TABLE OF CONTENTS Stereoisomerism is due to the difference in the relative positions of atoms or groups in space. The isomers which differ in relative arrangement of atoms or groups in space around the carbon atom are known as stereoisomers and the phenomenon is known as stereoisomerism.
The branch of organic chemistry which deals with the three dimensional structure of the molecule and its effects on chemical behavior is known as stereochemistry. Stereoisomers are roughly divided into two classes:
Configurational stereoisomers, are those which interconvert with difficulty under normal conditions and thus can be isolated. Interconversion in these stereoisomers usually involves a bond-breaking process. This type of isomerism exists either due to rigidity in a molecule (geometrical isomers) or due to right-or-left-handed arrangement of atoms around carbon atom (optical isomers).
Conformational stereoisomers, are those which interconvert easily at room temperature through rotations about single bonds.
Before studying the three types of stereoisomers (optical, geometric and conformers) we must consider the three dimensional structure of tetrahedral carbon.
Kekule (1858) proposed that four valencies of carbon are distributed in one plane and hence the carbon compound can be represented as a square with the carbon atom at the center of the square (Figure 5.1(a)). Le Bel and Van't Hoff (1874) proposed separately that the four valencies of carbon distributed in three dimensions and if we join the ends of these four valencies by straight lines, we get a regular tetrahedron (Figure 5.lb).
