Isomers and IsomerismIn 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 isomerismIsomers have same molecular formula but differ in arrangement of atoms within a molecule

Types of isomerismMainly isomerism is of two types1. Structural isomerism2. Stereoisomerism1-Structural isomersAre compounds having same molecular formula but different structural formula

These are of further 5 typesChain isomerismPosition isomerismFunctional isomerismMetamerismTautomersim

StereoisomersAre compounds having same structural formula but different in arrangement of atoms in spaceStereoisomers are of two typesGeometrical or Cis-Trans isomerismOptical isomerism

Structural isomerismChain IsomerismSuch compounds have the same molecular formula but differ in the order in which the carbon atoms are bonded to each otherExamples 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 isomerismFunctional isomers have same molecular formula but differ in functional groups. For exampleEthyl alcohol and Dimethyl ether

MetamerismThis type of isomerism is due to unequal distribution of carbon atoms on either side of the functional groupSuch compounds are members of homologous seriesExample: Diethyl ether and Methyl propyl ether

TautomerismThis is a special type of functional isomerism in which isomers are in dynamic equilibrium with each otherFor example: ethyl acetoacetate is an equilibrium mixture of 2 forms-93% keto form and 6% enol form

Geometric isomerismIn alkenes two carbons are linked through sigma bond (SP2 hybridized orbitals) and a pi bond due to overlap of p orbitalsIn such compounds these two carbons along with other 4 substituent remain in plane and are lockedBecause 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 arrangementsCis 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 bondTrans isomer is that in which two similar groups are on the opposite side of the double bondConversion 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 isomerThis is because in trans bulky groups are on the opposite side, hence less repulsion or steric hindranceGeometrical isomers have different physical and chemical properties. These can be separated by conventional techniques like distillation and gas chromatography etc.

Geometrical isomerism in cyclic compoundsGeometrical isomerism is possible in cyclic compoundsThere should be restriction of rotation if two carbons are linked with a cyclic structure