Engineering Chemistry S Datta

166 ENGINEERING CHEMISTRY
Stereoisomers which are not pairs of enantiomers are known as diastereomers.
Another interesting phenomenon arises among the compounds containing similar chiral
carbon atoms [Fig. 7.8(d)].
CH 3
H
H
Cl
Cl
CH 3
Fig. 7.8 (d) Optically inactive isomer (meso form).
Lower part of the molecule is mirror image of the upper part of the molecule and the
molecule becomes optically inactive due to internal compensation. This type of isomer is known
as mesoform.
Geometrical Isomerism
This type of isomerism arises out of restricted rotation about C = C bond. When similar
groups are attached to the same side of the molecule, the configuration is known as
cis-isomers (Z-isomers). When similar groups are attached or the opposite side of the C = C
bond, the configuration is known as trans isomers (E-isomers). The geometrical isomerism
is also a case of diastereoisomerism [Fig. 7.8(e)].
HC 3 CH 3
H
CH 3
C C
C C
H H
CH 3
H
cis-2-butene
Trans-2-butene
(Z-2-butene)
(E-2-butene)
Fig. 7.8 (e)
R-S System of Nomenclature
R-S system of nomenclature was developed by Cahn, Ingold and Prelog to designate the
configurations of stereoisomers having chiral centres. The system is often termed as “absolute
configuration” assignment as this type of specifying configuration is independent of any reference
compound.
There are certain methods of assigning R-S notations to any chiral centre of a
molecule. These are as follows:
• The number of stereocentres in the molecule are identified.
• The four different groups (ligands) attached to each of the stereocentres are found out
and given priority 1, 2, 3, 4 or a, b, c, d, etc. based on ‘sequence rule’, such that 1 → 2
→ 3 → 4 or a → b → c → d.
• Then the molecule is viewed from the position far from the lowest priority group and a
hypothetical path is drawn moving from 1 → 2 → 3 or a → b → c.
If the path provides a clockwise motion, then the stereocentre is designated to have ‘R’
configuration. If the path describes a counter clockwise or anticlockwise path, then the
stereocentre is said to have ‘S’ configuration [Fig. 7.9].
The (R) or (S) designation is written in italics within the parentheses followed by a
hyphen before the name of the compound.