State and explain the term diastereoisomers and mesomers.

Explanation

Diastereoisomers : Stereoisomers which are not related to each other as mirror images are called diastereomers.
Mesomers : Optical isomers which are optically inactive due to the presence of plane of symmtry i.e., do not rotate the plane of polarised light are called mesomers. For example , in case of 2, 3-dichlorobutane the pair (1) and (II) represents a pair of enantiomers, pair (I) and (III) or pair (II) and (III) represent pairs of diastereoisomers. Here (III) is a meso compound and is optically inactive. It is achiral. As all these thrce isomers behave differently towards the polarised light, they are termed as optical isomers.