NPTEL – Biotechnology – Cell Biology
Joint initiative of IITs and IISc – Funded by MHRD Page 1 of 48
Module 8 Stereochemistry
Lecture 20 Stereochemistry I
Stereochemistry is the study of the relative arrangement of atoms or groups in a molecule
in three dimensional space. Stereochemical isomers are molecules, which have the same
chemical formula and bond connectivity but different relative arrangement in three-
dimensional space. In contrast, constitutional isomers have same molecular formula but
different bond connectivity. Thus, n-butane and isobutane are structural isomers while the
isomers of limonene, the compound which gives different taste to lemon and orange are
examples of stereochemical isomers (Figure 1).
H
3
CCH
3
H
3
CCH
3
CH
3
n-butaneisobutane
structural isomers
CH
3
H
3
C
(+)-limonene
CH
3
H
3
C
(-)- limonene
Stereochemical isomers
"orangy taste""lemony taste"
Figure 1
To understand the difference between the two isomers of limonene, introduction to some
new terms and concepts are required. The most important being the concept of chirality.
A chiral object is one that cannot be superposed on its mirror image. The term originates
from the greek term for “hand”. As it is with human hands, the left hand cannot be
superimposed on the right hand. It is the same with chiral molecules. They are non-
superimposable mirror images of each other. Achiral objects, on the other hand, are easily
superimposable on each other. A tennis racquet and a spoon are examples of achiral
objects.
The next question that comes to the mind is how to determine whether a molecule is
chiral or achiral. At times, it becomes extremely difficult to determine with increasing
molecular complexity, to determine the non superimposibity of a compound with its
mirror image. Thus, a mathemathical concept known as group theory can be applied to
determine the symmetry elements in a molecule. There are four symmetry elements
which needs to be considered for this purpose:
