ISOMERISM IN ORGANIC COMPOUNDS

The term ‘isomerism’ was given by Berzelius, and its represents of existence of two or more
compounds with the same molecular formula but different structure and properties (physical,
chemical, or both). Compounds exhibiting this isomerism are called isomers. The difference in
properties of two isomers is due to difference in (bond connectivity or spatial arrangement) the
arrangement of atoms within their molecules.

TYPES OF ISOMERISM

Isomerism is broadly divided into two types.

i. Constitutional isomerism
ii. Stereoisomerism
  Constitutional isomers (Formerly structural isomers):

This type of isomers have same molecular formula but differ in their bonding sequence. Structural
or constitutional isomerism is further classified into following types.

(a) Chain or nuclear or skeletal isomerism:

These isomers differ in the way in which the carbon atoms are bonded to each other in a carbon
chain or in other words isomers have similar molecular formula but differ in the nature of the
carbon skeleton (ie. Straight or branched)

(b) Position isomerism:

If different compounds belonging to same homologous series with the same molecular formula
and carbon skeleton, but differ in the position of substituent or functional group or an unsaturated
linkage are said to exhibit position isomerism.
Example:

(c) Functional isomerism:

Different compounds having same molecular formula but different functional groups are said to
exhibit functional isomerism.

Example:
(d) Metamerism:

This type of isomerism is a special kind of structural isomerism arises due to the unequal
distribution of carbon atoms on either side of the functional group or different alkyl groups
attached to the either side of the same functional group and having same molecular formula. This
isomerism is shown by compounds having functional group such as ethers, ketones, esters and
secondary amines between two alkyl groups.

(e) Tautomerism:

It is a special type of functional isomerism in which a single compound exists in two readily inter
con-vertible structures that differ markedly in the relative position of atleast one atomic nucleus,
generally hydrogen. The two dif-ferent structures are known as tautomers.

There are several types of tautomerism and the two important types are dyad and triad systems.

(i) Dyad system:

In this example hydrogen atom oscillates between carbon & nitrogen atom

(ii) Triad system: In this system hydrogen atom oscillates between three polyvalent atoms. It
involves 1,3 migration of hydrogen atom from one polyvalent atom to other within the mole-cule.
The most important type of triad system is keto–enol tautomerism and the two groups of tautomers
are ketoform and enol-form. The polyvalent atoms involved are one oxygen and two carbon atoms.
Enolization is a process in which keto-form is converted to enol form. Both tautomeric forms are
not equally stable. The less stable form is known as labile form

Example:

Nitro-aci tautomerism.
(f) Ring chain isomerism:

In this type of isomerism, compounds having same molecular formula but differ in terms of
bonding of carbon atom to form open chain and cyclic structures for eg:

 Stereoisomerism:

The isomers which have same bond connectivity but different arrangement of groups or atoms in
space are known as stereoisomers. This branch of chemistry dealing with the study of three-
dimensional nature (spactial arrangement) of molecules is known as stereo chemistry. The
metabolic activities in living organisms, natural synthesis and drug synthesis involve various
stereoisomers.

(a) Geometrical isomerism:

Geometrical isomers are the stereoisomers which have different arrangement of groups or atoms
around a rigid frame work of double bonds. This type of isomerism occurs due to restricted rotation
of double bonds, or about single bonds in cyclic compounds.

(i)Alkenes

In alkenes, the carbon-carbon double bond is sp2 hybridized. The carbon-carbon double bond
consists of a σ bond and a π bond. The σ bond is formed by the head on overlap of sp 2 hybrid
orbitals. The π bond is formed by the side wise overlap of ‘p’ orbitals. The presence of the π bond
lock the molecule in one position. Hence, rotation around C=C bond is not possible. This
restriction of rotation about C-C double bond is responsible for geometrical isomerism in alkenes.
These two compounds are termed as geometrical isomers and are distinguished from each other
by the terms cis and trans. The cis isomer is one in which two similar groups are on the same side
of the double bond. The trans isomers is that in which the two similar groups are on the opposite
side of the double bond, hence this type of isomerism is often called cis-trans isomerism.

(ii)Oximes and azo compounds:

Restricted rotation around C=N (oximes) gives rise to geometrical isomerism in oximes. Here
‘syn’ and ‘anti’ are used instead of cis and trans respectively. In the syn isomer the H atom of a
doubly bonded carbon and –OH group of doubly bonded nitrogen lie on the same side of the double
bond, while in the anti isomer, they lie on the opposite side of the double bond. For eg:

(b) Optical Isomerism

Compounds having same physical and chemical property but differ only in the rotation of plane of
the polarized light are known as optical isomers
and the phenomenon is known as optical
isomerism.
Some organic compounds such as glucose have the ability to rotate the plane of the plane polarized
light and they are said to be optically active compounds and this property of a compound is called
optical activity. The optical isomer, which rotates the plane of the plane polarized light to the right
or in clockwise direction is said to be dextrorotary (dexter means right) denoted by the sign (+),
whereas the compound which rotates to the left or anticlockwise is said to be levorotatory (leavues
means left) denoted by sign(-). Dextrorotatory compounds are represented as ‘d’ or by sign (+)
and levorotatory compounds are represented as ‘l’ or by sign (-).

Enantiomerism and optical activity

An optically active substance may exist in two or more isomeric forms which have same physical
and chemical properties but differ in terms of direction of rotation of plane polarized light, such
optical isomers which rotate the plane of polarized light with equal angle but in opposite direction
are known as enantiomers and the phenom-enon is known as enantiomerism. Isomers which are
non-super impossible mirror im-ages of each other are called enantiomers.

Conditions for enantiomerism or optical isomerism

A carbon atom whose tetra valency is satisfied by four different substituents (atoms or groups) is
called asymmetric carbon or chiral carbon. It is indicated by an asterisk as C*. A molecule
possessing chiral carbon atom and non-super impossible to its own mirror image is said to be a
chiral molecule or asymmetric, and the property is called chirality or dissymmetry.