what are the types of isomerism

Aug 26, 2022, 16:45 IST

ISOMERISM

In the study of organic chemistry we come across many cases when two or more compounds are made of equal number of like atoms. A molecular formula does not tell the nature of organic compound; sometimes several organic compounds may have same molecular formula.  The compounds possess the same molecular formula but differ from each other in physical or chemical properties, are called isomers and the phenomenon is termed isomerism (Greek, isos = equal; meros  = parts). Since isomers have the same molecular formula, the difference in their properties must be due to different modes of the combination or arrangement of atoms within the molecule. Broadly speaking,

Types of isomerism

(i) Structural Isomerism

(ii) Stereoisomerism

(iii) Structural isomerism

When the isomerism is simply due to difference in the arrangement of atoms within the molecule without any reference to space, the phenomenon is termed structural isomerism. In other words, while they have same molecular formulas they possess different structural formula. This type of isomerism which arises from difference in the structure of molecules, includes:

(a) Chain or Nuclear Isomerism

(b) Positional Isomerism

(c) Metamerism 

(d) Functional Isomerism

(e) Ring Chain Isomerism

(f) Tautomerism

(ii) Stereoisomerism:

When isomerism is caused by the different arrangements of atoms or groups in space, the phenomenon is called Stereoisomerism (Greek, Stereos = occupying space). The stereoisomers have the same structural formula but differ in the spatial arrangement of atoms or groups in the molecule. In other words, stereoisomerism is exhibited by such compounds which have identical molecular structure but different configurations.

Stereoisomerism is of three types:

(a) Geometrical or cis-trans isomerism.

(b) Optical Isomerism.

(c) Conformational isomerism.

Chain or Nuclear Isomerism

This type of isomerism arises from the difference in the structure of carbon chain which forms the nucleus of the molecule. It is, therefore, named as chain, nuclear isomerism or skeletal isomerism. For example, there are known two butanes which have the same molecular formula (C₄H₁₀) but differ in the structure of the carbon chains in their molecules.

Tautomerism

The term tautomerism is used for isomers that are fairly readily interconvertible and that differ from each other only (a) in electron distribution and (b) in the position of a relatively mobile atom or group, The mobile atom is generally hydrogen and the phenomenon is then called as prototropy.

Both acids and bases catalyse such interconversions. Possible limiting mechanisms are those 

(a) in which proton removal and proton acceptance (from the solvent) are separate operations and a carbanion intermediate is involved. i.e. an intermolecular pathway and 

(b) in which one and the same proton is transferred intramolecularly.