Engineering Chemistry S Datta

MECHANISM OF ORGANIC REACTIONS 165
H
H
C
HO 2 C
CH 3
OH
C
HC 3 CO 2
H
HO
Mirror
Fig. 7.8 (a) The mirror image forms of lactic acid.
Asymmetric Molecules
If we translate the three-dimensional molecule to a two-dimensional figure, we get
Fischer projection formula of the molecule.
COOH
COOH
H
OH
HO
H
CH 3
D(–)-Lactic acid mirror
Fig. 7.8 (b)
CH 3
L(+)-Lactic acid
Then the test of chirality is very simple. If we rotate Fischer projection formula of one
molecule on the plane of the paper by 180° none of the configurations arrived at which is
superimpossible on the other. These types of molecules rotate plane of polarised light (see Fig. 7.6)
either right or left. The two configurations of lactic acid rotate plane of polarised light in the
same degrees but in the opposite directions. These two forms of stereoisomers (configurations)
are known as enantiomers.
Highlights:
• Number of possible stereoisomers = 2 n .
where n = number of chiral carbon atoms in the molecule.
• A mixture of equal parts of enantiomers is called racemic modification. The prefix
(±) is used to designate the racemic mixture. They can be separated by various
physical and chemical methods.
For compound containing two different chiral carbon atoms, the stereoisomers
(I) and (II), (III) and (IV) are pairs of enantiomers, (I) and (III), (II) and (IV) are diastereomers.
CH 3
CH 3
CH 3
CH 3
H
Cl
Cl
H
H
Cl
Cl
H
Cl
H
H
Cl
H
Cl
Cl
H
CH 2 5
CH 2 5
CH 2 5
(I) (II) (III) (IV)
Fig. 7.8 (c) Stereoisomers of 2, 3-Dichloropentane.
CH 2 5