Isomers and Isomerism

• In organic chemistry, we come across some compounds which have similar number of atoms but differ in physical and chemical properties and are called isomers and the phenomenon is called isomerism

• Isomers have same molecular formula but differ in arrangement of atoms within a molecule

Types of isomerism• Mainly isomerism is of two types

– 1. Structural isomerism– 2. Stereoisomerism

1-Structural isomers– Are compounds having same molecular formula but

different structural formula

–These are of further 5 types• Chain isomerism• Position isomerism• Functional isomerism• Metamerism• Tautomersim

• Stereoisomers–Are compounds having same structural

formula but different in arrangement of atoms in space

–Stereoisomers are of two types• Geometrical or Cis-Trans isomerism• Optical isomerism

Structural isomerism• Chain Isomerism

– Such compounds have the same molecular formula but differ in the order in which the carbon atoms are bonded to each other

– Examples are • n-butane and isobutane

Positional isomers• Positional isomers have same molecular formula but differ in the position of a functional group on the carbon chain. For example

2-methylbutane and 2, 2-dimethylpropane

•1-bromobutane and 2-bromobutane

Functional isomerism• Functional isomers have same molecular

formula but differ in functional groups. For example– Ethyl alcohol and Dimethyl ether

Metamerism• This type of isomerism is due to unequal

distribution of carbon atoms on either side of the functional group

• Such compounds are members of homologous series

• Example: Diethyl ether and Methyl propyl ether

Tautomerism• This is a special type of functional isomerism

in which isomers are in dynamic equilibrium with each other

• For example: ethyl acetoacetate is an equilibrium mixture of 2 forms-93% keto form and 6% enol form

Geometric isomerism

• In alkenes two carbons are linked through sigma bond (SP2 hybridized orbitals) and a pi bond due to overlap of p orbitals

• In such compounds these two carbons along with other 4 substituent remain in plane and are locked

• Because rotation will break the double bond

• This restriction of rotation is responsible for geometric isomerism in alkenes

• Consider the case of 2 butene. It exists in two special arrangements

• Cis 2 butene and Trans 2 butene

• Such compounds are referred as geometrical isomers

• Cis isomer is one in which two similar groups are on the same side of a double bond

• Trans isomer is that in which two similar groups are on the opposite side of the double bond

• Conversion of such isomers into each other is only possible if heated at high temperature or absorb light. Energy around 62 Kcal/mole is needed to break pi bond

• Trans isomers are more stable than the corresponding cis isomer

• This is because in trans bulky groups are on the opposite side, hence less repulsion or steric hindrance

• Geometrical isomers have different physical and chemical properties. These can be separated by conventional techniques like distillation and gas chromatography etc.

Geometrical isomerism in cyclic compounds

• Geometrical isomerism is possible in cyclic compounds

• There should be restriction of rotation if two carbons are linked with a cyclic structure