Organic chemistry, Organic compounds, Isomerism

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