit cell, identical atoms lie at each corner as well as
in the centre of each face. Those atoms in the corners touch those in the faces
but not each other. The atoms in the face centre is being shared by two unit
cells, each atom in the face centers makes 1/2 contribution to the unit cell.
∴ Number of atoms in a
fcc unit cell
1.17.
hexagonal close packing
Crystalline solids exhibit a regular and repeating pattern of
constituent particles. The diagrammatic representation of three-dimensional
arrangements of constituent particles in a crystal, in which each particle is
depicted as a point in space is known as a crystal lattice. In a crystal
lattice, the atoms are very closely packed, leaving very little space between
them. This arrangement of elements in solids also helps us in the determination
of the formula of a compound. We have learned three-dimensional solid
packing can be packed in two ways viz., cubical close packing (CCP) and
hexagonal close packing (HCP).
In hexagonal close packing (HCP) too, there are two
basic kinds of voids are involved, namely, octahedral voids and tetrahedral
voids. We know that the number of tetrahedral voids present in a lattice is
twice the number of close-packed particles. While the number of octahedral
voids generated is equal to the number of close-packed particles. The
arrangement of particles in these voids depends on other factors too. For
example, in ionic solids, the bigger ions from the close-packed structure and
the smaller ions occupy the voids. Tetrahedral voids are occupied if the latter
ions are small. Whereas if the latter ions are bigger, octahedral voids are
occupied. The fraction of octahedral or tetrahedral voids occupied by the
molecules helps us in the determination of the formula of the compound.
Problems on hexagonal close packing Formula
Question: Atoms of element Y form
hexagonal close packing lattice and those of the element X occupy 1/4th of
tetrahedral voids. What is the formula of the compound formed by the elements X
and Y?
Solution: The number of
tetrahedral voids formed = 2 × (number of atoms of element Y)
Since only 1/4th of these voids are
occupied by X, the ratio of elements of X to Y can be given by:
2 × (1/4):1 or 1:2
Thus, the formula of the compound is XY2.
1.18.
Co-ordination number in typical structures NaCl, CsCl, ZnS, TiO2
The
ionic crystal of NaCl is shown in Figure. Each sodium ion is surrounding by six
chloride ions and each chloride ions is surrounded by six sodium ions. The
maximum intensity of reflection occurs at the glancing angle of 5.90,
8.40 and 5.20 for 100, 110 and 111 planes, respectively
for first order reflection.
For face-centred cubic system the planes can be passed
through the atom having Miller indices 100, 110 and 111 at the relative spacing
a/2:a/2 √
2: a/ √ 3
So d100:d110:d111 = a/2:a/2 √ 2:a/ √ 3
= 1:0.707:1.154
This ratio is almost identical with the ratio we have
calculated from experimental observations. Hence NaCl crystal is face-centred
cubic system.
Structure of CsCl crystal
Cesium chloride, CsCl, has a body centred cubic
structure. In its crystal lattice, each
Cs+ ion is surrounded by 8 Cl-
ions and its coordination number is 8. The value of distance between Cs+
ion and Cl- ion as determined by Bragg’s spectrometer is 3.5100A
1.19.
comparison of structure and properties of diamond and graphite
The diamond
lattice consists of a series of atoms, each of which is placed between four
neighbours. The latter occupies the angular points of a regular tetrahedral,
while atom under consideration lies in the centre. The type of structure runs
throughout the crystal. The C-C bond distance is 0.154 nm. The whole lattice is
continuous. The diamond crystal is regarded as giant molecule. The crystal is
very hard because the covalent links runs without a break throughout the whole
crystal. The crystal can be cut only by breaking the covalent links. High
melting point can also be explained by stating that the atoms are very firmly
attached within the crystal.
Though
diamond and graphite are both covalent crystals. The great difference between
graphite and diamond can be understood in terms of the crystal lattice. Graphite
has hexagonal networks in sheets like benzene rings. The distance between atoms
in the plain is 142 pm but the distance between these atomic layer planes is
335 pm. In two directions the carbon atoms are tightly held as in diamond, but
in the third direction, the force of attractions appreciably less. As a result
one layer can slip over the other. The crystal is flatty.
|
DIAMOND |
GRAPHITE |
|
Diamonds have a
strong three-dimensional network structure |
Graphites have
a two-dimensional sheet-like structure |
|
The networks
are formed because of the presence of covalent bonds. |
They are formed due to weak Van der Waal's forces of attraction. |
|
They are hard
in nature. |
They are soft
in nature. |
|
Molecules are closely packed, as a result, they have high density. |
There is a
large gap between molecules. Therefore, they have low density. |
|
There is no
presence of free carbon atoms in diamonds. |
There is
presence of free Carbon atoms in graphite. |
|
Diamonds do not conduct electricity. |
Graphite conducts electricity. |
1.20.
numerical problems involving core concepts
Q.2 Metallic gold Au=197 is face centred cubic lattice. Calculate (a)
how many atoms occupy the gold unit cell and (b) what is the mass number of a
gold unit cells.
Solution:
1.21.
Defects in solids –
According
to the law of nature nothing is perfect, and so crystals need not be
perfect. They always found to have some
defects in the arrangement of their constituent particles. These defects affect
the physical and chemical properties of the solid and also play an important
role in various processes. For example, a process called doping leads to a
crystal imperfection and it increases the electrical conductivity of a
semiconductor material such as silicon. The ability of ferromagnetic material
such as iron, nickel etc., to be magnetized and demagnetized depends on the
presence of imperfections. Crystal defects are classified as follows
1)
Point defects
2)
Line defects
3)
Interstitial defects
4)
Volume defects
In
this portion, we concentrate on point defects, more specifically in ionic
solids.
Point
defects are further classified as follows
1.22.
stoichiometric defects.
This
defect is also called intrinsic (or) thermodynamic defect. In stoichiometric
ionic crystals, a vacancy of one ion must always be associated with either by
the absence of another oppositely charged ion (or) the presence of same charged
ion in the interstitial position so as to maintain the electrical neutrality.
Schottky defect:
Schottky
defect arises due to the missing of equal number of cations and anions from the
crystal lattice. This effect does not change the stoichiometry of the crystal. Ionic
solids in which the cation and anion are of almost of similar size show Schottky
defect. Example: NaCl.Presence of large number of Schottky defects in a
crystal, lowers
its
density. For example, the theoretical density of vanadium monoxide (VO)
calculated using the edge length of the unit cell is 6.5 g cm-3, but the actual
experimental density is 5.6 g cm-3. It indicates that there is
approximately 14% Schottky defect in VO crystal.
Presence
of Schottky defect in the crystal provides a simple way by which atoms or ions
can move within the crystal lattice.
Frenkel
defect:
Frenkel
defect arises due to the dislocation of ions from its crystal lattice. The ion
which is missing from the lattice point occupies an interstitial position. This
defect is shown by ionic solids in which cation and anion differ in size.
Unlike Schottky defect, this defect does not affect the density of the crystal.
For
example, AgBr, in this case, small Ag+ ion leaves its normal site and occupies
an interstitial position as shown in the figure.
1.23.
Non-stoichiometric defects.
Non-stoichiometric
compounds are those compounds in which proportion of cation and anion is not
similar and defects of these compounds are known as non-stoichiometric defects.
In this defect a large number of positive and negative charges are present. As
we know, crystals remain neutral so, if a spare amount of negative charge is
present then it will be maintained by extra positive charge presence. Because
of this positive and negative charge, solid crystal structure shapes will
become improper, and solid become imperfect.
This
defect can be happened due to 2 reasons:
In
a lattice, substances have a cation whose proportion is more than in an anion.
Hence, known as a metal excess defect.
In
a lattice, substances have a cation whose proportion is lesser than in an
anion. And hence known as the metal deficiency defect.
Metal excess defect:
Metal
excess defect arises due to the presence of more number of metal ions as
compared to anions.
Alkali
metal halides NaCl, KCl show this type of defect.
The
electrical neutrality of the crystal can be maintained by the presence of
anionic vacancies equal to the excess metal ions (or) by the presence of extra
cation and electron present in interstitial position.
For
example, when NaCl crystals are heated in the presence of sodium vapour, Na+
ions are formed and are deposited on the surface of the crystal. Chloride ions
(Cl-) diffuse to the surface from the lattice point and combines with Na+ ion.
The electron lost by the sodium vapour diffuse into the crystal lattice and
occupies the vacancy created by the Cl- ions. Such anionic vacancies which are
occupied by unpaired electrons are called F centers. Hence, the formula of NaCl
which contains excess Na+ ions can be written as Na1+x Cl.
ZnO
is colourless at room temperature. When it is heated, it becomes yellow in
colour.
On
heating, it loses oxygen and thereby forming free Zn2+ ions. The
excess Zn2+ ions move to interstitial sites and the electrons also
occupy the interstitial positions.
Metal deficiency defect:
Metal
deficiency defect arises due to the presence of less number of cations than the
anions. This defect is observed in a crystal in which, the cations have
variable oxidation states.
For
example, in FeO crystal, some of the Fe2+ ions are missing from the
crystal lattice.
To
maintain the electrical neutrality, twice the number of other Fe2+
ions in the crystal is oxidized to Fe3+ ions. In such cases, overall
number of Fe2+ and Fe3+ ions is less than the O2-
ions. It was experimentally found that the general formula of ferrous oxide is
FexO, where x ranges from 0.93 to 0.98.
Impurity defect:
A
general method of introducing defects in ionic solids is by adding impurity
ions. If the impurity ions are in different valance state from that of host,
vacancies are created in the crystal lattice of the host. For example, addition
of CdCl2 to AgCl yields solid solutions where the divalent cation Cd2+
occupies the position of Ag+. This will disturb the electrical
neutrality of the crystal. In order to maintain the same, proportional number
of Ag+ ions leaves the lattice. This produces a cation vacancy in
the lattice, such kind of crystal defects are called impurity defects.
1.24.
Liquid crystals
There are certain solids which on heating undergo two sharp phase
changes one after the other. They first fuse sharply yielding turbid liquids
and again equally sharply at higher temperature yielding clear liquids. These
changes get reversed on cooling at the same temperature. The turbid liquid show
anisotropy i.e. they have different physical properties from different
directions. Anisotropy is particularly seen in the optical behaviour of
liquids. In an anisotropic substance, the physical property are different in
different direction. On the other hand true liquids are isotropic ie same
physical properties in different directions. As anisotropic properties are
associated with crystalline state, the turbid liquids are known as liquid
crystals.
This
liquid crystal term, however, is not satisfactory since the substances in this
state do not have properties of crystalline state. Actually, they are more like
liquids in having properties like mobility, surface tension, viscosity etc.
Amongst other names that have been suggested are crystalline liquids and
anisotropic liquids, but these are also not satisfactory. The term mesomorphic
state (meaning intermediate form) probably fits best. But, the older term
liquid crystal continues to be used even in the present day literature.
Substances which show the above behaviour are usually some long chain
organic molecules either terminating in groups such as-OR, -COOR or having
groups like -C=N-,-N=NO-,-C=C- in the middle. The first solid showing this
peculiar property was discovered in 1888 was cholesteryl benzoate C6H5COOC27H45. It fuses sharply at 1450C
to form turbid liquid and on further heating changes into clear liquid at 1780C.
If we cool, the above changes are reversed i.e., the clear liquid when cooled
first changes into turbid state at 1780C and then into the solid
state at 1450C
Later on, p-azoxyanisole and p-azoxyphenetone were found to exhibit the
same properties. In 1991 P.G. De Genees, a French physicist got the Nobel Prize
in Physics for contribution to liquid crystals and polymers.
1.25.
classification and applications.
In
a liquid the moleucules have random arrangement and they are able to move fast
each other. In a solid crystal the molecules have an ordered arrangement and
are in fixed positions. In a liquid crystal, however, molecules are arranged
parallel to each other and can flow like a liquid. Thus liquid crystals have
the fluidity of a liquid and optical properties of solid crystals. Accordingly,
to their molecular arrangement, the liquid crystals are classified into three
types
Nematic liquid
crystals:
in
nematic liquid crystals molecules are parallel to each other like soda straws
but they are free to slide or roll individually.
Smetic liquid crystals:
The
molecules in this type of liquid crystals are also parallel but these are
arranged in layers. These layers can slide past each other.
Cholesteric liquid
crystals:
As
in nematic crystals in this type liquid crystals the molecules are parallel but
arranged in layers. The molecules in successive layers are slightly rotated
with respect to the layers above and below so as to form spiral structure.
Application of liquid crystals:
On
account of their remarkable optical and electrical properties, liquid crystal
found several practical applications. Some of these are given below.
Number display:
When
a thin layer of nematic liquid crystal is placed between two electrodes and an
electrical field is applied, the polar molecules are pulled out of alignment.
This cause the crystal to be opaque. Transparency returns when electrical
signal is removed. This property is used in the number displays of digital
watches, calculators, and other instruments.
Monitoring body temperature:
Like
the solid crystals, liquid crystals can diffract light. Only one of the
wavelengths of the white light is refracted by the crystal which appears coloured.
As the temperature changes the distance between the layers of molecules also
changes. Therefore, the colours of the reflected light changes correspondingly.
These colesteric liquid crystal undergoes a series of colour changes with
temperature. These crystals are used in indicator tapes to monitor body
temperature or to spot areas of overheating in mechanical systems.
Uses of Liquid Crystals
Because
liquid crystals have unusual physical and optical properties, they are used in
many ways. As proof –
(i) Color of cholesteryl
crystals depends on temperature. Therefore, to calculate the temperature, the
temperature calculation helps a lot.
(ii) Used in gas-liquid
colorimetric analysis.
(iii) As they consume very
little power, they are used in many electronic devices (calculator, clock,
digital display).
(iv) Used as solvents in
programmatic studies to investigate the design of molecules with vectorial
properties.
(v) Cholesterol crystals
can be used to detect and detect lesions in the body.
Vitreous State
Boron
trioxide, silicon dioxide, germanium dioxide, etc., do not give solid crystals
when cooled suddenly in the molten state of athene. Instead, they give a glassy
substance. This state is Vitreous State.
Objects
that give a mirror state
(i) Boron trioxide
(ii) Silicon dioxide
(iii) Germanium dioxide
(iv) Arsenic oxide
(v) Phosphorus pentoxide
(vi) Beryllium fluoride
(vii) Glycerol
(viii) Glucose
When
examining the properties of objects in the glassy state
They
have been found to have both solid and liquid properties. As evidence
(A)
Solid state properties
(i) High rigidity (ii)
Inflexible
(iii) Bearing all forces.
(B)
Liquid phase properties
(i) Light penetration takes
place
ii)
have similar optical properties in all directions.
Therefore,
the glassy state can be considered as an intermediate state between the solid
state and the liquid state.
Unlike
perfect solid crystals, glassy materials do not melt at a certain temperature.
Therefore, molecules and their structure in glassy materials can be considered
to be disordered. Therefore, glassy materials can be considered
non-crystalline, fine powders. When glassy materials are left alone at high
temperatures for long periods of time, crystals emerge from them. Then, the
mirror state disappears. Therefore, glassy materials can be considered as
either amorphous solids or supercooled liquids.
Short Answer Questions:
1.
Define surface tension. What is its unit?
2.
How does vapour pressure varies with temperature.
3.
Explain why
(i)
Drops of liquids are spherical in shape.
(ii)
At the boiling point, the temperature of liquid does not rise although
it is being heated.
(iii)
Glycerol is more viscous than water.
4.
Write a note on specific refraction.
5.
Write a note on liquid crystals.
6.
Explain the term viscosity of a liquid.
Long Answer Questions:
1.
Define the terms surface tension and surface energy. Discuss capillary
rise method for determination of surface tension in the laboratory.
2.
What are liquid crystals? How are they classified? How would you account
for turbidity observed in liquid crystals? What are the uses of liquid
crystals?
3.
Why do you use the same viscometer for the liquid and water during the
experimental determination of the viscosity of the liquid by Ostwald
viscometer? Describe the experiment.
4.
Write notes on the following
· Vapour pressure
· Optical oxaltation
· Ramsay- Shields
equation
UNIT-III
2.
Nuclear Chemistry
2.1. Natural radioactivity -
α, β and ɣ rays
Radioactivity : The phenomenon of spontaneous disintegration of
certain atomic nuclei resulting in the emission of radioactive rays is called
radioactivity. Radioactivity is a nuclear phenomenon and it is not affected by
external factors such as temperature, pressure etc. This phenomenon was
discovered by Henry Becqurel.
To
explain the spontaneous decay of radioactive elements, Rutherford and Soddy put
forward the theory of radioactive disintegration. According to this theory the
quantity of a radioactive element which disappears in unit time is directly
proportional to the amount (atoms) of radioactive substance present at that
time.
Based
on the above theory, the following equation is derived which confirms that all
radioactive reactions follow I order
2.2.
Half-life period
The
time required to disintegrate one half of any radioactive substance is called
half life period (t1/2). The half life period (t1/2) of a
radioactive substance is independent of initial concentration. It depends only
on the disintegration constant (X) of the radioactive element. t/ is used to
indicate the relative stability of radioactive substance. If t1/2 is
the shorter, faster is the rate of decay and hence the substance is more
unstable and viceversa.
Since
radioactivity is a nuclear phenomenon, it must be connected with the
instability of the nucleus.
An
a - particle is equal to the bundle of two protons and two neutrons and hence
it is equal to the Helium nucleus (2He4).
p-particle
is a fast moving electron.
ɣ—radiation is a waver of very short wavelength with
very high energy.
Radioactive
decay series: Radioactive heavy nuclei decay by a series of α -
emission or β emissions, finally resulting in the formation of a
stable isotope of lead. There are about 4 decay series.
4n
- Thorium series
4n+1
- Neptunium series
4n+2
- Uranium series
4n+3
- Actinium series
2.3.
Fajan–Soddy group displacement law
Soddy and Kasimir Fajans independently unraveled the
pattern of transformations that accompanied α and β ra- dioactive decay. They
gave a law, to know the position of new element formed after the emission of α
& β-particle.
According to the Group displacement law...
• If an α-particle is emitted by a radio active
element from its nucleus, the atomic no.(Z) of new element or daughter element
formed is decreased by 2 units & the mass number (A) is decreased by 4
units. Therefore, the position of new element formed is displaced by two groups
towards the left in the periodic table.
• If a β-particle is emitted by a radioactive element,
the atomic number of daughter element or new element is increased by one unit.
Therefore, the position of new element is displaced by one group towards the
right in periodic table.
• If an α-particle is emitted from the nucleus of
radioactive element and then 2β-particles are emitted in next two
transformations, the daughter element is an isotope of parent element. The
daughter & parent element has the same atomic number. Hence according to
Group displacement law position of daughter & parent element in the
periodic table will remain same.
• Group displacement law is not applicable to
lanthanides & actinides. (ie for f-block elements).
2.4.
Geiger–Nattal rule
In
Gamow’s theory of α-decay we have considered an alpha particle in a nucleus as
a particle in a box. The particle is in a bound state because of the presence
of the strong interaction potential. It will constantly bounce from one side to
the other, and due to the possibility of quantum tunneling by the wave through
the potential barrier, each time it bounces, there will be a small likelihood
for it to escape. But once it comes out of the nucleus how far will it travel
before getting detected. Or in a way where should we place our detector so that
we can have an α-particle detection.
See
in the Gamow’s theory the disintegration constant depends on the energy of the
α-particle meaning it’s the energy content of α-particle because of which it
will travel. Geiger and Nuttall made experimental study between the decay
constant (λ) and the range of the α-particle (Rα) for different α-emitters. What
they have found is the following.
For
an α-emitting radioactive substance the logarithm of the decay constant (λ) and
the logarithm of
Dr.
Upakul Mahanta, Department of Physics, Bhattadev University
the
range of the α-particle (Rα) in air are in linear relation to each other.
To
put it in a mathematical way
where C1, C2 are constants. But
the above expression is an empirical one. Then again they have also showed that
the range of the α-particle (Rα) also depends upon the velocity of the
α-particle in air and in fact they have found it is proportional to the cubed
of the velocity of the α-particle.
Rα ∝ v3 = kv3
k is proportionality constant. Again
which is the Geiger-Nuttall Law in terms of energy
with the assumption that c1 3/2 = A and c1 B1
+ c2 = B. Thus the GeigerNuttall law also relates the decay constant
of a radioactive isotope with the energy of the a-particles emitted. And the
thumb rule is that the short-lived nuclei emit more energetic alpha particles
than long-lived ones.
2.5.
isotopes, isobars, isotones
Isotopes are variants of a particular element with
different numbers of neutrons. For example, the two isotopes of Uranium
are, 23592 U and 23992 U.
You will see here that the number of protons is the same in both the isotopes,
but they contain 143 and 147 neutrons, respectively. The presence of an extra
neutron significantly changes the behaviour of that particular atom. There are
two different types of isotopes, stable and radioactive. Stable isotopes can
exist in their free state without breaking down spontaneously. Radioactive
isotopes are too unstable to sustain themselves, and they spontaneously break
down into two lighter daughter elements with the emission of particles such as
alpha, beta, and gamma rays.
Isobars are elements that have the same number of nucleons
(sum of protons and neutrons). The series of elements with 40 Mass numbers
serve as a good example; 4016S, 4017Cl, 4018Ar, 4019K,
and 4020Ca. The nucleus of all the above-mentioned
elements contain the same number of particles in the nucleus but contain
varying numbers of protons and neutrons.
Isotones are atoms that have the same neutron number but
different proton number. For example, 3616S, 3717Cl, 3818Ar, 3919K,
and 4020Ca are all isotones of 20 since they all
contain 20 neutrons.
2.6.
Mirror nuclei
A mirror nucleus is
defined as the nucleus that contains a number of protons and a number of
neutrons that are mutually interchangeable when compared to another nucleus are
called a mirror nucleus.
The mirror nucleus
pairs have equal spins and the same parity.
If the number of
protons of the first isotope is designated by (Z1) then the number
of neutrons of the second isotope is designated as (N2) are equal.
Z1=N2
If the number of
protons of the second isotope is designated by (Z2) then the number
of neutrons of the first isotope is designated as (N1) are equal.
Z2=N1
The mass number of
both nuclei are the same.
Z1+N1=N2+Z2
Examples:
(I)14𝐶 𝑎𝑛𝑑 14𝑂 are mirror nuclei
|
Isotope 1 |
Z1 |
N1 |
Isotope 2 |
Z2 |
N2 |
|
14𝐶 |
6 |
8 |
14𝑂 |
8 |
6 |
So, mass number of
both the nucleus is equal.
Z1+N1=N2+Z2
6+8=8+6
(II) 15𝑁 𝑎𝑛𝑑 15𝑂 are mirror nuclei
|
Isotope
1 |
Z1 |
N1 |
Isotope
2 |
Z2 |
N2 |
|
15𝑁 |
7 |
8 |
15𝑂 |
8 |
7 |
So, the mass number of
both the nucleus is equal.
Z1+N1=N2+Z2
7+8=8+7
2.7.
iso
diaphers
Isodiaphers are defined by the number of protons and
neutrons in an atom’s nucleus.
Any element with a specific number of protons and
neutrons in its nucleus is called a nuclide. The number of neutrons in a
nuclide may differ from the number of protons.
These nuclides are employed in nuclear reactions
because their nuclei contain energy for the same reason. Isodiaphers are a pair
of nuclides with the identical number of protons and neutrons in the atom’s
nucleus.
Although the nuclei of both nuclides have different
numbers of protons and neutrons, the difference between them is the same.
Take Uranium 92U238 and thorium 90Th234,
for example.
Uranium’s nucleus contains 92 protons and 146
neutrons, hence the difference is:
146 – 92 = 54
Thorium’s nucleus contains 90 protons and 144
neutrons, hence the difference is:
144 – 90 = 54
As a result, isodiaphers are nuclei with the same
amount of protons and neutrons in their nucleus.
Take Uranium 92U238 and thorium 90Th234,
for example. They have the difference of 54.
The mass number of any element is equal to the sum of
its protons and neutrons, but the atomic number of any element is equal to the
number of protons in that atom plus the number of electrons. Nuclides carry out
nuclear reactions such as fission and fusion, which divide or combine atoms to
generate new ones, releasing a lot of energy.
2.8.
Nuclear isomerism
Nuclear isomerism is caused due to the two lowest nuclear states with
zero angular momentum and the opposite party. Their lifetime is estimated under
these conditions of two quanta and for the ejection of two elections from the K
or L shell. It is also exhibited by all diatomic molecules having an odd Z
value.
2.9.
radioactive decay series
The spontaneous change of an unstable nuclide into another is radioactive
decay. The unstable nuclide is called the parent nuclide; the nuclide that
results from the decay is known as the daughter nuclide. The daughter
nuclide may be stable, or it may decay itself. The radiation produced during radioactive
decay is such that the daughter nuclide lies closer to the band of stability
than the parent nuclide.
Types of Radioactive Decay
Ernest Rutherford’s experiments involving the
interaction of radiation with a magnetic or electric field helped him determine
that one type of radiation consisted of positively charged and relatively
massive αα particles; a second type was made up of negatively charged
and much less massive β particles; and a third was uncharged
electromagnetic waves, γ rays. We now know that αα particles
are high-energy helium nuclei, β particles are high-energy electrons,
and γ radiation compose high-energy electromagnetic radiation. We
classify different types of radioactive decay by the radiation produced.
Alpha particles, which are attracted to the negative
plate and deflected by a relatively small amount, must be positively charged
and relatively massive. Beta particles, which are attracted to the positive
plate and deflected a relatively large amount, must be negatively charged and
relatively light. Gamma rays, which are unaffected by the electric field, must
be uncharged. A diagram is shown. A gray box on the left side of the diagram
labeled “Lead block” has a chamber hollowed out in the center in which a sample
labeled “Radioactive substance” is placed. A blue beam is coming from the
sample, out of the block, and passing through two horizontally placed plates
that are labeled “Electrically charged plates.” The top plate is labeled with a
positive sign while the bottom plate is labeled with a negative sign. The beam
is shown to break into three beams as it passes in between the plates; in order
from top to bottom, they are red, labeled “beta rays,” purple labeled “gamma
rays” and green labeled “alpha rays.” The beams are shown to hit a vertical plate
labeled “Photographic plate” on the far right side of the diagram.
Alpha (αα) decay is the emission of an α particle
from the nucleus. For example, polonium-210 undergoes α decay:
84Po210⟶2He4+82Pb206
or 84Po210 ⟶ 2α4 + 82Pb206
Alpha decay occurs primarily in heavy nuclei (A >
200, Z > 83). Because the loss of an α particle gives a daughter nuclide
with a mass number four units smaller and an atomic number two units smaller
than those of the parent nuclide, the daughter nuclide has a larger n:p ratio
than the parent nuclide. If the parent nuclide undergoing α decay lies below the
band of stability, the daughter nuclide will lie closer to the band.
Beta (β) decay is the emission of an electron from a
nucleus. Iodine-131 is an example of a nuclide that undergoes β decay:
53I131⟶ −1e0+ 54X131 or 53I131 ⟶ −1β0+54Xe131
Beta decay, which can be thought of as the conversion
of a neutron into a proton and a β particle, is observed in nuclides with a
large n:p ratio. The beta particle (electron) emitted is from the atomic
nucleus and is not one of the electrons surrounding the nucleus. Such nuclei
lie above the band of stability. Emission of an electron does not change the
mass number of the nuclide but does increase the number of its protons and
decrease the number of its neutrons. Consequently, the n:p ratio is decreased,
and the daughter nuclide lies closer to the band of stability than did the
parent nuclide.
Gamma emission (γ emission) is observed when a
nuclide is formed in an excited state and then decays to its ground state with
the emission of a γ ray, a quantum of high-energy electromagnetic radiation.
The presence of a nucleus in an excited state is often indicated by an asterisk
(*). Cobalt-60 emits γ radiation and is used in many applications including
cancer treatment:
27Co∗60⟶0γ0+27Co60
There is no change in mass number or atomic number
during the emission of a γ ray unless the γ emission accompanies one of the
other modes of decay.
Positron emission (β+ decay) is the
emission of a positron from the nucleus. Oxygen-15 is an example of a nuclide
that undergoes positron emission:
8O15⟶+1e0+7N15
or 8O15⟶+1β0+7N15
Positron emission is observed for
nuclides in which the n:p ratio is low. These nuclides lie below the band of
stability. Positron decay is the conversion of a proton into a neutron with the
emission of a positron. The n:p ratio increases, and the daughter nuclide lies
closer to the band of stability than did the parent nuclide.
Electron capture occurs when one
of the inner electrons in an atom is captured by the atom’s nucleus. For
example, potassium-40 undergoes electron capture:
19K40+−1e0⟶18Ar40
Electron capture occurs when an inner shell electron
combines with a proton and is converted into a neutron. The loss of an inner
shell electron leaves a vacancy that will be filled by one of the outer
electrons. As the outer electron drops into the vacancy, it will emit energy.
In most cases, the energy emitted will be in the form of an X-ray. Like
positron emission, electron capture occurs for “proton-rich” nuclei that lie
below the band of stability. Electron capture has the same effect on the
nucleus as does positron emission: The atomic number is decreased by one and
the mass number does not change. This increases the n:p ratio, and the daughter
nuclide lies closer to the band of stability than did the parent nuclide.
Whether electron capture or positron emission occurs is difficult to predict.
The choice is primarily due to kinetic factors, with the one requiring the
smaller activation energy being the one more likely to occur.
2.10.
magic numbers
1.
Pairs of nucleons frequently form inside. A single unpaired nucleon can
be taken out of the nucleus more easily than a paired one. Two protons and two
neutrons combine to form a very stable nucleus. The fact that a particle has a
significant binding energy of approximately 28.3 MeV lends support to this.
2.
The graph between binding energy per nucleon and atomic number exhibits
numerous kinks, one of which is illustrated as an example in the range A= 126
to 150 in Fig. These kinks are associated with a sharp increase in the binding
energy per nucleon.
These kinks or discontinuities have been found to
occur whenever either the Neutron number or the proton number or both take the
values 2,8,20,50,82 and 126. Nuclei containing 2,8,20,50,82 and 126 nucleons of
the same kind known as Magic numbers, have a very high stability. For
example, 2He4 with Z=N=2 and 8O16 with Z = N = 8 are highly stable. Similarly,
the nuclei with 14,28 and 40 nucleons (semi-magic numbers) are slightly less
stable but are more stable than the rest.
3. The most
numerous nuclei are those with even numbers of both protons and neutrons; the
least abundant are those with odd numbers of both protons and neutrons; and the
intermediate types are those with odd numbers of one type and even numbers of
the other type. High natural abundance is, of course, related to stability.
Brown provided information regarding the relative abundances of nuclei in 1949
using information about the elements that make up the sun, the earth, and the
stars.
The relative abundances of naturally occurring
isotopes with nuclei that contain magic numbers of neutrons or protons are
typically larger than 60%. For instance, the relative abundances of the
isotopes 88Sr (N = 50), 138Ba (N = 82), and 140Ce (N = 82) are 82.56%, 71:66%,
and 88.48%, respectively. The stable end product of the natural radioactive
series, Lead 82Pb208 has underline Z= 82 and N=126 both magic
numbers.
1.
In contrast to other elements, an element with a magic number of protons
typically has a higher number of stable isotopes. For instance, whereas argon
(Z = 18) and titanium (Z = 22) have 3 and 5 stable isotopes, respectively,
calcium (Z = 20) has 6 stable isotopes. Again, the greatest number of stable
isotopes is found in tin with Z = 50. This value is 10, as opposed to 8 for
tellurium (Z = 52) and cadmium (Z = 48).
2.
Compared to the nearby isotones, the number of naturally occurring
isotones with the magic numbers of neutrons is typically high. For instance, at
N = 82 there are seven stable isotones as opposed to three and two at N = 80
and two at N = 84, respectively. Similar circumstances exist at N = 20, 28, and
50, which have 5, 5, and 6 isotones, respectively. These numbers are larger
than in the cases of the nearby isotones.
3.
The nuclei with the magic numbers of neutrons typically have low neutron
capture crosssections. The probability of these nuclei acquiring an extra
neutron is low because their neutron shells are already full, as seen in Fig.
Similar to this, the cross sections for proton capture are small for nuclei
with the magic proton numbers.
4.
If the heavy nuclei's disintegration energies are plotted as functions
of mass number A for a given Z, a regular fluctuation is typically seen up
until the magic neutron number N = 126, at which point there is a sharp
discontinuity. This demonstrates the neutron number 126's magical properties.
5.
P-emitters exhibit discontinuities at the magic proton or neutron values
6.
The three lead isotopes, which all have the same magic number Z = 82 of
protons in their nuclei, are the stable end products of all three naturally
occurring radioactive series.
7.
The earliest excited states of nuclei with magic numbers of neutrons or
protons occur at energies higher than those of the nearby nuclei.
2.11.
Units
The
most commonly used unit is the curie. It was originally based on the rate of
decay of a gram of radium. Experiments have yielded the result that there
are-about 3.7 x 1010 disintegrations per second per gram of radium. This number
is taken as a standard and is called the curie. Thus by definition,
A
Curie of activity is a very strong source of radiation.
Thus, one has
and
1 millicurie = 1 mCi = 10-3Ci
1 microcurie = 1μCi = 10-6Ci.
Sometimes one uses another unit for activity, called
the rutherford.
Activity can also be defined in terms of N as
The radiation exposure is measured by the unit called roentgen(R). One
roentgen is defined as the quantity of radiation which produces
1.6 × 1012 pairs of ion in 1 gram of air.
2.12.
nuclear stability - neutron- proton ratio
Some nuclei are stable, some are not. A stable nucleus
is one that will remain in its current state indefinitely unless an outside
agent interacts with it; an unstable nucleus is one that will spontaneously
change to another. Various modes of decay will be covered below. In general,
stable nuclei have approximately equal number of neutrons as protons, and a
strong excess of one or the other will result in an unstable nucleus. The ratio
of neutrons to protons in a stable nucleus is thus around 1:1 for small nuclei
(Z < 20). The ratio increases slowly with atomic number up to about 1.58 at
high Z. There are only two stable nuclei with Z > N (more protons than
neutrons): 1H and 3He, each of which has one more proton than it has neutrons.
There are some tendencies, or rules, that stable
nuclei observe. By definition A = N + Z, i.e., it is the total number of
nucleons in a nucleus.
Stable nuclei of even Z are more numerous than those
with odd Z.
Stable nuclei of even N are more numerous than those
with odd N.
Stable nuclei of even A are more numerous than those
with odd A.
Stable nuclei of even A usually have even Z. Among the
exceptions to this rule are the following light, stable nuclei: 2H, 6Li,
10B, and 14N.
Only two stable structures are known for which Z >
N: see above.
For any Z there is a range of stable N. At low Z, N ≈
Z; as Z increases, the value of N/Z for which stability occurs gradually rises
to around 1.58 for the heaviest stable nuclei, namely, those of lead (Z = 82).
For the most part, alpha and beta decays tend to occur
in ways that move a nucleus toward the range of N/Z shown in the figure above.
Thus in a typical alpha decay,
236Ra → 222Rn + α + γ
+ Q
Z = 88 86 (number of protons)
N = 138 136 (number of neutrons)
N / Z = 1.5682
1.5814
2.13.
Binding energy
The rest
mass of the stable nucleus of a stable atom is always less than the sum of the
masses of constituent nucleons. The difference is called the mass defect Δm
(i.e., Δm.c2) is utilised in keeping the nucleons bound
together. This energy is known as the binding energy. In order to break
the nucleus into its constituent nucleons an amount of energy equal to its
binding energy has to be supplied to the nucleus. The mass defect per
nucleon Δm/A = P, is called the packing fraction of the nucleus.
Atomic
mass is the mass of a single atomic particle or molecule. It is the sum of
protons and electrons present in the atom of an element. It is expressed in
mole. It is simply a collection of nuclides that make up a chemical element. it
is a whole number.
Atomic
weight is the ratio of atom of an element. The average weight of an atom is
relative to the 1/12 weight of the carbon -12 atom. It is also referred to as
relative atomic mass.The value is not necessarily a whole number.
Atomic weight = [Mass(a)
The
nucleons are bound together in a nucleus and the energy has to be supplied in
order to break apart the constituents into free nucleons. The energy with which
nucleons are bounded together in a nucleus is called as Binding Energy
(B.E.). In order to free nucleons from a bounded nucleus this much of
energy (= B.E.) is to be supplied.
It is
observed that the mass of a nucleus is always less than the mass of constituent
(free) nucleons. This difference in mass is called as mass defect and
is denoted as Dm.
If mn: mass of a neutron;
mp: mass of a proton
M (Z, A):
mass of bounded nucleus
Then, Δm =
Z . mp + (A – Z). mn – M (Z, A)
This
mass-defect is in form of energy and is responsible for binding the nucleons
together. From Einstein's law of inter-conversion of mass into energy:
E = mc2 (c: speed of light; m: mass)
Binding
energy,
Generally, Δm
is measured in amu units. So let us calculate the energy equivalent to 1 amu.
It is calculated in eV (electron volts; 1 eV =1.6 x10–19J)
E (= 1 amu = 1.67 × 10–27 (3×108)2 / 1.6 × 10–19) eV = 931 × 108 eV = 931 MeV
=> B.E.
= Δm (931) MeV
There is
another quantity which is very useful in predicting the stability of a nucleus
called as Binding energy per nucleons.
B.E. per
nucleons = Δm (931) / A MeV
Observation from the plot of
B.E./nucleons Vs mass number (A):-
(i) B.E./nucleons
increases on an average and reaches a maximum of about 8.7 MeV for Aº 50 –
80.
(ii) For
more heavy nuclei, B.E./nucleons decreases slowly as A increases. For the
heaviest natural element U238 it drops to about 7.5 MeV.
(iii) From
above observation, it follows that nuclei in the region of atomic masses 50-80
are most stable.
2.14.
Packing fraction
The packing fraction describes the distribution of
nucleons inside the nucleus. The link between the mass defect and the number of
nucleons is what it’s called. The mass defect is the difference between the
actual isotopic mass (M) and the mass number (M) (A). As a consequence, A is
the correct answer.
P= M-A
Where A is the mass number and M is the actual
isotopic mass, is an isotope’s packing fraction (P).
The packing fraction might be positive, negative or 0
percent. If the packing fraction is more than one, the nucleus is unstable and
will undergo fusion or fission, depending on the packing fraction. The nuclei
are particularly stable if the packing fraction is negative and vice versa. In
this scenario, mass defects reveal the presence of binding energy. The
monoisotopic elements have a mass number that matches the isotopic mass, as
shown by the zero-packing fraction.
In nuclear physics, the numbers 2, 8, 20, 28, 50, 82
and other uncommon numbers are employed. “Magic numbers” are what they’re
called. The nuclei are deemed to be particularly stable if their atomic number
or neutron number equals one of the magic numbers. The nucleus will try to
reduce the number of neutrons while increasing the number of protons if the
neutron to proton ratio is larger. In a similar vein, a nucleus with a lower
neutron-to-proton ratio would try to increase the number of neutrons while
lowering the number of protons to improve stability. As a consequence of this
process, they will emit radioactive emissions. That is why, in our reactor
designs, we use hydrogen isotopes for fusion and uranium for fission.
2.15.
Mass defect.
The nuclear binding energy holds a significant
difference between the nucleus’s actual mass and its expected mass depending on
the sum of the masses of isolated components.
Hence, energy and mass are related based on the following
equation:
E=mc2
Where c is the speed of light. In nuclei, the binding
energy is so high that it holds a considerable amount of mass.
The actual mass is less than the sum of individual
masses of the constituent neutrons and protons in every situation because
energy is ejected when the nucleus is created. This energy consists of mass
which is ejected from the total mass of the original components and is called a
mass defect. This mass is missing in the final nucleus and describes the energy
liberated when the nucleus is made.
Mass defect is determined as the difference between
the atomic mass observed (Mo) and expected by the combined masses of its
protons (mp, every proton has a mass of 1.00728 AMU) and neutrons (mn, 1.00867
AMU).
Md=(mn+mp)-mo
Bond Energy or Bond-dissociation Energy
We are talking about bond energy and bond-dissociation
energy, which are basically measures of the binding energy between the atoms in
a chemical bond. Bond energy is the energy that is used to disassemble a
molecule into its constituent atoms. It appears in the form of chemical energy
released during chemical explosions, the burning of chemical fuel and other
processes.
Applications
Binding energy is also applied in determining whether
fusion or fission will be favourable. For elements that are lighter than
iron-56, the fusion releases energy since the nuclear binding energy rises with
the hike in mass. Elements that are heavier than iron-56 release energy on
fission since the lighter elements consist of higher binding energy. Hence,
there exists a peak at iron-56 according to the nuclear binding energy curve.
2.16.
Simple calculations involving mass defect and B.E., decay constant and
t1/2 and radioactive series.
Problem 1:-:
If mass of proton = 1.008 amu and mass of neutron =
1.009 amu, then the binding energy per nucleon for 4Be9
(mass = 9.012 amu) will be:
(A) 0.0672 MeV (B) 0.672 MeV
(C) 6.72 MeV (D) 67.2 MeV
Solution:-
Mass defect,
Δm = (4 ×
1.008 + 5 × 1.009) – 9.012
= 9.077 – 9.012 =0.065 amu
BE/A = 0.065 × 931 / 9 = 6.72 MeV
Problem 2:-:
The energy released in the following b-decay process
will be:
Given that,
mn = 1.6747 × 10–27 kg
mp = 1.6725 × 10–27 kg
me = 0.00091 × 10–27 kg
(A) 0.931
MeV (B) 0.731 MeV
(C) 0.511
MeV (D) 0.271 MeV
Solution:-
Mass defect Δm = (1.6747 – 1.6725 – 0.0091) × 10–27 = 0.0012
× 10–27 kg
ΔE = 0.0012 × 10–27 × (3 × 108)2 / 1.6 × 10–12
= 0.731 MeV
Problem 3:-:
If the mass of 3Li7
is 7.01653 amu, then find out binding energy per nucleon for 3Li7
.
(A) 5.6
MeV (B) 39.25 MeV
(C) 1
MeV (D) zero.
Solution:-
E = ΔE / A = Δm × 931 / A MeV
Δm = (3mp +
4mn) – mass of Li7
= (3 × 1.00759 + 4 × 1.008898) – 7.01653
= 0.04216
ΔE = 0.04216
× 931 / 7 = 39.25 / 7 = 5.6 MeV
Problem 4:-:
How much energy is released in the following reaction?
1H2 + 1H2 = 2He4
If the B.E./Nucleon of 1H2 and 2He4
are 1.123 MeV and 7.2 MeV respectively.
(A) 12
MeV (B) 24.3 MeV
(C) 36
MeV (D) zero
Solution:-
B. E. of 1H2,
ΔE = 1.125
E = A × ΔE
E = 2 × 1.125 = 2.25 MeV
B.E. of two 1H2 = 2.25
Ed = 4.5 MeV
B.E. of an α -particle = 4 × 7.2
Ea = 28.8
Energy released ER = Ea – Ed
ER = 28.8 – 4.5 = 24.3 MeV
From the above observation we conclude that, option
(B) is correct.
PROBLEM - 5
Calculate the (i) mass defect, (ii) binding energy and
(iii) the binding energy per nucleon for a 6C12 nucleus.
Nuclear mass of 6C12=12.000000 a.m.u., mass of
hydrogen nucleus =1.007825 a.m.u. and mass of
neutron =1.008665 a.m.u.
Solution
Given,
Mass of one proton = 1.007825 a.m.u
Mass of one neutron= 1.008665 a.m.u
Nuclear mass of 6C12 =
12 a.m.u
(i) 6C12 has 6 proton,
6 electron and 6 neutron.
Mass of nucleus = Mass of 6 proton + Mass of 6 neutron
=(6×1.007825)+(6×1.008665)
=12.09894u
Mass defect (Δm) =12.09849u−12u=0.098931u
(ii) Binding Energy = Mass Defect × 931.5MeV
=0.09849u×931.5MeV/u=92.15MeV
(iii) Binding Energy per Nucleon
= Binding Energy /
Number of Nucleons
Number of nucleon = Mass Number = 12
So, Binding Energy per nucleon =92.1512=7.68MeV
PROBLEM- 6
After
24 hours, only 0.125 g out of the initial quantity of 1g of a radioisotope
remains behind. what is half-life period?
PROBLEM-7 Half-life period of a radioactive element is 100
seconds. Calculate the disintegration constant and average life period. How
much time will it take for 90% decay?
2.17.
Isotopes – uses
a) Study of reaction mechanism
Mechanism of photosynthesis in plants
A small quantity of Radioactive CO 2 containing radioactive
oxygen O18 is mixed with ordinary carbondioxide and the process is
carried out. It has been found that oxygen gas evolved along with sugar
formation is non-radioactive. Therefore O2 produced comes from water
and not from carbondioxide. So the correct mechanism is as follows.
6CO2 + 6H2O→C6H12O6 + 6O2
Study of hydrolysis of ester
By labelling oxygen, the mechanism of ester hydrolysis can be studied by
using water labelled with O18. The hydrolysis of an ester by water
enriched with radioactive oxygen is indicated as :
Therefore
it is the acid and not alcohol produced which is radioactive confirming the
above mechanism.
2.18.
Nuclear energy; nuclear fission and fusion
Nuclear Fission
Nuclear fission is the process in which a heavy
nucleus breaks up into two lighter nuclei of almost equal size with the release
of an enormous amount of energy. This type of nuclear fission reaction was
first observed by German Chemists Otto Hahn, F.Strassman and Meitner by
bombarding 92U235 with slow moving neutrons. The process
is usually accompanied by emission of neutrons. The nuclear fission has been
produced in heavy nuclei such as 235U,238U, 232Th
by neutrons, protons, deuterons.
Mechanism of fission
In the fission process, the heavy nucleus absorbs a
neutron and forms an unstable compound nucleus. The compound nucleus then
breaks up more or less in the middle to give fission product.
Example
A typical example of the fission process in the
fission of uranium by neutrons is explained by the following equation.
92U235 + 0n1→ 56Ba141 + 3 0n1
+ 200 MeV
Further, the neutrons released (say three) from the
fission of first uranium atoms can hit three other uranium atoms. In this way a
chain reaction is set up resulting into the liberation of an enormous amount of
energy. In the case of nuclear fission, 92U236 formed
breaks up in several ways.
This fission process is self multiplying process and
hence a tremendous amount of energy is released in a very short interval of
time. Therefore, explosion takes place. Atom bomb is based on nuclear fission
process.
Energy released in nuclear fission reaction
92U235 +0n1
→ 42Mo95+57La139+20n1+
71e0
The isotopic mass of U235 =235.118 amu
The isotopic mass of 42Mo95 = 94.936 amu
The isotopic mass of 57La139 = 138.95 amu
The isotopic mass of 0n1 =1.009 amu
: 235.118 + 1.009 → 94.936 + 138.95 + 2 x
1.009
236.127 amu → 235.906 amu
: The mass converted into energy is
= (236.127 - 235.906) amu
= 0.213 amu
Since 1amu (atomic mass unit) = 931 MeV, for one 235U
fission energy released = 0.213 x 931.48 = 200 MeV
Nuclear Power Generator
A nuclear reactor or nuclear power generator is a kind
of furnace for carrying out the controlled fission of a radioactive material
like U235 for producing power.
The core of the nuclear reactor produces heat through
nuclear fission. Heavy water at high pressure takes heat away from the core. In
the heat exchanger, the heavy water inside the reactor gives up its heat to
water outside the reactor, which boils to form steam. The steam is taken away
to drive turbines that make electricity. In Tamilnadu atomic power stations
generating electricity are situated at Kalpakkam and Koodamkulam.
Nuclear Fusion
When lighter nuclei moving at a high speed are fused
together to form a heavy nucleus, the process is called nuclear fusion.
In fusion reaction, the mass of heavier nucleus formed
is less than the total mass of two lighter nuclei. Thus, just like a fission
reaction, the source of energy in a fusion reaction is also the disappearance
of mass, which gets converted into energy.
Nuclear fusion reaction takes place at very high
temperature of about 108K. Therefore, this reaction is called
thermonuclear reaction.
1H2 +
1H3 → 2He4 +
0n1 + Energy
Deuterium Tritium Helium
The Mass loss is equal to 0.018 amu and the
corresponding energy released is 1.79 x 109 KJmol-1.
Hydrogen Bomb
The highly destructive hydrogen bomb is also based on
the fusion reactions of hydrogen to form helium producing large amount of
energy. Hydrogen bomb consists of an arrangement for nuclear fission in the
centre surrounded by a mixture of deuterium (1H2) and
lithium isotope (3Li6). Fission reaction provides the
high temperature necessary to start the fusion.
Fusion reactions take place in hydrogen bomb.
2.19.
major nuclear reactors in India
|
Nuclear Power Plants in
India – Operational |
|||
|
Name Of Nuclear Power Station |
Location |
Operator |
Capacity |
|
Kakrapar Atomic Power Station – 1993 |
Gujarat |
NPCIL |
440 |
|
(Kalpakkam) Madras Atomic Power Station – 1984 |
Tamil Nadu |
NPCIL |
440 |
|
Narora Atomic Power Station- 1991 |
Uttar Pradesh |
NPCIL |
440 |
|
Kaiga Nuclear Power Plant -2000 |
Karnataka |
NPCIL |
880 |
|
Rajasthan Atomic Power Station – 1973 |
Rajasthan |
NPCIL |
1,180 |
|
Tarapur Atomic Power Station – 1969 |
Maharashtra |
NPCIL |
1,400 |
|
Kudankulam Nuclear Power Plant – 2013 |
Tamil Nadu |
NPCIL |
2,000 |
Nuclear power in India has suffered from generally low
capacity factors. As of 2021, the lifetime weighted energy availability factor of
the Indian fleet is 66.1%. However, capacity factors have been improving in
recent years. The availability factor of Indian reactors was 74.4% in the years
2019–2021. One of the main reasons for the low capacity factors is lack of
nuclear fuel.
India has been making advances in the field of
thorium-based fuels, working to design and develop a prototype for an atomic
reactor using thorium and low-enriched uranium, a key part of India's three
stage nuclear power programme.
2.20.
Radiation hazards, disposal of radioactive waste and safety measures.
TACKLING RADIOACTIVE WASTES EFFICIENTLY
Any activity related to the nuclear fuel cycle, that
produces or uses radioactive materials generates radio-active waste. The
management of radiation emitting radioactive material is a matter of concern
and is what sets nuclear wastes apart. Public acceptance of nuclear energy
largely depends on the public assurance for safe management of radioactive
wastes. Not all nuclear wastes are particularly hazardous or difficult
to manage as compared to other toxic industrial wastes.
Safe management of radioactive waste has been accorded
high priority right from the inception of our nuclear energy program. In
accordance with international guidelines, a coherent comprehensive and
consistent set of principles and standards are being practiced all over the
world for waste management system. Radioactive waste would be managed in a
manner so as not to cause any undue radiation risk to the workers, the public
(present as well as future generation) and the environment.
Management of these wastes covers the entire range of
activities right from handling, treatment, conditioning, transport, storage and
disposal.
The recent technological developments in India
realize the recovery of valuable radionuclide from radioactive waste for
societal application besides ensuring the highest level of safety in the
management of radioactive waste.
UNDERSTANDING RADIOACTIVE WASTES
Radioactive wastes are generated during various
operations of the nuclear fuel cycle as well as production and use of
radionuclide for various societal applications. The activities like mining and
processing of uranium ore, fabrication of nuclear fuel, generation of power in
nuclear reactor, processing of spent nuclear fuel, management of radioactive
waste, production and use of radionuclide for various industrial and medical
applications, research associating with radioactive material etc. generates the
different types of radioactive waste. Radioactive waste can be in gas, liquid
or solid form, and its level of radioactivity can vary. The waste can remain
radioactive for a few hours or several months or even hundreds of thousands of
years. Depending on the level and nature of radioactivity, radioactive wastes
can be classified as exempt waste, Low & Intermediate level waste and High
Level Waste. The most important and advantageous property of radioactive waste
is 'Its radioactive hazard potential reduces with time depending on the
half lives of radionuclide present in the waste'. Such feature
differentiates them significantly from conventional chemical or industrial
waste, hazard potential or toxicity of which does not alter with time and
remains constant till its transformation to other suitable form.
LOW AND INTERMEDIATE LEVEL WASTE (LILW)
Low and Intermediate Level Waste (LILW) radioactive
waste are generated in radiation facilities and nuclear fuel cycle operations
ranging from uranium processing, fuel fabrication, nuclear power plants,
research reactors, radiochemical facilities and fuel reprocessing. LILW have
generally high volumes and low levels of radioactivity. They are segregated
based on their physical nature and different management techniques have been
established based on their nature for their effective treatment. They are
further classified based on their radioactivity as well as also based on half
life of radionuclide, as short lived and long lived wastes.
Significant quantum of LILW of diverse nature gets generated in different
nuclear installations.
They are essentially of two types
Primary Wastes comprising of radioactively contaminated
equipment (metallic hardware) spent radiation sources etc.
Secondary wastes resulting from
different operational activities, protective rubber and plastic wears,
cellulosic and fibrous material, organic ion exchange resins filter cartridges
and others.
HIGH LEVEL WASTE
High level radioactive liquid waste (HLW) containing
most (~99%) of the radioactivity in the entire fuel cycle is produced during
reprocessing of spent fuel. A major stream of this waste is the aqueous
radioactive waste generated from the first cycle extraction of the spent fuel
processing. Also solid waste not suitable for disposal in near surface disposal
facilities due to significant concentration of long-lived radionuclides or
decay heat above the prescribed limits may also need to be regarded as high
level waste. Issue of the long lived radioactive waste has been the focal point
of debate for the success of nuclear power. Planning for management of HLW thus
takes into account the need for their effective isolation from the biosphere
and their continuous surveillance for extended periods of time spanning several
generations. To meet this objective in the long term, waste isolation systems
comprising multiple barriers are employed so as to prevent the movement of
radionuclides back to the human environment.
MANAGEMENT OF RADIOACTIVE WASTES
Utmost emphasis is given to waste minimization, and
volume reduction in the choice of processes and technologies adopted in
radioactive waste management plants. As a waste management philosophy, no
waste in any physical form is released / disposed to the environment unless the
same is cleared, exempted or excluded from regulations. A comprehensive
radioactive waste management is established taking into account the operational
capability for the management of radioactive waste and an independent
regulatory capability for its overview.
In consideration to the primary objective of
protecting human health, environment and future generations, the overall
philosophy for safe management of radioactive wastes in India, is based on the
concept of
- Delay and Delay
- Dilute and Disperse
- Concentrate and Contain
- Recycle and Reuse
Effective management involves segregation,
characterization, handling, treatment, conditioning and monitoring prior to
final disposal.
SOLID WASTE
Substantial amount of LIL wastes of
diverse nature, gets generated in different nuclear installations as
radioactive solid waste. Treatment and conditioning of solid wastes are
practiced, to reduce the waste volume in ways, compatible to minimizing the
mobility of the contained radioactive materials. A wide range of treatment and
conditioning processes are available today with mature industrial operations
involving several interrelated steps and diverse technologies.
Proper disposal of Solid waste is essential to ensure
protection of the health and safety of the public and quality of the
environment including air, soil, and water supplies. Radiological hazards
associated with short lived wastes < 30 years half life get significantly
reduced over a few hundred years by radioactive decay. Disposal of waste is carried
out in specially constructed engineering modules such as stone lined
trenches, reinforced concrete trenches and tile holes at Near Surface Disposal
Facility (NSDF). These disposal structure are located both above and
under-ground in access - controlled areas and are designed based on multi
barrier principle for ensuring effective containment and isolation of the
radioactivity till it decays to innocuous level. The NSDFs where the disposal
structures are located are kept under constant surveillance with the help of
bore-wells laid out in a planned manner by routinely monitoring the underground
soil and water samples to confirm effective confinement of radioactivity
present in the disposed waste.
The high level solid wastes contain
large concentration of both short and long lived radionuclide's, warranting
high degree of isolation from the biosphere and usually calls for final
disposal into Geological Disposal Facility (GDF). A key idea was that long-term
disposal would be best carried out by identifying suitable sites at which the
waste could be buried, a process called deep geological disposal.
LIQUID WASTE (LIL)
Liquid waste streams are pre-treated by various
techniques, such as filtration, adsorption, chemical treatment, evaporation,
ion exchange; reverse osmosis etc., prior to immobilization in suitable matrix
depending upon the nature, volume & radioactivity content.
GASEOUS WASTE
Gaseous waste is treated at the source of generation.
Various techniques involoving adsorption on activated charcoal, absorption /
scrubbing, filtration by high efficiency particulate air filter etc., are used
for effective treatment of gaseous waste
MANAGEMENT OF HIGH LEVEL WASTE
High level radioactive waste gets generated during
reprocessing of spent fuel. Most of the radioactive isotopes in high level
waste emit large amounts of radiation and have long half-lives. The management
of high level waste in the Indian context, is carried out in the following
three stages:
1. Immobilisation of high
level liquid waste into inert vitrified borosilicate glasses through process
called 'vitrification'.
2. Engineered interim
storage of the vitrified waste for passive cooling & surveillance over a
period of time, qualifying it for subsequent disposal.
3. Disposal of the
vitrified waste in a deep geological repository.
VITRIFICATION
India is one of the few countries to have mastered the
technology of vitrification. Over the years BARC has developed the technology
for vitrification of HLW. India has a unique distinction of having operating
vitrification plant at Trombay, Tarapur and Kalpakkam.
In our existing plant at Trombay vitrification process
is essentially batch operation consisting of heating and fusing of
pre-concentrated waste and glass forming additives and is carried out in Induction
Heated Metallic Melter based on induction heating.
While the plant at Trombay is based on pot glass
technology, the concept of Joule Heated Ceramic Melter (JHCM) is
utilized at the facility at Tarapur. The Joule Melter Technology is essentially
a single step process, where immobilisation of HLW in a borosilicate glass
matrix is achieved in a refractory-lined melter. The Joule Heated Ceramic
Melter (JHCM) process exploits the high temperature behaviour of glass whereby
it becomes an electrical conductor at elevated temperatures and favourable
changes in its viscosity near the pour point, helps in product withdrawal and
shut off. The distinctive features of the Advanced Vitrification System (AVS)
of Tarapur and Waste Immobilisation Plant, Kalpakkam, employing JHCM for
vitrification of HLW, are increased throughput, availability of higher furnace
temperature and minimum dependence on operator skills.
Cold Crucible Induction Melter (CCIM) is emerging as a
futuristic technology for vitrification of high level liquid waste. Besides
being compact and advantageous as in-cell equipment, it offers flexibility,
susceptibility to treat various waste forms with better waste loading and
enhanced melter life. The CCIM is manufactured from contiguous segments forming
a cylindrical volume, but separated by a thin layer of electrically insulating
material. The number and the shape of the segments and the insulating gap
between them must be optimized to minimize the power dissipation by induced
currents in the crucible, while ensuring cooling of the crucible.
- INTERIM STORAGE OF VITRIFIED WASTE
The vitrified product is encapsulated in suitable
containers and over packs and stored for dissipation of radioactive decay, heat
and surveillance for a period of 15-20 years. Sufficient data can be generated
on the product behavior and the radiation and thermal conditions of the product
are expected to get stabilized to a level where transport of the product
becomes viable. On the basis if safety and techno-economic considerations, a
natural draught air cooling system has been designed for the storage vault.
WEALTH FROM WASTE
High level radioactive liquid waste contains various
useful fission product such as 137Cs, 90Sr,106Ru
etc., which have many industrial as well as medical applications. The energy
associated with these isotopes can be used for blood irradiation, food
preservation, sewage treatment, therapeutic applications, brachy therapy &
various other industrial applications. Separation and recovery of these useful
isotopes from radioactive waste and their deployment for societal application
makes the waste as a material of resource.
137CESIUM GLASS PENCILS
FOR IRRADIATION
137Cs can be used as a prominent alternate
irradiation source to 60C° for various applications like blood irradiator, food
irradiator, irradiation of sewage sludge etc.
Due to longer half-life of 137Cs as
compare to 60Co, the radiation sources need to be replaced at
lesser frequency. 137Cs is available in large quantity in
radioactive waste as one of the principal fission product.
In-house development of selective extractants and
their deployment has resulted into recovery of bulk of 137Cesium
from waste. The recovered 137Cs solution is converted into non-
dispersible cesium glass pencil to be used as blood irradiator.
Few lac Ci of 137Cs have been
recovered successfully and are converted into Cs glass pencils each having
activity of 2.0 to 5.0 Ci/gm of 137Cs at Waste Immobilization
Plant Trombay. These pencils have been supplied to various hospitals through
BRIT after ensuring rigorous quality assurance.
Research and Development is being pursued to make use
of Cs glass pencils for other irradiation process such as food irradiation.
90STRONIUM FOR MILKING OF 90YTTRIUM FOR
RADIOPHARMACEUTICAL APPLICATION
90Sr, another isotope present in waste, decays
to 90Y by beta decay having its application as a
radiopharmaceutical product for therapeutic use during treatment of cancer.
In-house developed strontium selective extractant has been successfully
deployed for separation/ recovery of strontium from HLW and converting into
Yttrium generator. 90Y is milked out from purified 90Sr using in-house
developed membrane technology and supplied for radiopharmaceutical application.
106Ru FOR EYE CANCER
TREATMENT
106Ru has an important application for eye cancer
treatment as a brachy therapy. Till date, 106Ru plaques are
imported. Technology for recovery of 106Ru from nuclear waste
and fabrication of 106Ru containing silver plaque has been
successfully developed as an import substitute for eye cancer treatment along
with cost effectiveness. Ru plaques, containing about 300-600 microcurrie of
Ru-106 activity, are produced and supplied to various eye hospitals through
BRIT for eye cancer treatment. The indigenously developed Ru-106 eye plaques
are cost effective and thier performance is at par the international standard.
Safe management of radioactive waste has been accorded
high priority right from the inception of our nuclear energy program. As a
result of rugged design with 'defense in depth' concept, well established
practices and safety review by independent agency, an excellent track record
for safe management of radioactive waste in India has been demonstrated for
more than five decades. Consistent efforts in R&D has enabled indigenous
development of novel processes and technologies in the field of management of
radioactive waste and their deployment to realise the waste volume
minimization, effective isolation of radionuclide in engineered matrix,
minimization of discharges and extracting wealth from waste by separating
useful radionuclide from radioactive waste for societal applications. Such
developments enable the country to be front-runner in the field of radioactive
waste management in the world.
UNIT-IV
3.
Halogen derivatives Aliphatic halogen derivatives
When
one or more hydrogen atoms of aliphatic or aromatic hydrocarbons are replaced
by the corresponding number of halogens like fluorine, chlorine, bromine or
iodine, the resultant compounds are called either haloalkanes or halo arenes.
They serve as starting materials for many organic synthesis.
Halogen
substituted organic compounds are widely spread in nature and find application
in our day to day life as well as in industry. Certain compounds like
chloramphenicol produced by soil microbes are used in the treatment of typhoid;
chloroquine is used in the treatment of malaria, halothane is used as an
anesthetic, and halogenated solvents like trichloroethylene are used for
cleaning electronic equipments.
Mono
halogen derivatives of alkanes are called haloalkanes. Haloalkanes are
represented by general formula R - X, Where, R is an alkyl group (CnH2n+1)
- and X is a halogen atom (X=F, Cl, Br or I). Haloalkanes are further
classified into primary, secondary, tertiary haloalkane on the basis of type of
carbon atom to which the halogen is attached.
3.1. Nomenclature and
classes of alkyl halides
Common system
In
the common system, haloalkanes are named as alkyl halides. It is derived by
naming the alkyl group followed by the halide.
IUPAC system
Let
us write the IUPAC name for the below mentioned haloalkanes by applying the
general rules of nomeclature
3.2.
Physical properties
1.
Pure haloalkanes are colourless.
Bromo and iodo alkanes are coloured in the presence of light.
2.
Haloalkanes having one, two or
three carbon atoms are in the gaseous state at normal temperature. Haloalkanes
having more than three carbon atoms are liquids or solids.
3.
Boiling point and Melting point
Haloalkanes
have higher boiling point and melting point than the parent alkanes having the
same number of carbons because the intermolecular forces of attraction (dipole
- dipole interaction and vander Waals forces) are stronger in haloalkane.
The
boiling point and melting point of haloalkanes decreases with respect to the
helogen in the following order.
Example
CH3I
> CH3Br > CH3Cl > CH3F
The
boiling points of chloro, bromo and iodo alkanes increase with the increase in
the number of halogen atoms.
For Example:
CCI4
> CHCI3 > CH2CI2 > CH3CI
4. The boiling point and melting point of mono
haloalkane increase with the increase in the number of carbon atoms.
Example CH3CH2CH2Cl
> CH3CH2Cl > CH3Cl
5.
Among isomeric alkyl halides the boiling point decreases with the increase in
branching in the alkyl group; with increase in branching, the molecule attains
spherical shape with less surface area. As a result the inter molecular forces
become weak, resulting in lower boiling points.
6. Solubility
Haloalkanes
are polar covalent compounds soluble in organic solvents, but insoluble in
water because they cannot form hydrogen bonds with water molecules
7. Density
The
density of liquid alkyl halides are higher than these of hydrocarbons of
comparable molecular weight.
3.3.
Chemical reactions. Nucleophilic substitution reactions – SN1, SN2 and
SNi mechanisms with stereochemical aspects and effect of solvent.
Nature of C - X bond in haloalkane
Carbon
halogen bond is a polar bond as halogens are more electro negative than carbon.
The carbon atom exhibits a partial positive charge (δ+)
and halogen atom a partial negative charge (δ-).
The
C -X bond is formed by overlap of sp3 orbital of carbon atom with
half filled p-orbital of the halogen atom. The atomic size of halogen increases
from fluorine to iodine, which increases the C - X bond length. Larger the
size, greater is the bond length, and weaker is the bond formed. The bond
strength of C - X decreases from C - F to C - I in CH3X. The changes
in the value of bond length, bond enthalpy and bond polarity, as we move from C
-F to C -I.
Haloalkanes
are one of the most reactive classes of organic compounds. Their reactivity is
due to the presence of polar carbon - halogen bond in their molecules. The
reactions of haloalkane may be divided into the following types
·
Nucleophilic substitution reactions
·
Elimination reactions
·
Reaction with metals
·
Reduction
1) Nucleophilic substitution reactions
We know that the C6+ - X6- present in halo alkane
is polar and hence the nucleophilic reagents are attracted by partially
positively charged carbon atoms resulting in substitution reactions.
Reaction with aqueous alkali or moist silver
oxide.(Hydrolysis)
Haloalkane reacts with aqueous solution of KOH or moist silver oxide (Ag2O/H2O) to form
alcohols.
i) Reaction with alcoholic ammonia (Ammonolysis)
Haloalkanes
react with alcoholic ammonia solution to form alkyl amines.
Example
However,
with excess of halo alkane, secondary and tertiary amines along with quartenary
ammonium salts are obtained
Ambident Nucleophiles
Nucleophiles
such as cyanide and nitrite ion which can attack nucleophilic centre from two
sides of the nucleophile are called ambident nucleophiles. These nucleophiles
can attack with either of the two sides depending upon the reaction conditions
and the reagent used.
ii) Reaction with alcoholic KCN
Haloalkanes
react with alcoholic KCN solution to form alkyl cyanides.
Example
iii) Reaction with alcoholic AgCN
Haloalkanes
react with alcoholic AgCN solution to form alkyl isocyanide.
Example
iv)
Reaction with sodium or potassium nitrite
Haloalkanes
react with alcoholic solution of NaNO2 or KNO2 to form
alkyl nitrites.
Example
v)
Reaction with silver nitrite
Haloalkanes
react with alcoholic solution of AgNO2 to form nitro alkanes.
v)
Reaction with silver nitrite
Haloalkanes
react with alcoholic solution of AgNO2 to form nitro alkanes.
Example
vi)
Reaction with sodium or potassium hydrogen sulphide
Haloalkanes
react with sodium or potassium hydrogen sulphide to form thio alcohols.
Example
vii) Williamson ether synthesis
Haloalkane,
when boiled with sodium alkoxide gives corresponding ethers.
Example
This
method can be used to prepare mixed (unsymmetrical) ethers also.
Mechanism
of Nucleophilic substitution reaction
The
mechanism of nucleophilic substitution reaction is classified as
a)
Bimolecular Nucleophilic
substitution
reaction (SN2)
b)
Unimolecular Nucleophilic substitution reaction (SN1)
The
rate of SN2 reaction depends upon the concentration of both alkyl
halide and the nucleophile.
Rate
of reaction
=
k2 [alkylhalide][nucleophile]
It
follows second order kinetics and occurs in one step.
This
reaction involves the formation of a transition state in which both the
reactant molecules are partially bonded to each other. The attack of
nucleophile occurs from the back side (i.e opposite to the side in which the
halogen is attacked). The carbon at which substitution occurs has inverted
configuration during the course of reaction just as an umbrella has tendency to
invert in a wind storm. This inversion of configuration is called Walden
inversion; after paul walden who first discovered the inversion of
configuration of a compound in SN2 reaction.
SN2
reaction of an optically active haloalkane is always accompanied by inversion
of configuration at the asymmetric centre. Let us consider the following
example
When
2 - Bromooctane is heated with sodium hydroxide, 2 - octanol is formed with
invesion of configuration. (-)– 2 – Bromo octane is heated with sodium
hydroxide (+) – 2 – Octanol is formed in which
– OH group occupies a position opposite to what bromine had occupied,
a. (-) 2 - Bromo octane
b. Transition State
c. (+) 2 - Octanol
(product)
SN1 Mechanism
SN1 stands for unimolecular
nucleophilic
substitution
‘S’
stands for substitution
‘N’
stands for nucleophilic
‘1’
stands for unimolecular (one molecule is involved in the rate determining step)
The
rate of the following SN1 reaction depends upon the concentration of
alkyl halide (RX) and is independent of the concentration of the nucleophile
(OH-).
Hence
Rate of the reaction
=
k[alkyl halide]
R-Cl
+ OH- → R - OH + Cl-
This
SN1 reaction follows first order kinetics and occurs in two steps.
We
understand SN1 reaction mechanism by taking a reaction between
tertiary butyl bromide with aqueous KOH.
This
reaction takes place in two steps as shown below
Step - 1 Formation of carbocation
The
polar C - Br bond breaks forming a carbocation and bromide ion. This step is
slow and hence it is the rate determining step.
The
carbocation has 2 equivalent lobes of the vacant 2p orbital, so it can react
equally rapidly from either face
Step - 2
The
nucleophile immediately reacts with the carbocation. This step is fast and
hence does not affect the rate of the reactions.
As
shown above, the nucleophilic reagent OH- can attack carbocation
from both the sides.
In
the above example the substrate tert-butyl bromide is not optically active,
hence the obtained product is optically inactive. If halo alkane substrate is
optically active then, the product obtained will be optically inactive racemic
mixture. As nucleophilic reagent OH- can attack carbocation from
both the sides, to form equal proportion of dextro and levorotatory optically
active isomers which results in optically inactive racemic mixture.
Example
Hydrolysis
of optically active 2 - bromo butane gives racemic mixture of ± butan-2-ol.
The
order of reactivity of haloalkanes towards SN1 and SN2
reaction is given below.
3.4.
Di, Tri & Tetra Halogen derivatives:
Carbon
compounds containing more than one halogen atoms are called poly halogen
compounds. Some of the important poly halogen compounds are described below.
They
are classified as
a)
gem - dihalides
CH3CHCl2
Ethylidene chloride 1,1 - Dichloro ethane
b) vic - dihalides
For
Example
3.5.
Preparation
a) gem- dihalides
Ethylidene
dichloride (1, 1 - Dichloro ethane) is prepared by
(i) Treating acetaldehyde with PCl5
(ii) Adding hydrogen chloride to acetylene
b) vic- dihalides
Ethylene
dichloride (1, 2 - Dichloro ethane) is prepared by the following methods.
i) Addition of chlorine to ethylene
ii)
Action of PCl5 (or HCl) on ethylene glycol
3.6.
Properties
Physical Properties
i) They are sweet smelling, colourless liquids
having relatively high boiling points.
ii) The boiling point of ethylidene chloride is
less than that of ethylene dichloride.
Chemical properties
1)
Hydrolysis with aqueous NaOH or KOH
Gem-Dihalides,
on hydrolysis with aqueous KOH give an aldehyde or a ketone vic-Dihalides, on
hydrolysis with aqueous KOH gives glycols.
This
reaction can be used to distinguish the gem- Dihalides and vic- Dihalides.
2)
Reaction with Zinc (Dehalogenation)
Gem-
Dihalides and vic- Dihalides on treatment with zinc dust in methanol give
alkenes.
3)
Reaction with Alcoholic KOH (Dehydrohalogenation)
gem-
Dihalides and vic- Dihalides on treatment with alcoholic KOH give alkynes.
Methylene chloride (Di chloromethane)
Preparation
Methylene
chloride is prepared by the following methods
1)
Reduction of chloroform
a)
Reduction of chloroform in the presence of Zn + HCl gives methylene chloride.
b)
Reduction of chloroform using H2/Ni
2)
Chlorination of methane
Chlorination
of methane gives methylene chloride
Uses
of methylene chloride
Methylene
chloride is used as
1) aerosol spray propellant
2) solvent in paint remover
3) process solvent in the manufacture of drugs
4) a metal cleaning solvent
TRIHALOALKANE
Trihaloalkanes
are compounds obtained by replacing three hydrogen atoms of a hydrocarbon by
three halogen atoms.
Example
CHCl3 CHI3
Chloroform Iodoform
1) CHLOROFORM
Chloroform is an
important trihaloalkane. Dumas named CHCl3 as chloroform as it gives formic acid on
hydrolysis.
Preparation: Chloroform
is prepared in the laboratory by the reaction between ethyl alcohol with
bleaching powderfollowed by the distillation of the product chloroform.
Bleaching powder act as a source of chlorine and calcium hydroxide. This
reaction is called haloform reaction. The reaction proceeds in three steps as
shown below.
Step - 1: Oxidation
Ethyl alcohol Acetaldehyde
Step - 2: Chlorination
Acetaldehyde Trichloro acetaldehyde
Step - 3: Hydrolysis
Chloral chloroform
Physical properties
(i)
Chloroform is a colourless liquid with peculiar sickly smell and a
burning taste
(ii)
The vapours of chloroform when inhaled it causes unconsciousness
(depress the central nervous system) and hence it is used as an anaesthetic.
Chemical properties
1)
Oxidation
Chloroform
undergoes oxidation in the presence of light and air to form phosgene (carbonyl
chloride)
Since
phosgene is very poisonous, its presence makes chloroform unfit for use as
anaesthetic.
2)
Reduction
Chloroform
undergoes reduction with zinc and HCl in the presence of ethyl alcohol to form
methylene chloride.
3)
Nitration
Chloroform
reacts with nitric acid to form chloropicrin. (Trichloro nitro methane)
It
used as an insecticide and soil sterilising agent.
4)
Carbylamine reaction
Chloroform
reacts with aliphatic or aromatic primary amine and alcoholic caustic potash,
to give foul smelling alkyl isocyanide (carbylamines)
Methylamine Chloroform Methylisocyanide
This reaction is used
to test primary amine.
TETRA HALOALKANE
Carbon tetrachloride
is a good example for tetra haloalkane Carbon tetrachloride
Preparation
1. Chlorination of methane
The reaction of
methane with excess of chlorine in the presence of sunlight will give carbon
tetrachloride as the major product.
2.
Action of carbondisulphide with chlorine gas
Carbon
disulphide reacts with chlorine gas in the presence of anhydrous AlCl3
as catalyst giving carbon tetrachloride
Physical properties
(i)
Carbon tetrachloride is a colourless liquid with its specific smell
(ii)
It is insoluble in water and soluble in organic solvents
Chemical properties
(i) Hydrolysis
Carbon
tetrachloride reacts with hot water or with hot water vapour producing the poisonous
gas, phosgene.
(ii) Reduction
Carbon
tetrachloride is reduced by iron powder in dilute HCl medium to form chloroform
Freons
(CFC)
The
chloro fluoro derivatives of methane and ethane are called freons.
Nomenclature
Freon
is represented as Freon-cba
Where
c = number of carbon atoms – 1
b
= number of hydrogen atoms + 1
a
= total number of fluorine atoms
Example
|
Formula |
C-1 |
H+1 |
F |
Name |
|
CFC13 |
1-1=0 |
0+1 = 1 |
1 |
Freon-11 |
|
CF2C12 |
1-1=0 |
0+1 = 1 |
2 |
Freon-12 |
|
C2F2CI4 |
2-1=1 |
0+1 = 1 |
2 |
Freon-112 |
|
C2F3Cl 3 |
2-1=1 |
0+1 = 1 |
3 |
Freon-113 |
Freon
– 12 is prepared by the action of hydrogen fluoride on carbon tetrachloride in
the presence of catalylic amount of antimony pentachloride. This is called swartz
reaction.
Physical properties
Freons
are highly stable, unreactive, non corrosive, non toxic, easily liquefiable
gases.
Uses:
(i)
Freons are a used as refrigerants in refrigerators and air conditioners.
(ii)
It is used as a propellant for aerosols and foams
(iii)
It is used as propellant for foams to spray out deodorants, shaving
creams, and insecticides.
DDT (p,p’-dichloro diphenyl trichloro ethane)
DDT, the first
chlorinated organic pesticide was prepared in 1873,
and in 1939 Paul Muller discovered the effectiveness of
DDT as an insecticide. He was awarded Noble prize in medicine and physiology in
1948 for this discovery.
DDT can be prepared by
heating a mixture of chlorobenzene with chloral (Trichloro acetaldehyde) in the
presence of Conc.H2SO4.
3.7.
Aromatic halogen compounds
Haloarenes
are the compounds in which the halogen is directly attached to the benzene
ring.
3.8.
Nomenclature
In
the IUPAC nomenclature, the halo arenes are named by adding prefix halo before
the name of the aromatic hydrocarbon. For naming disubstituted arenes, the
relative position of the substituent 1,2; 1,3 and 1,4 are indicated by the
prefixes ortho, meta and para, respectively.
For
poly haloarenes the numbering should be done in such a way that the lowest
possible number should be given to the substituents and the name of the
halogens are arranged in alphabetic order.
Nomenclature
can be well understood from the following examples.
3.9.
Preparation
1) Direct halogenation
Chlorobenzene
is prepared by the direct chlorination of benzene in the presence of lewis acid
catalyst like FeCl3
2) From benzene diazonium chloride
Chloro
benzene is prepared by Sandmeyer reaction or Gattermann reaction using benzene
diazonium chloride.
(i)
Sandmeyer reaction
When
aqueous solution of benzene diazonium chloride is warmed with Cu2Cl2
in HCl gives chloro benzene
3) Preparation of iodobenzene
Iodobenzene
is prepared by warming benzene diazonium chloride with aqueous KI solution.
4) Preparation of fluorobenzene
Fluoro
benzene is prepared by treating benzenediazonium chloride with fluoro boric
acid. This reaction produces diazonium fluoroborate which on heating
produces fluorobenzene. This reaction
is called Balz – schiemann reaction.
5) Commercial preparation of chloro benzene (Raschig process)
Chloro
benzene is commercially prepared by passing a mixture of benzene vapour, air
and HCl over heated cupric chloride .This reaction is called Raschig process.
3.10.
Properties
Melting and boiling points
The
boiling points of monohalo benzene which are all liquids follow the order
Iodo > Bromo > Chloro
The
boiling points of isomeric dihalobenzene are nearly the same
The
melting point of para isomer is generally higher than the melting points of
ortho and meta isomers. The higher melting point of p-isomer is due to its
symmetry which leads to more close packing of its molecules in the crystal
lattice and consequently strong intermolecular attractive force which requires
more energy for melting
p
-Dihalo benzene > o- Dichloro benzene > m-Dichloro benzene
Solubility
Haloarenes
are insoluble in water because they cannot form hydrogen bonds with water ,but
are soluble in organic solvents
Density
Halo
arenes are all heavier than water and their densities follow the order.
Iodo
benzene > Bromo benzene > Chloro benzene
Chemical properties
A. Reactions invoving halogen atom
1. Aromatic nucleophilic substitution reaction
Halo
arenes do not undergo nucleophilic substitution reaction readily. This is due
to C-X bond in aryl halide is short and strong and also the aromatic ring is a
centre of high electron density.
The
halogen of haloarenes can be substituted by OH- , NH2-,
or CN- with appropriate nucleophilic reagents at high temperature
and pressure.
For
Example
This
reaction is known as Dow’s Process
2. Reaction with metals
a) Wurtz Fittig reaction
Halo
arenes reacts with halo alkanes when heated with sodium in ether solution to
form alkyl benzene. This reaction is called Wurtz Fittig reaction.
b) Fittig reaction
Haloarenes
react with sodium metal in dry ether, two aryl groups combine to give biaryl products.
This reaction is called Fittig reaction
B) Reaction
involving aromatic ring
3. Electrophilic substitution reaction
Haloarenes
undergo aromatic electrophilic substitution reactions. The rate of eleclophilic
substitution of halobenzene is lower than that ofbenzene. halogen is
deactivating due to - I effect of halogen. The lone pair of electrons on the
chlorine involves in resonance with the ring. It increases the electron density
at ortho and para position. The halogen attached to the benzine ring with draw
electron and thereby and hence the halogen which is attached to the benzene
directs the incoming, electrophile either to ortho or to para position in electrophilie
substitution reaction
4) Reduction
Haloarenes
on reduction with Ni- Al alloy in the presence of NaOH gives corresponding
arenes.
5) Formation of Grignard reagent
Haloarenes
reacts with magnesium to form Grignard reagent in tetra hydrofuran (THF).
3.11.
Uses
i)
Chloro benzene is used in the manufacture of pesticides like DDT
ii)
It is used as high boiling solvent in organic synthesis.
iii)
It is used as fibre - swelling agent in textile processing.
3.12.
Mechanism of nucleophilic aromatic substitution – benzyne intermediate.
we saw that electron-poor aromatic rings
containing a leaving group can undergo substitution with electron-rich
nucleophiles. We saw that the mechanism proceeds through addition of
a nucleophile to the aromatic ring (via an electron-rich
intermediate) followed by loss of a leaving group, in a process sometimes
called, “addition-elimination”.
Importantly, the only substitution product is the one
where the nucleophile ends up attached to the same carbon as that
bearing the leaving group. (This differentiates it from electrophilic
aromatic substitution, where a mixture of ortho-, para–
and meta- products can be obtained.)
2. A “Nucleophilic Aromatic Substitution” In
Name, But by A Different Mechanism
Although the “addition-elimination” mechanism for
nucleophilic aromatic substitution has been known since at least 1902, it
became increasingly clear in the first half of the twentieth century that
certain reactions classified as “nucleophilic aromatic substitution” appeared
to proceed through a different mechanism altogether.
For example, it was found that treating chlorobenzene
with sodium amide (NaNH2) in liquid ammonia (boiling point = –33°C)
resulted in the rapid formation of aminobenzene (“aniline”):
An addition-elimination mechanism here doesn’t seem
right, considering that nucleophilic aromatic substitution reactions with far
stronger electron withdrawing groups (e.g. NO2, rather than Cl)
require higher temperatures and longer reaction times.
Another observation was that no reaction occurred
under these conditions when the ortho- positions were
attached to alkyl groups. A hydrogen is necessary at one of these positions for
the reaction to proceed.
(note – NaNH2 and KNH2 can
be considered to be essentially the same for our purposes)
A second observation was that in the case below only
the ortho- and meta- products formed, and never the para–
.
3. The Benzyne
Intermediate
Various intermediates were proposed to explain these
results, but then in 1953 John D. Roberts (then at MIT) nailed
it by publishing one of the most elegant chemical experiments of
all time.
He and his team synthesized chlorobenzene but with a
special difference: the carbon attached to the chlorine was a radioactive
isotope of carbon (14C), not carbon (12C).
This radioactive carbon atom served as an atomic
“label”, which allowed them to conclusively determine if substitution happened
exclusively at the carbon bearing the leaving group.
Roberts’ group carried out the reaction under
conditions reported previously, and found that about 50% of the product ended
up with the NH2 attached to the labelled carbon, and the other
50% had the NH2 on the carbon adjacent to the
label.
This is not consistent with an
addition-elimination mechanism!
In fact, the roughly 50:50 ratio of products implies
the involvement of a symmetrical intermediate which is attacked
equally on either side.Roberts’ proposal – which has stood the test of time –
was the involvement of a short-lived intermediate bearing a carbon-carbon
triple bond: “Benzyne”!
At first glance, this seems crazy. A triple
bond in an aromatic ring?
Well, it’s not quite a true triple
bond in the way that we’re familiar with (i.e. with alkynes). Instead of an
overlap between two 2p orbitals (as in an alkyne) the “triple
bond” is formed through overlap of two adjacent sp2 orbitals
in the plane of the ring (i.e. at right angles to, and completely
independently of, the aromatic pi system).
Since these orbitals actually point away from each
other, the overlap between them is poor, resulting in a “triple bond” that is
actually very weak.
The strain energy of benzyne has been estimated to be
about 50 kcal/mol – more strained than cyclopropane (28 kcal/mol), and only
slightly less strained than cyclopropene (54 kcal/mol).
An intuitive way to think about it is to imagine the
involvement of two resonance structures (far left and far right, below) that
make strong (and equal) contributions to the overall resonance hybrid, such
that both carbons can be considered “electrophilic”.
[A more rigorous way to treat it is from a
molecular orbital perspective – a weak bond results in a low-energy LUMO, and
therefore a lower energetic barrier to attack by nucleophiles].
However strange it might look, the benzyne
intermediate explains all of these important observations, and more.
In the first step (elimination) a strong base removes
a hydrogen from the carbon adjacent to that bearing the leaving group,
resulting in an elimination reaction that forms the triple bond. This explains
why no reaction occurs when both positions adjacent to the leaving group lack
hydrogen!
- In the second step (addition), attack of the can come at either side
of the triple bond, resulting in about a 1:1 mixture of the product with
NH2 attached to the labelled carbon (A) and NH2 adjacent
to the labelled carbon (B).
Although it’s tempting to use –NH2 as
nucleophile, the more likely nucleophile here is the solvent, NH3,
which readily reacts with the extremely reactive benzyne intermediate. After
attack of NH3, proton transfer occurs to result in the neutral
product.
3.1. Aryl alkyl halides
Nomenclature
Alkyl halides are named in two ways. In the common
system, the alkyl group is named first followed by the appropriate word
chloride, bromide, etc. The common name of an alkyl halide is always written as
two separate words.
In the IUPAC system, alkyl halides are named
haloalkanes. The other rules followed in naming compounds is that
·
Select the longest chain of carbon atoms containing the halogen atom.
·
Number the chain to give the minimum number to the carbon-carrying
halogen atom.
·
If multiple bonds (double or triple bonds) are present, then it is given
the preference in numbering the carbon chain.
·
The IUPAC name of any halogen derivative is always written as one word.
|
Compound |
Common Name |
IUPAC Name |
|
CH3-Cl |
Methyl Chloride |
Chloromethane |
|
CH3-CH2-Br |
Ethyl bromide |
Bromoethane |
|
CH3-C(CH3)2-Br |
tert-Butyl bromide |
2-Bromo-2-methylpropane |
|
CHCl3 |
Chloroform |
Trichloromethane |
|
CH3-CH(Br)2 |
Ethylidene bromide |
1,1-Dibromoethane |
|
CH2=CH-CH2-I |
Allyl iodide |
3-Iodoprop-1-ene |
- Aryl halides are named by prefixing “halo” to the name of the
parent aromatic hydrocarbon.
- If there is more than one substituent on the ring then the relative
positions of the substituents are indicated by mathematical numerals.
- In the common system, the relative position of two groups is shown
by prefixes ortho, meta or para.
The common and IUPAC names of some representative
haloarenes are given below.
3.2.
Alcohols
Alcohol,
an organic compound containing hydroxyl (-OH ) functional group. Many organic
compounds containing –OH group play an important role in our body. For example,
cholesteryl alcohol commonly known as cholesterol is an important component in
our cell membrane. Retinol, the storage form of vitamin A, finds application in
proper functioning of our eyes. Alcohols also find application in many areas
like medicine, industry, etc., For example, methanol is used as an industrial
solvent, ethyl alcohol an additive to petrol, isopropyl alcohol as a skin
cleanser for injection, etc., The hydroxyl group of alcohol can be converted to
many other functional groups.
Hence,
alcohols are important resource in synthetic organic chemistry. In this unit,
we will learn the preparation, properties and uses of alcohols, phenols and
ethers.
3.3.
Nomenclature
Naming
the organic compounds according to IUPAC guidelines
·
Select the longest continuous chain of carbon atoms (root word) containing
the functional group ( -OH ).
·
Number the carbon atoms in the chain so that the carbon bearing the -OH
group has the lowest possible number.
·
Name the substituent (if any)
·
Write the name of the alcohol as below.
Prefix
+ Root word + Primary suffix + Secondary suffix
(substituents)
(longest chain) (Saturation /unsaturation) (ol)The following table
illustrates the IUPAC nomenclature of alcohols.
3.4.
Classification
Alcohols
can be classified based on the number of hydroxyl groups and the nature of the
carbon to which the functional group (–OH) is attached.
3.5.
Preparation
1.
From Alkyl halides: Alkyl halides on heating with dilute aqueous NaOH
gives alcohols. Primary alkyl halides undergo substitution by SN2
reaction. Secondary and tertiary alkyl halides usually undergo nucleophilic
substitution by SN1 mechanism.
If
R =t-butyl, the reaction proceeds
through the formation of t-butyl carbocation
From alkenes: Addition of water across the double bond of an alkene
in presence of concentrated sulphuric acid gives alcohols. This addition
reaction follows Markownikoff’s rule.
From Grignard reagent: Nucleophilic addition of Grignard reagent to
aldehydes/ketones in presence of dry ether followed by the acid hydrolysis
gives alcohols. Formaldehyde gives primary alcohol and other aldehydes give
secondary alcohols. Ketones give tertiary alcohols.
4. Hydroboration:
Diborane
reacts with an alkene to form trialkyl borane which on treatment with H2O2
in presence of NaOH gives an alcohol. (Refer reactions of diborane) The
overall reaction is hydration of an alkene. This reaction yields an
anti-Markownikoff's product.
1)
Reduction of carbonyl compounds:
Reduction
of aldehydes/ketones with LiAlH4 in the presence of solvents like
THF (Tetrahydrofuran) followed by hydrolysis gives alcohols. Unlike other
reducing agents such as Raney Ni, Na-Hg/H2O, the lithium aluminium
hydride does not reduce the carbon-carbon double bond present in
unsaturated carbonyl compound and hence it is a best reagent to prepare
unsaturated alcohols.
3.6.
Properties
Physical properties
i.
Lower alcohols are colourless liquids and the higher members are waxy
solids.
ii.
They have higher boiling points than the corresponding other organic
compounds such as alkanes, aldehydes, ethers etc., this is due to the presence
of intermolecular hydrogen bonding present in alcohols.
iii.
Among isomeric alcohols primary alcohols have higher boiling point and
the tertiary alcohols have lower boiling points.
iv.
The lower members are highly soluble in water due to the formation of
intermolecular hydrogen bonding with water.
Boiling
point of alcohols in comparision with other organic compounds.
Chemical properties of alcohols
Nucleophilic substitution reactions of alcohols
Alcohol
has a strong basic leaving group (OH-). So, -OH group is first
converted into -OH2 group by adding an acid. The -OH2
group in the protonated alcohol can be easily displaced by a nucleophile such
as Br- to give alkyl halides.
Example: Alcohols undergo nucleophilic substitution reaction with hydro halic
acids to form alkyl halides. In case of tertiary alcohols heating is required.
2. Elimination reactions of alcohols
When
alcohols are heated with a suitable dehydrating agents like sulphuric acid, the
H and OH present in the adjacent carbons of alcohols are lost, and it results
in the formation of a carbon - carbon double bond. Phosphoric acid, anhydrous
ZnCl2, alumina etc., can also be used as dehydrating agents.
Mechanism
Primary
alcohols undergo dehydration by E2 mechanism
Tertiary
alcohols undergo dehydration by E1 mechanism. It involves the
formation of a carbocation.
Protonation of alcohol
3.7.
Uses
Uses of methanol :
1.
Methanol is used as a solvent for paints, varnishes, shellac, gums, cement,
etc.
2.
In the manufacture of dyes, drugs, perfumes and formaldehyde.
Uses of ethanol:
1.
It is also used in the preparation of a)Paints and varnishes. b) Organic
compounds like ether, chloroform, iodoform, etc., c)Dyes, transparent soaps.
2.
As a substitute for petrol under the name power alcohol used as fuel for
aeroplane
3.
It is used as a preservative for biological specimens.
Uses of ethylene glycol:
1.
Ethylene glycol is used as an antifreeze in automobile radiator
2.
Its dinitrate is used as an explosive with TNG.
Uses of glycerol
1.
Glycerol is used as a sweetening agent in confectionary and beverages.
2.
It is used in the manufacture of cosmetics and transparent soaps.
3.
It is used in making printing inks and stamp pad ink and lubricant for watches
and clocks.
4.
It is used in the manufacture of explosive like dynamite and cordite by mixing
it with china clay
3.8.
Test for hydroxyl groups
The following tests are used to distinguish between
1°, 2° and 3° alcohols.
a) Lucas test:
When
alcohols are treated with Lucas agent (a mixture of concentrated HCl and
anhydrous ZnCl2) at room temperature, tertiary alcohols react
immediately to form a turbidity due to the formation of alkyl chloride which is
insoluble in the medium. Secondary alcohols react within 10 minutes to form a
turbidity of alkyl chloride where primary alcohols do not react at room
temperature.
b) Victor Meyer’s test:
This
test is based on the behaviour of the different nitro alkanes formed by the
three types of alcohols with nitrous acid and it consists of the following
steps.
i) Alcohols are converted into alkyl iodide by
treating it with I2/P .
ii) Alkyl iodide so formed is then treated with
AgNO2 to form nitro alkanes.
ii) Nitro alkanes are finally treated with HNO2
(mixture of NaNO2 / HCl) and the resultant solution is made alkaline
with KOH.
Result:
•
Primary alcohol gives red colour
•
Secondary alcohol gives blue colour.
•
No colouration will be observed in case of tertiary alcohol.
3.9.
Oxidation of diols by periodic acid
Ethylene
glycol on treatment with periodic acid gives formaldehyde. This reaction is
selective for vicinal 1,2 – diols and it proceeds through a cyclic periodate
ester intermediate.
3.10.
Oxidation of diols by lead tetraacetate.
The
Criegee oxidation is a glycol cleavage reaction in which vicinal diols are
oxidized to form ketones and aldehydes using lead tetraacetate. It is analogous
to the use of periodate (Malaprade reaction) but uses a milder oxidant. This
oxidation was discovered by Rudolf Criegee and coworkers and first reported in
1931 using ethylene glycol as the substrate.
Two
mechanisms are proposed for the Criegee oxidation, depending on the
configuration of the diol. If the oxygen atoms of the two hydroxy groups are
conformationally close enough to form a five-membered ring with the lead atom,
the reaction occurs via a cyclic intermediate. If the structure cannot adopt
such a conformation, an alternate mechanism is possible, but is slower.
Trans-fused five member rings are heavily strained, thus trans-diols that are
on a five-membered ring will react slower than cis-alcohols on such a
structure.
UNIT-V
4.
Phenols
Phenols
are organic compounds in which a -OH group is directly attached to a benzene
ring. The carbon bearing the -OH group is sp2 hybridized.
4.1.
Nomenclature;
4.2.
Preparation from diazonium salts
Aniline
is diazotized with nitrous acid (NaNO2 +HCl ) at 273-278K to give
benzene diazonium chloride which on further treatment with hot water in the
presence of mineral acid gives phenol.
4.3.
Preparation from Cumene
A
mixture of benzene and propene is heated at 523K in a closed vessel in presence
of H3PO4 catalyst gives cumene (isopropylbenzene). On
passing air to a mixture of cumene and 5% aqueous sodium carbonate solution,
cumene hydro peroxide is formed by oxidation. It is treated with dilute acid to
get phenol and acetone. Acetone is also an important byproduct in this
reaction.
4.4.
Dow’s process
From
halo arenes(Dows process)
When
Chlorobenzene is hydrolysed with 6-8% NaOH at 300 bar and 633K in a closed vessel,sodium phenoxide
is formed which on treatment with dilute HCl gives phenol.
4.5.
Raching process;
Generally we call it the Raschig - Hooker process.
This process is used for production of phenol. We use Rasching’s process for
the preparation of chlorobenzene. After that we convert the obtained
chlorobenzene to phenol by hydrolysis that is by adding water.
In the first step we convert the benzene into
chlorobenzene. That is when benzene reacts with hydrochloric acid in the
presence of oxygen we will get chlorobenzene. In this step we use either copper
or iron chloride catalyst and we expose the material to air at 4000C
In step two we convert the obtained chlorobenzene into
phenol by hydrolyses. In this step we expose the material (chlorobenzene) to a
steam at 4500C
Phenol is a useful precursor to a huge collection of drugs,
most notably aspirin but also several herbicides and pharmaceutical drugs.
Phenol by-products have been used in the making of cosmetics including hair
colouring, sunscreens and in skin lightening preparations.
4.6.
Properties
Physical Properties
Phenol
is colourless, needle shaped crystal, hygroscopic, corrosive and poisonous. It
turns pink on exposure to air and light. The simplest phenols are liquids or
low melting solids, they have quite high boiling points. Phenol is slightly
soluble in water because of hydrogen bonding. However other substituted phenols
are essentially insoluble in water.
Chemical Properties:
Reactions
involving -OH group.
a) Reaction with Zn dust:
Phenol
is converted to benzene on heating with zinc dust. In this reaction the hydroxyl
group which is attached to the aromatic ring is eliminated.
b) Reaction with ammonia:
Phenol
on heating with ammonia in presence of anhydrous ZnCl2 gives
aniline.
c)Formation of esters:
Schotten-Baumann reaction :
Phenol
on treatment with acid chlorides gives esters. The acetylation and benzoylation
of phenol are called Schotten-Baumann reaction.
d)Formation of ethers:
Williamson ether synthesis:
An
alkaline solution of phenol reacts with alkyl halide to form phenyl ethers. The
alkyl halide undergoes nucleophilic substitution by the phenoxide ion in the
presence of alkali.
d) Oxidation:
Phenol
undergoes oxidation with air or acidified K2Cr2O7
with conc. H2SO4 to form 1,4-benzoquinone.
e)Reduction:
Phenol
on catalytic hydrogenation gives cyclohexanol.
4.7.
acidic character and effect of substitution on acidity.
Acidity of Phenol
Phenol
is more acidic than aliphatic alcohols. Unlike alcohols it reacts with bases
like sodium hydroxide to form sodium phenoxide. This explains the acidic
behaviour of phenol.let us consider the aqueous solution of phenol in which the
following equilibrium exists.
Ka
value for the above equilibrium is 1×10-10 at 25oC.
This Ka value indicates that it is more acidic than aliphatic alcohols. This
increased acidic behaviour can be explained on the basis of the stability of
phenoxide ion. We have already learnt in XI standard that the phenoxide is more
stabilised by resonance than phenol.
In
substituted phenols, the electron withdrawing groups such as -NO2,
-Cl enhances the acidic nature of phenol especially when they are present at
ortho and para positions. In such cases, there is a possibility for the
extended delocalisation of negative charge on the phenoxide ion. On the otherhand
the alkyl substitued phenols show a decreased acidity due to the electron
releasing +I effect of alkyl group.
|
Table: pKaValues of some alcohols and phenols |
||
|
S.No. |
Compound |
pKa Value |
|
1 |
methanol |
15.5 |
|
2 |
ethanol |
15.9 |
|
3 |
propan - 2- ol |
16.5 |
|
4 |
2 - methyl propan 2 - ol |
18.0 |
|
5 |
Cyclohexanol |
18.0 |
|
6 |
Phenol |
10.0 |
|
7 |
o - nitrophenol |
7.2 |
|
8 |
p - nitrophenol |
7.1 |
|
9 |
m - nitrophenol |
8.3 |
|
10 |
o - cresol |
10.2 |
|
11 |
m - cresol |
10.1 |
|
12 |
p - cresol |
10.2 |
4.8.
Fries Rearrangement
·
Fries Rearrangement is an organic rearrangement reaction in which an
aryl ester is transformed into a hydroxy aryl ketone with the help of a Lewis
acid catalyst and an aqueous acid.
·
In this reaction, an acyl group belonging to the phenolic ester
migrates to the aryl ring.
·
It is important to note that Fries rearrangement is ortho and para
selective, i.e. the acyl group attaches itself at the ortho or para positions
of the aryl ring.
·
The selectivity of the reaction can be directed by modifying the
reaction conditions (such as the temperature under which the reaction is
conducted, or the solvent used in the reaction).
An illustration detailing the Fries rearrangement
undergone by phenyl acetate (acetoxy benzene) is provided above. Note that the
products feature ortho and para migrations of the acyl group.
Fries Rearrangement
Mechanism
Initially, the carbonyl oxygen belonging to the acyl
group forms a complex with the Lewis acid catalyst (usually AlCl3).
The formation of the complex with the carbonyl oxygen is favoured over the
complexation of the phenolic oxygen since the carbonyl oxygen is richer in
electrons and is, therefore, a better Lewis base.
Now, the bond between the phenolic oxygen and the acyl
complex becomes polarised, resulting in the rearrangement of the AlCl3 bond
to the phenolic oxygen. This results in the generation of an acylium
carbocation.
The acylium carbocation goes on to attack the aromatic
ring via an electrophilic aromatic substitution reaction. It is
important to note that the orientation of this electrophilic aromatic
substitution is temperature-dependent. Low reaction temperatures favour
substitutions at the para position and relatively high temperatures favour
ortho substitution.
The mechanism of the Fries rearrangement reaction is
illustrated above. The use of a non-polar solvent in this reaction also favours
the formation of ortho-substituted products. Highly polar solvents favour para
substitution in this reaction.
- Limitations of Fries Rearrangement
The key limitations of Fries rearrangement are listed
below.
- Owing to its relatively harsh reaction conditions, only esters with
relatively stable acyl components can be used in this reaction.
- Low yields are obtained when heavily substituted acyl components
exist.
- The presence of deactivating or meta-directing groups on the
aromatic ring results in low yields.
4.9.
Claisen
rearrangement
Claisen rearrangement is an organic chemical reaction
that offers a powerful method of the formation of carbon-carbon bonds. The
reactant of this reaction – allyl vinyl ether, is converted into a gamma,
delta-unsaturated carbonyl compound when subjected to heat or a Lewis acid.
The Claisen rearrangement reaction is named after its
discoverer, the German chemist Rainer Ludwig Claisen, who discovered it in
1912. This reaction belongs to the “sigmatropic rearrangement” category
of reactions wherein the mechanism of the reaction is concerted (i.e. all the
bonds break and form simultaneously).
An interesting fact about this reaction is that it was
the first ever recorded example of a [3,3]- sigmatropic rearrangement reaction.
An example of the Claisen rearrangement reaction of an
allyl vinyl ether is given below.
Claisen Rearrangement Reaction
The reaction can also be performed with allyl phenyl
ethers. In this rearrangement, the regio selectivity is affected by the meta-substitution.
The [3,3]-sigmatropic rearrangement of the allyl phenyl ether gives an
intermediate. This intermediate now undergoes tautomerization to give a phenol
which is substituted at the ortho position. An example for the
[3,3]-sigmatropic rearrangement of an allyl phenyl ether is given below.
Mechanism of Claisen Rearrangement
This rearrangement reaction has an exothermic nature.
As discussed earlier, the reaction mechanism is concerted. The reaction
kinetics of this rearrangement reaction is of the first order. The reaction is
accelerated by polar solvents. Hydrogen-bonding solvents provide further
acceleration of reaction speed and greater rate constants.
1. Allyl Vinyl Ethers
Here, heat is the catalyst of the reaction. When the
allyl vinyl ether is subjected to heat, it forms a transition state. Now, a
[3,3]-sigmatropic rearrangement takes place leading to the formation of the
required gamma, delta-unsaturated carbonyl compound product.
This mechanism is illustrated below.
2. Allyl Phenyl Ethers
The electrons are pushed around the six-membered ring
in an electrocyclic process. The resulting dienone now undergoes
tautomerization to give its aromatic enol form. This form is more stable than
the dienone form. The required compound is therefore formed.
This mechanism can be illustrated as follows.
ELECTROPHILIC SUBSTITUTION REACTIONS
4.10.
Reimer – Teimen Reaction
On
treating phenol with CHCl3/NaOH, a -CHO group is introduced at ortho
position. This reaction proceeds through the formation of substituted benzal
chloride intermediate.
4.11.
Kolbe-Schmidt Reaction
In
this reaction, phenol is first converted into sodium phenoxide which is more
reactive than phenol towards electrophilic substitution reaction with CO2
. Treatment of sodium phenoxide with CO2 at 400K, 4-7 bar pressure
followed by acid hydrolysis gives salicylic acid.
4.12.
Gatermann synthesis
Formylation of aromatic substrates like benzene,
phenol or ether on reaction with hydrogen cyanide and gaseous hydrochloric acid
in the presence of anhydrous aluminium chloride or zinc chloride is known as
Gattermann aldehyde synthesis. Zinc chloride with HCN and HCl forms zinc
cyanide.
EXAMPLE:1
EXAMPLE:2
Mechanism
1.Formation
of formiminochloride aluminium chloride intermediate by the action ofAlCl3 on HCN
and HCI (a).
2. Electrophilic reaction of the intermediate on
aromatic compound leads to the formation of cationic intermediate (b).
3. Deprotonation and aromatisation forming iminoformyl
hydrochloride (c).
4. Hydrolysis of the product (c) to get aldehyde.
4.13.
Libermann reaction
While phenol is
reacted with NaNO2 and concentrated H2SO4, it
provides a deep green or blue colour which changes to red on dilution with
water. while generated alkaline along with NaOH original green
or blue colour is restored. This reaction is termed as Liebermann's nitroso
reaction and is employed as a test of phenol.
4.14.
phthalein reaction
On
heating phenol with phthalic anhydride in presence of con.H2SO4,
phenolphthalein is obtained.
4.15.
Resorcinol
Resorcinol compound consists of elements Carbon,
Hydrogen, and Oxygen. Carbon is a nonmetal present in group-14 of the periodic
table. Its atomic number is 6 and is represented with the symbol C. Hydrogen is
a colorless, odorless, tasteless, and flammable gas. Its atomic number is 1 and
is represented with the symbol H. Oxygen is a highly reactive nonmetal and a
good oxidizing agent. It is present in the chalcogen group of the periodic
table. Its atomic number is 8 and is represented by the symbol O.
Resorcinol Formula
Resorcinol is a white crystalline solid organic
compound with a faint odor and sweetish to bitter taste. Its chemical formula
is C6H6O2. It is soluble in
water, alcohol, and ether, but it doesn’t dissolve Chloroform and Carbon
disulfide. It is difficult to ignite. C6H6O2 is
one of the three isomeric benzenediols. It is a 1,3-isomer of benzenediol,
i.e., benzene dihydroxylated at 1 and 3 positions. The other names of
Resorcinol are Resorcin, 3-Hydroxyphenol, m-Benzenediol, and
m-Dihydroxybenzene. Resorcin doesn’t occur naturally in a free state but is
found in argan oil. It turns into pink color on exposure to air and light.
Preparation of Resorcinol
Resorcinol crystallizes from benzene as colorless
needles. Resorcinol is prepared by a classic sulfonate fusion process. Firstly
benzene is treated with sulfuric acid at 100°C, which gives mono sulfonic acid.
It is converted into m-disulfonic acid with 65% oleum at 85°C. Now
m-Benzenedisulfonate melts in Sodium Hydroxide(NaOH) at 300 °C to give
Resorcinol and Sodium sulfite.
Structure of Resorcinol
Physical Properties of Resorcinol
- Resorcinol appears in white solid form.
- Odor of Resorcinol is a faint Benzene odor.
- The molecular weight of Resorcinol is 110.1 g/mol.
- The melting point of Resorcinol is 110°C.
- Its boiling point is 277°C.
- The density of Resorcinol is 1.28 g/cm3.
Chemical Properties of Resorcinol
- Resorcinol (C6H6O2) on partial
hydrogenation gives Dihydroresorcinol (C6H8O2),
which is also known as 1,3-cyclohexanedione.
- Sodium amalgam (NaHg) reduces Resorcinol (C6H6O2)
to dihydro resorcinol (C6H8O2). The
resultant is heated to 150 to 160 °C with a concentrated barium hydroxide
solution to give γ-acetylbutyric acid (C6H10O3).
- Resorcinol reduces Fehling’s solution and ammoniacal silver
solutions.
Uses of Resorcinol
- Resorcinol is used as a sensitizer and an erythropoietin inhibitor.
- It is used in the production of resins.
- Resorcinol is widely used in the making plastics and
pharmaceuticals.
- It is used as an intermediate in the synthesis of organic
compounds.
- Resorcinol is used in the treatment of acne.
- It is a disinfectant and analytical reagent.
4.16.
Quinol
The
chemical formula for hydroquinone (Quinol) is C6H6(OH)2𝐶6𝐻6(𝑂𝐻)2. It is a crystalline solid at room temperature. The crystal structure of
hydroquinone is monoclinic. The unit cell contains 4 molecules composed of two
and two not dependent on space group symmetry.
Let's take a look at the structure of deprotonated hydroquinone. A
deprotonated hydroquinone has one hydrogen atom missing.
The resonance structures for this compound are-
Hydroquinone
can be prepared in two main methods at a large scale, these methods are-
The
first method is quite similar to the cumene method, in this benzene undergoes
dialkylation reaction with propene and produces 1,4-diisopropyl benzene.
1,4-diisopropylbenzene, reacts with air to produce bishydroperoxide, it has
structure similar to cumene hydroperoxide. Bishydroperoxide rearranges itself
in the presence of acid to yield hydroquinone and acetone.
The
second method proceeds by the hydroxylation of phenol in the presence of a
catalyst. Hydrogen peroxide is used in the reaction and a mixture of
hydroquinone and catechol is produced. The chemical reaction for the
preparation of hydroquinone is as follows-
C6H5OH + H2O2 → C6H4(OH)2+H2O𝐶6𝐻5𝑂𝐻 + 𝐻2𝑂2
→ 𝐶6𝐻4(𝑂𝐻)2+𝐻2𝑂
Hydroquinone uses
- It is commonly
used as a reducing agent.
- It is used as a
biomarker for benzene exposure
- It is used by
photographic developers.
- It is used to
treat acne scarring.
- It is used in
skin lightening and whitening creams.
- It finds
application in photographic film and paper.
- It is used as a
polymerization barrier.
- It finds its
application as a preservative for resins and improves its shelf life.
- It acts as a
biomarker in benzene exposure.
4.17.
picric acid
Properties of Picric Acid
It’s chemical structure consists of a benzene ring
with three nitro groups (-NO2) attached to it. Picric acid is highly
explosive and can be toxic if handled improperly.
Physical Properties of Picric Acid
The physical properties of picric acid are mentioned
below:
- Picric acid, also known as 2,4,6-trinitrophenol, is a yellow
crystalline solid at room temperature.
- It has a distinct odor, often described as similar to bitter
almonds.
- Picric acid is sparingly soluble in water but dissolves well in
organic solvents like ethanol and ether.
- It is highly explosive when dry and can detonate upon impact or friction.
- The compound is sensitive to heat and shock, making it hazardous to
handle.
- Picric acid is commonly used in the manufacture of dyes,
explosives, and as a reagent in chemical laboratories.
- It can form sensitive and unstable compounds with metals like lead,
copper, and iron, increasing its risk of detonation.
- Exposure to picric acid, especially through inhalation or skin
contact, can cause irritation, burns, and other health hazards.
Chemical Properties of Picric Acid
The chemical properties of picric acid are mentioned
below:
- Picric acid is a yellow yet crystalline compound.
- It is one of the most exhaustive ideal gases, and thus, even in
small quantities, it is highly dangerous.
- It can form solution in both, water and many organic solvents.
- Picric acid, is generally applied in the setting up of dyes,
metalutic substances(explosives) and as a reagent in chemical.
- It has a tendency to be ‘troublemaker’ reacting violently with
metals, bases, and reducing agents.
- This exhibits its characteristisc when heated, can decompose
violently.
- The corrosion on skin or eyes can be very dangerous and can lead to
severe burns.
- Its strong reactive power makes it rated thus caution is required
when handling and therefor storage.
Preparation of Picric Acid
The method of preparation of picric acid is discussed
below:
Formation of Phenol Sulphuric Acid:
First, phenol is mixed with concentrated sulfuric
acid. This mixture is gently heated. During this reaction, sulfuric acid acts
as a catalyst, facilitating the formation of phenol sulfate.
Nitration Reaction:
In the second step, the phenol sulfate formed in the
previous step is further treated with concentrated nitric acid.
Nitric acid introduces nitro groups (NO2)
onto the phenol ring. These nitro groups preferentially attach
themselves to the ortho and para positions of the phenol ring due to electronic
effects, resulting in the formation of 2,4,6-trinitrophenol, which is picric
acid.
Uses of Picric Acid
The uses of picric acid are mentioned below:
- Picric acid and its derivatives like Dunnite and TATB are common
explosives.
- Picric acid, especially as Picral, has been used in metallurgy, but
its use is declining due to risks.
- Picric acid is used in organic chemistry to create picrates, aiding
in identification.
- Bouin solution, containing picric acid, enhances histology staining
but may cause DNA hydrolysis.
- Picric acid reacts with hydrogen cyanide to produce red
isopurpurate for cyanide quantification.
- Picric acid was used in early 20th-century hospitals as an
antiseptic and for various treatments.
- Picric acid historically measured blood glucose levels using the
Lewis and Benedict system.
- Picric acid has been used in fly tying to dye materials for fishing
lures, despite toxicity concerns.
4.18.
Aromatic alcohols Nomenclature
|
STRUCTURE |
NAME |
|
|
4-Fluoro-2-methylphenol |
|
|
2-Chlorobenzyl
alcohol |
|
|
3-Bromobenzyl alcohol |
|
|
4-Bromobenzyl alcohol |
4.19.
Benzyl alcohol
Benzyl alcohol with the chemical formula C6H5CH2OH
is an aromatic alcohol. The "Bn" group of benzyls is often
abbreviated (not to be mistaken with "Bz" used for benzoyl), and
benzyl alcohol is referred to as BnOH. Benzyl alcohol is a colourless liquid
with a faint aromatic scent. Its polarity, low toxicity, and low vapour
pressure make it a useful solvent. Benzyl alcohol has modest water solubility
(4 g/100 mL), and alcohol and diethyl ether are miscible. The anion formed by
alcohol group deprotonation is called benzylate, or benzyl oxide.
IUPAC Name: Phenyl methanol
Synonyms:Phenylcarbinol, Benzenemethanol
Chemical Formula: C7H8O
METHODS OF PREPARATION
4.20.
HYDROLYSIS
Obtained by the chlorination of Toluene followed by
hydrolysis with aqueous NaOH.
The formation of benzyl alcohol from benzyl chloride involves
nucleophilic substitution.
This method is used in the large scale preparation of
benzyl alcohol.
4.21.
Reduction of benzaldehyde
Phenylmethanol can be prepared from benzaldehyde by
treating it with zinc in presence of HCl
This is categorised as a reduction reaction as
hydrogen is added to the molecule during this reaction.
Given below is the chemical reaction for the same.
4.22.
Cannizzaro reaction
In
the presence of concentrated aqueous or alcoholic alkali, aldehydes which do
not have α - hydrogen atom undergo self oxidation and reduction (disproportionation)
to give a mixture of alcohol and a salt of carboxylic acid. This reaction is
called Cannizaro reaction.
Benzaldehyde
on treatment with concentrated NaOH (50%) gives benzyl alcohol and sodium
benzoate.
This
reaction is an example disproportionation reaction
Crossed Cannizaro reaction
When
Cannizaro reaction takes place between two different aldehydes (neither
containing an α hydrogen atom), the reaction is called as crossed cannizaro
reaction.
In
crossed cannizaro reaction more reactive aldehyde is oxidized and less reactive
aldehyde is reduced.
4.23.
Grignard synthesis
4.24.
Physical properties
It is a colourless pleasant smelling liquid with b.p.
478 K. It is sparingly soluble in water, because of the presence of hydrophobic
phenyl group (larger in size when compared to methyl or ethyl group). But it is
soluble in organic solvents like benzene and alcohol.
Chemically it resembles aliphatic primary alcohol. 1.
It is not so acidic as to dissolve in sodium hydroxide but reacts with sodium
metal forming sodium benzylate or sodium benzoxide.
Sodium benzoxide brings about nucleophilic
substitution at methyl carbon atom of the methyl iodide forming ethers.
2. Reagents like PCl5, SOCl2 and
HCl readily forms benzyl chloride with benzyl alcohol.
4. On heating with phosphorous and hydriodic acid, it
is reduced to Toluene. Benzyl iodide is the intermediate in this reaction.
Iodine is removed by phosphorous.
Hydrogen in presence of Palladium is the other reagent
that can reduce benzyl alcohol to Toluene.
5. Benzyl alcohol forms esters with carboxylic
acids in
presence of conc. sulphuric acid, and also with acid chlorides and acid
anhydrides.
6. Oxidation : (i) With mild oxidising
agents like copper nitrate or lead nitrate, benzyl alcohol is converted to
benzaldehyde.
Decomposition of the metallic nitrate provide the
source for the above oxidation.
(ii) In the oxidation with acidified potassium
dichromate or alkaline potassium permanganate, benzaldehyde is first formed
which undergoes further oxidation to benzoic acid.
7. In addition to the above reactions, it undergoes
reactions characteristic of the benzene ring-namely electrophilic substitution
reactions like halogenation, nitration, sulphonation etc. In all these cases
substitution takes place in the benzene ring. Like CH3- and CH2C1-
groups, CH2OH- is also ortho, para directing group. Hence
ortho or para substituted products are formed.
Uses
(i) Used as a local anaesthetic in intravenus
subcutaneous injections.
(ii) as an antiseptic in ointments.
(iii) as esters in perfumery. (Benzyl acetate has
fragrance of Jasmine)
(iv) as benzyl benzoate in the treatment of asthma and
whooping cough.
(v) in the manufacture of synthetic resins.
4.25.
Thiols
Thiols
are often called “mercaptans,” a reference to the Latin term mercurium
captans (capturing mercury), since the -SH group forms strong bonds with
mercury and its ions. Thiols are analogous to alcohols. Thiols are weakly
acidic (pKa ~ 10) and are much stronger acids than alcohols (pKa ~ 16).
However, thiols usually do not form hydrogen bonds due to the sulfur atom not
have sufficient electronegativity. Thiols named using the same rules as
alcohols except the parent chain is named as alkane with the suffix -thiol
added. As a substituent the -SH group is called a mercapto group.
Thiols are usually prepared by using the hydrosulfide
anion (-SH) as a nucleophile in an SN2 reaction with alkyl
halides.
One problem with this reaction is that the thiol
product can deprotonate and undergo a second SN2 reaction with an
additional
alkyl halide to produce a sulfide side product. This
problem can be solved by using thiourea, (NH2)2C=S, as
the nucleophile. The SN2 reaction first produces an alkyl
isothiourea salt as an intermediate. This salt is then hydrolyzed to form
the thiol by a reaction with aqueous base.
Preparation of Thiols
Thiols are prepared by the nucleophilic
substitution reaction where hydrosulfide anion is a nucleophile which
reacts with alkyl halides.
For example- Reaction of sodium hydrosulfide with
alkyl halides.
Hydrosulfide is used in excess amounts to prevent the
reaction of thiol with the alkyl halide and the formation of sulphide and thioether
due to its high nucleophilicity.
One more reaction uses alkyl halides with thiourea as
a sulphur nucleophile. This reaction produces alklisohiouronium salts which are
later hydrolyzed to thiols.
In this reaction, the lone pair of sulphur attacks on
the alkyl group of bromoalkane and the alkyl bromide bond is broken. Then
s-alklisothiouronium bromide is reacted with sodium hydroxide. The
intermediate is then made to react with hydronium ions in the presence of
sodium hydroxide. Therefore, the formation of thiol takes place.
Properties of Thiol
The physical and chemical properties of thiol are as
follows-
Physical Properties
- They are colourless liquids.
- Their odour is similar to that of garlic.
- They have low boiling points.
- They are less soluble in water and other polar solvents.
- Thiol is responsible for the characteristic fragrance of
grapefruit.
Chemical Properties
The reaction of thiol with bromine yields organic
disulfides. The reaction takes place as follows-
2R−SH + Br2 →R−S−S−R + 2HBr
2𝑅−𝑆𝐻 + 𝐵𝑟2 →𝑅−𝑆−𝑆−𝑅 + 2𝐻𝐵𝑟
Oxidation of thiols with powerful oxidising agents
sodium hypochlorite or hydrogen peroxide yields sulphonic acids. The reaction
proceeds as follows-
R−SH + 3H2O2 →RSO3H + 3H2O
GENERAL CHEMISTRY-III
Model Question Paper
SECTION A – (10 × 2 =
20 marks)
Answer ALL questions
1. Define Boyle’s temperature.
2. What is RMS velocity.
3. Define liquid crystals.
4. State laws of crystallography.
5. Define Nuclear binding energy.
6. What are radioactive series.
7. What is nucleophilic substitution reaction.
8. Give the reason why benzene will not undergoes
nucleophilic substitution reaction.
9. Write the reaction of nitration of phenol.
10. What is catalysis hydrogenation.
SECTION B – (5 × 5 = 25
marks)
Answer ALL questions
11. A)Discuss the Maxwell distribution of molecular
velocities (or)
B) Derive the kinetic gas equation.
12. A) What is mean by seven crystal system. Explain
in detail.
(or)
B) What are liquid crystals? How are they
classified.
13. A) Difference between Nuclear fission and Nuclear
fusion (or)
B) What are the types of nuclear reactions? Give
example.
14. A) Explain the mechanism of SN1 reaction. (or)
B) Describe the Aromatic Nuclear Substitution
reaction with example
15. A) Briefly explain the acidic character of phenol (or)
B) Write the notes on I) Remer Tiemann reaction
II) Houben Hoesh reaction.
SECTION C – (3 × 10 =
30 marks)
Answer any THREE questions
16. A) What is mean by viscosity and surface tension.
What is the effect of temperature on it.
B) Write notes on liquid crystals.
17. Write the notes on Bravis Space lattice.
18. Write an account on application of Nuclear
Chemistry.
19. Write preparation, properties and uses of Benzyl
chloride.
20. Briefly explain the following reactions
I) Kolbe’s reaction.
II) Gatterman reaction.
III) Claisen
rearrangement.
IV) Cannizaro reaction.