H
H
H
H
H
H
H
H
H
H
H
R
R
H
H
H
H
H
H
H
H
H
H
H
Ring-flip in chair conformation
of monosubstituted cyclohexane
R = Any substituent group
or atom other than H
R in axial position
R in equatorial position
Configurational isomers
Configurational isomers differ from each other only in the arrangement of
their atoms in space, and cannot be converted from one into another by
rotations about single bonds within the molecules. Before we look into the
details of various configurational isomers, we need to understand the
concept of chirality.
Chirality Many objects around us are handed. For example, our left and
right hands are mirror images of each other, and cannot be superimposed on
each other. Other chiral objects include shoes, gloves and printed pages. Many
molecules are also handed, i.e. they cannot be superimposed on their mirror
images. Such molecules are called chiral molecules. Many compounds that
occur in living organisms, e.g. carbohydrates and proteins, are chiral
The most common feature in chiral molecules is a tetrahedral (i.e. sp
3
-
hybridized) carbon atom with four different atoms or groups attached. Such
a carbon atom is called a chiral carbon or an asymmetric carbon. Chiral
molecules do not have a plane of symmetry.
X
Y
z
W
X
z
z
W
C *
Chiral carbon
Four different groups/atoms present
C
Achiral carbon
At least two same groups/atoms (Z) present
When there are two or more atoms/groups that are the same, the carbon is
called achiral. Achiral molecules often have a plane of symmetry. If a
molecule can be divided by a plane into two equal halves that are mirror
images of each other, the plane is a plane of symmetry, and the molecule is
42
CH3 STEREOCHEMISTRY