(ii) Position isomerism

It is due to the difference in the position of the substituent atom or group or an unsaturated linkage in the same carbon chain. Examples are

(iii) Functional isomerism

This type of isomerism is due to difference in the nature of functional group present in the isomers, e.g.,

(iv) Metamerism

It is due to the difference in nature of alkyl groups attached to the same functional group. This type of isomerism is shown by compounds of the same homologous series. For example,

(v) Tautomerism

Tautomerism may be defined as the phenomenon in which a single compound exists in two readily interconvertible structures that differ markedly in the relative position of at least one atomic nucleus, generally hydrogen. The two different structures are known as tautomers of each other.

Sometimes the term tautomerism is also called as desmotropism (Greek desmos-bond; tropos-turn), since the interconversion of the two forms involves a change of bonds or dynamic isomerism as the two forms are in dynamic equilibrium with each other. Other names for tautomerism are kryptomerism, allelotropism or merotropy; however, tautomerism is the most widely accepted term.

There are several types of tautomerism of which keto-enol tautomerism is the most important. In this type, one form (tautomer) exists as a ketone while the other exists as an enol. The two simplest examples are of acetone and phenol.

However, the most widely studied example of keto-enol tautomerism is that of acetoacetic ester (ethyl acetoacetate).