There are two main types of isomers: structural isomers and stereoisomers. Structural isomers have the same molecular formula but different structural formulas due to differences in how atoms are arranged within the molecule. There are five types of structural isomers: chain isomers, position isomers, functional isomers, metamerism, and tautomers. Tautomers are structural isomers that can interconvert and exist in dynamic equilibrium, such as the keto and enol forms of aldehydes and ketones which involve the migration of a proton between carbon and oxygen atoms.

1. By Dr. Alhaiqa Farooqi
R Ph , Pharm D , MS, FCPS ,
MCPS

2. 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.

3. Two main types
STRUCTURAL ISOMERISM
STEREOISOMERISM

5. 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.

6. Five types
Chain isomerism
Position isomerism
Functional isomerism
Metamerism
Tautomerism

7. Same molecular formula but differ in the
order in which carbon atoms are bonded to
each other.
E.g.

8. Same molecular formula but differ in the
position of functional group on carbon
chain.

9. Same molecular formula but different
functional groups.

10. This type of isomerism arises due to
unequal distribution of carbon atoms on
either side of the functional group.

11. When two structural isomers are
mutually interconvertible & exist in
dynamic equilibrium they are called
tautomers & the phenomenon is called
tautomerism.
Uracil

12. 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.

13. 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.

14.  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)

15. Ethyl acetoacetate