Cl Cl
H H
Cl Cl
Cl Cl
H H
Cl Cl
H H
Stereoisomerism in1,2-dichlorocyclohexane
Meso compound
Enantiomers
Geometrical isomers
Geometrical isomerism is found in alkenes and cyclic compounds. In
alkenes, there is restricted rotation about the double bond. When there are
substituent groups attached to the double bond, they can bond in different
ways, resulting in trans (opposite side) and cis (same side) isomers. These
are called geometrical isomers. They have different chemical and physical
properties. Each isomer can be converted to another when enough energy is
supplied, e.g. by absorption of UV radiation or being heated to temperatures
around 300
C. The conversion occurs because the p bond breaks when
energy is absorbed, and the two halves of the molecule can then rotate with
respect to each other before the p bond forms again.
C C
H
G
H
G
C C
G
H
H
G
trans-Isomer
Substituent G is on opposite sides
on the double-bonded carbons
cis-Isomer
Substituent G is on same side
on the double-bonded carbons
When there is the same substituent attached to the double bonded carbons,
as in the above example, it is quite straightforward to designate trans or cis.
However, if there are more than one different groups or atoms present, as in
the following examples, the situation becomes a bit more complicated for
assigning cis and trans.
C C
H
Cl
Br
Cl
C C
Cl
H
Br
Cl
C C
H
Cl
I
F
Alkenes with different substituents on the double-bonded carbons
To simplify this situation, the E/Z system is used for naming geometrical
isomers. Z stands for German zusammen, which means the same side, and E
for German entgegen, meaning on the opposite side.
In the E and Z system, the following rules or steps are followed.
(a) On each C atom of the double bond, we have to assign the priority of the
atoms bonded. The priority should be on the same basis as the (R)/(S)
system (i.e. on the basis of atomic number).
3.2 ISOMERISM
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