Presented by Niamat ullah

Post Rn BScN

Berzelius coined the term isomerism (Greek: isos =

equal; meros = part) to describe the relationship

between two clearly different compounds having the

same elemental composition. Such pairs of

compounds differ in their physical and chemical

properties and are called isomers. For example,

Ethyl alcohol (CH3CH2OH) and

Dimethyl ether (CH3OCH3) are isomers.

3

Stereochemistry

• The two major classes of isomers are constitutional isomers and

stereoisomers.

Constitutional/structural isomers have different IUPAC names, the

same or different functional groups, different physical properties and

different chemical properties.

Stereoisomers differ only in the way the atoms are oriented in space.

They have identical IUPAC names (except for a prefix like cis or

trans). They always have the same functional group(s).

• A particular three-dimensional arrangement is called a configuration.

Stereoisomers differ in configuration.

The Two Major Classes of Isomers

Fig 3.1 Different types of isomerism of organic compounds

ISOMERISM

Stereoisomerism

Structural or

Constitutional

Isomerism

Cinfigurational

Stereoisomerism

Conformational

Stereoisomerism

Geometric

Isomerism

Optical

IsomerismRotational

Isomerism

Amine

Inversion

Chain or Skeletal or

Nuclear Isomerism

Position

Isomerism

Functional

IsomerismMetamerism Tautomerism

Ring-chain

Isomerism

These differ from each other in the way their

atoms are connected, i.e., in their

structures. It’s six types signifying the

main difference in the structural features

of the isomers are:

I. Chain/Skeletal/Nuclear Isomerism

II. Position Isomerism

III. Functional Isomerism

IV. Metamerism

V. Tautomerism

VI. Ring Chain Isomerism

These have same molecular formula but different

arrangement of carbon chain within the molecule.

C4H10

n-Butane

(straight chain)Same molecular

formula 2-Methylpropane (Isobutane)

(Branched chain)

H3C—CH2—CH2—CH3 H3C—CH—CH3

CH3

CH3CH2CH2CH2CH3

C5H12 H3C CH CH2 CH3

CH3

n-Pentane

Same

molecular formula 2-Methylbutane

(Iso -pentane)2, 2-Dimethylpropane

(Neo-pentane)

CH3

H3C C CH3

CH3

These have same carbon skeleton but differ in

the position of attached atoms or groups or

in position of multiple (double or triple)

bonds.

Propan-1-ol (The OH group at

CH3CH2CH2OH

Propan-2-ol (The OH group at C1) C2)

OH

CH3—CH—CH3

Cl

Cl

Cl

Cl

Cl

Cl

o-Dichlorobenzene m-Dichlorobenzene p-Dichlorobenzene

These have same molecular formula but different

functional groups.

CH3CH2OH CH3 O CH3

Ethanol Dimethyl ether

CH3 C OH

O

Ethanoic acid

O

H C OCH3

Methyl methanoate

Propanal (Propionaldehyde)

CH3CH2 C H

O

Propanone (Acetone)

O

CH3 C CH3and

These have different number of carbon atoms (or alkyl

groups) on either side of a bifunctional group (i.e., -O- ,

-S-, -NH-, -CO- etc.). Metamerism is shown by members

of the same family, i.e., same functional groups.

CH3CH2CH2—O—CH3CH3CH2—O—CH2CH3 or

Ethoxy ethane

(Diethyl ether) 1-Methoxy propane

(Methyl n-propyl ether)

2-Methoxypropane

(Isopropyl methyl ether)

is a metamer of

CH3

CH3CH2CH—O—CH3

is a metamer of or

Pentan-3-one

(Diethyl ketone) Pentan-2-one

(Methyl n-propyl ketone)

3-Methylbutan-2-one

(Isopropyl methyl ketone)

CH3CH2—C—CH 2CH3 CH3CH2CH2—C—CH 3

OO

CH3—C—CHCH 3

O CH3

Structural or constitutional isomers existing in easy

and rapid equilibrium by migration of an atom or

group are tautomers (keto-enol tautomerism).

Vinyl alcohol (enol form)

(Negligible amount)

CH2 C—H

OH

Acetaldehyde (keto form)

O

CH3—C—H

Prop-1-ene-2-ol (enol form)

(Negligible amount)

CH2 C—CH3

OH

Acetone (keto form)

O

CH3—C—CH3

Open chain and cyclic compounds having

the same molecular formula are called ring -

chain isomers

CH3CH CH2Propene

and

Cyclopropane

Cyclopropene

and

Propyne

CH3 C CH

Isomers which have the same molecular

formula and same structural formula but

differ in the manner their atoms or groups

are arranged in the space are called

stereoisomers. It is of two types:

I. Configurational Isomerism

II. Conformational Isomerism

The stereoisomers which cannot be

interconverted unless a covalent bond is

broken are called configurationally isomers.

These isomers can be separated under

normal conditions.

The configurational isomerism is again of two

types:

a) Optical Isomerism or Enantiomerism

b) Geometrical Isomerism

The stereoisomers which are related to each

other as an object and its

non-superimposable mirror image are called

optical isomers or enantiomers (Greek:

enantion means opposite).

The optical isomers can also rotate the plane

of polarised light to an equal degree but in

opposite direction.

The property of rotating plane of polarised

light is known as optical activity.

The optical isomers have similar physical and

chemical properties.

Molecular formula C3H6O3 represents two

enantiomeric lactic acids as shown below:

H

CH3

HO

COOH

( +) - Lactic acid

(Rotates the plane of polarized

light towards right hand side i.e.

clockwise)

COOH

OH

CH3

H

( -) - Lactic acid

(Rotates the plane of polarized

light towards left hand side i.e.

anticlockwise)

Mirror

Geometric isomers are the stereoisomers

which differ in their spatial geometry due to

restricted rotation across a double bond.

These isomers are also called as cis-trans

isomers. For example, molecular formula

C2H2Cl2 corresponds to two geometric

isomers as follows:

C C

H

ClCl

H H

Cl

Cl

H

CC

cis-1,2-Dichloroethene trans-1,2-Dichloroethene

The stereoisomers which can beinterconverted rapidly at room temperaturewithout breaking a covalent bond are calledconformational isomers or conformers.

Because such isomers can be readilyinterconverted, they cannot be separatedunder normal conditions.

Two types of conformational isomers are:

a) Conformational isomers resulting fromrotation about single bond

b) Conformational isomers arising fromamine inversion

Because the single bond in a molecule

rotates continuously, the compounds

containing single bonds have many

interconvertible conformational isomers.e.g,

'boat' and 'chair' forms of cyclohexane.

H

H

H

H

H

H

H

H

H

H

H

H

Cyclohexane

(Chair form) Cyclohexane

(Boat form)

H

H

H

H

H

HH

H

H

H

H

H

Nitrogen atom of amines has a pair of non-

bonding electrons which allow the molecule

to turn "inside out" rapidly at room

temperature. This is called amine inversion or

Walden inversion.

Transition state

R3

N

R2

R1

R1R2

N

R3

R1

R2

N

R3