ISOMERISM IN ORGANIC CHEMISTRY
1.
Distinguish Enantiomers and Diastereomers.
Enantiomers
Diastereomers.
1. Optical isomers having the same
magnitude but different sign of
Optical isomers differ in the
magnitude of optical rotation.
optical rotation.
2. They have configuration with
They are never mirror images.
Non -super imposable object mirror
image relationship.
3. They are identical in all properties
except the sign of optical rotation.
4. Separation of enantiomers is a
tedious process.
They differ in all physical
properties.
Separation from the other pairs
of enantiomers is easy.
5. Example : (+) & (-) tartaric acid.
Example : meso-tartaric acid &
(+) tartaric acid.
2.
Explain the Optical isomerism of Tartaric acid.
 Tartaric acid is a Dihydroxy dicarboxylic acid
 Tartaric acid contains two identical chiral carbon atom.
 The number of optical isomers = 2n = 22 = 4
 But It has only three isomeric forms.
COOH
|
H
*
C
OH
|
HO
*
C
H
|
COOH
 Tartaric acid has two optical isomers with same magnitude but different sign of optical
rotation ( d , l pair )
 They have object – mirror image relationship.
 Both the chiral carbon rotates the plane polarized light into clockwise direction. So, it is
called dextrorotatory. d (+) Tartaric acid

Both the chiral carbon rotates the plane polarized light into anti clockwise direction.
So, it is called laevo rotatory. l (–) Tartaric acid
Racemic miture :
 When equal amounts of ‘d’ and ‘l’ isomers are mixed, the mixture is called Racemic
mixture.
 In this mixture, the rotation of d – isomer is compensated by rotation of l – isomer. So,
it becomes optically inactive. It is due to external compensation.
 It can be separated into two optically active forms.
Meso Tartaric acid :
 Meso Tartaric acid molecule has a symmetric plane,
which divides the molecule into two equal halves.
COOH
H
OH
 One asymmetric carbon atom is dextro rotatory and other - - - - - - - - - - - - - - - - is laevo rotatory both rotating to the same extent but in
H
OH
opposite direction.
 The net result is that this isomer becomes optically inactive.
COOH
 It is due to internal compensation.
 It has the configuration which is super impossible on its mirror image.
 d & l isomers which have the same magnitude but different sign of optical rotation are
called enantiomers
 Meso tartaric acid and d or l isomers are differed in the magnitude of optical rotation
and are called as Diastereomers.
3.
Describe the conformations of Cyclohexanol.
 Cyclohexanol exists in two chair forms.
 These two forms are inter convertible and exist in equilibrium.
 In conformation (I), the –OH group is axially oriented and in conformation (II) the –OH
group is equatorially oriented.
 The energy of the axial conformer is little higher than that of a equatorial. Hence, equatorial
conformer is present about 90% and axial conformer is only 10% at equilibrium.
 So, the axial conformer is unstable for the following reasons.
Reason :

The axial –OH group experiences steric interaction with the axial H – atoms present
at the third ‘C’ atom.

This is called 1:3 diaxial interaction. This decreases the stability of the axial
conformer.
4.
Explain the following terms with suitable exambles .
1. External compensation 2. Internal compensation
3. Assymetric carbon atom
External compensation
 When equal amounts of d – isomer and l – isomer are mixed one gets a racemic mixture
and this process is called racemisation.
 A racemic mixture becomes optically inactive.
 Because in this mixture rotation towards clockwise direction by the dextroisomer is
compensated by the rotatin towards the anticlockwise direction by the leavo isomer.
The optical inactivity of a racemic mixture is said to be due to external compensation.
Internal Compensation
 Meso isomer has two asymmetric carbon atom.The molecule have a symmetric plane.
Which divides the molecule into two equal halves. The molecule becomes achiral.
 The configuration of one carbon is the mirror image of the other(super impossible).
 The net result being , the molecule as a whole become symmetric
 one asymmetric carbon atom is dextro rotatory and other is laevo rotatory - both
rotating plane polarised light to the same extent in opposite directions. isomer
becomes optically inactive
 The optical inactivity of the meso isomer is due to internal compensation
Asymmetric carbon atom
A carbon atom attached to four different atoms or groups is called asymmetric caron
atom
Example : Lactic acid
The carbon with astreik is called the chiral carbon atom.
5. Distinguish Racemic form from Meso form.
Racemic form
Meso form
1. It is a mixture that contains It is a single compound
equal amount of d-isomer
and l-isomer.
2. It is a mixture that can be It is a single compound and hence
separated into two optically cannot be separated.
active forms.
3. It is optically inactive due to It is optically inactive due to internal
external compensation.
compensation.
4. Molecules of isomers present Molecules are achiral.
are chiral
5. E.g., equal amount of (+) & e.g., Meso-tartaric acid.
(-) tartaric acid.
6.
Write a short account on Cis-Trans isomerism
 Isomerism that arises out of difference in the spatial arrangement of atoms or groups
about the doubly bonded carbon atoms is called geometrical isomerism.
 The isomer in which similar groups lie on the same side is called cis isomer and similar
groups lie in the opposite direction is called trans isomer. This isomerism is called cistrans isomerism.
Example : 2 – butene
7.
Explain the following with examples. (a) Cis-Trans isomerism
(b) Optical isomerism.
a. Cis-Trans Isomerism:
 Isomerism that arises out of difference in the spatial arrangement of atoms or groups
about the doubly bonded carbon atoms is called geometrical isomerism.
 The isomer in which similar groups lie on the same side is called cis isomer and similar
groups lie in the opposite direction is called trans isomer. This isomerism is called cistrans isomerism.
Example : 2 – butene
b.
Optical Isomerism :
The compounds having the same molecular formula but possessing different spatial
arrangement of atoms or groups and differ in the rotation of plane polarized light.
Example : Lactic acid.
8.
Explain the Optical isomerism of Lactic acid.
 Lactic acid contains one chiral carbon atom.
 It has two isomeric forms.
H
i.
CH3
|
*
C
|
OH
COOH
d – Lactic acid :
1. It rotates the plane polarized light in clockwise direction.
2. So, it is called dextro rotatory.
CH3
H
OH
COOH
ii.
l – lactic acid :
1. It rotates the plane polarized light in anticlockwise direction.
2. So, it is called laevo rotatory.
Above both forms are called enantiomers because they differ only in sign of rotation
CH3
HO
H
.
COOH
iii.
Racemic mixuture :
 When equal amounts of ‘d’ and ‘l’ isomer are mixed, the mixture is called Racemic
mixture.
 In this mixture, the rotation of d – isomer is compensated by rotation of l – isomer. So,
it becomes optically inactive.
 It is due to external compensation.
 Separation of recemic mixture into optically active compound is called resolution