not chiral. With achiral molecules, the compound and its mirror image are
the same, i.e. they can be superimposed.
If you rotate the mirror image through 180
, it is identical to the original
structure.
Br
Cl
H
H
H
Cl
H
Br
Achiral molecule
Superimposable mirror image
Mirror image
Enantiomers The Greek word enantio means ‘opposite’. A chiral molecule and its mirror image are called enantiomers or an enantiomeric pair.
They are nonsuperimposable. The actual arrangement or orientation (in
space) of atoms/groups attached to the chiral carbon (stereogenic centre or
stereocentre) is called the configuration of a compound.
W
X
Y
z
X
W
Y
z
CH 3
H
OH
2
C
5
H
CH 3
C H 5
2
O
H
H
Enantiomers
Not superimposable
Mirror image
The arrangement of W, X, Y
and Z is configuration
*
*
Enantiomers of 2-butanol
(-)-2-Butanol
*
*
(+)-2-Butanol
Properties of enantiomers Enantiomers share same physical properties,
e.g. melting points, boiling points and solubilities. They also have same
chemical properties. However, they differ in their activities with plane
polarized light, which gives rise to optical isomerism, and also in their
pharmacological actions.
Drawing a chiral molecule (enantiomer) On a plane paper, chiral
molecules can be drawn using wedge bonds. There are also a few other
methods that use horizontal bonds representing bonds pointing out of the
paper and vertical bonds pointing into the paper. Some examples are given
below.
X
W
z
Y
C
Wedge bonds
Y
W
z
X
C
z
X
Y
W
z
X
Y
W
Fischer projection
3.2 ISOMERISM
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