Chain and Positional 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.

While n-butane has a continuous chain of four carbon atoms, isobutane has a branched chain. These chain isomers have somewhat different physical and chemical properties, n-butane boiling at -0.5^o and isobutane at -10.2^o.This kind of isomerism is also shown by other classes of compounds.

Positional Isomerism

It is the type of isomerism in which the compounds possessing same molecular formula differ in their properties due to the difference in the position of either the functional group or the multiple bond or the branched chain attached to the main carbon chain. For example, n-propyl alcohol and isopropyl alcohol are the positional isomers.

In the aromatic series, the di-substituted products of benzene also exhibit positional isomerism due to different relative positions occupied by the two substituents on the benzene ring. Thus Xylene, C₆H₄(CH₃)₂, exists in the following three forms which are positional isomers.