Isomerism

By Dr. Alhaiqa FarooqiR Ph , Pharm D , MS, FCPS ,

MCPS

Compounds that have same molecular

formula but differ from each other in

physical or chemical properties are called

ISOMERS & the phenomenon is called

ISOMERISM.

Isomers have same molecular formula but

difference in their properties must be due

to different modes of combination or

arrangement of atoms within a molecule.

Two main types

STRUCTURAL ISOMERISM

STEREOISOMERISM

When the isomerism is due to difference in

arrangement of atoms within the molecule,

without any reference to space, the

phenomenon is called Structural

isomerism.

Structural isomers are the compounds that

have same molecular formula but different

structural formulas.

Five types

Chain isomerism

Position isomerism

Functional isomerism

Metamerism

Tautomerism

Same molecular formula but differ in the

order in which carbon atoms are bonded to

each other.

E.g.

Same molecular formula but differ in the

position of functional group on carbon

chain.

Same molecular formula but different

functional groups.

This type of isomerism arises due to

unequal distribution of carbon atoms on

either side of the functional group.

When two structural isomers aremutually interconvertible & exist indynamic equilibrium they are calledtautomers & the phenomenon is calledtautomerism.

Uracil

Here the keto form changes to enol form

by migration of a proton to carbonyl

oxygen, the pair of electron shifts from C-H

bond to C-C bond.

Tautomerism involves equilibrium between

structures & different tautomeric forms

have real existence.

Tautomerism involves movement of both

electrons & atoms whereas resonance

structures differ in respect of position of

electrons only.

Aldehydes, ketones & other carbonyl

compounds ( e.g. esters) exhibit this special

type of tautomerism.

It involves migration of proton from α- carbon

to carbonyl oxygen.

The tautomer containing carbonyl group is

designated as Keto form & the other

containing hydroxy group attached to doubly

bonded carbon is referred to as Enol

form(alkENe+alcohOL)

Ethyl acetoacetate