This article throws light upon the two main types of isomerism in molecules. The types are: 1. Structural Isomerism 2. Stereo Isomerism.
Type # 1. Structural Isomerism:
When the isomers differ only in the arrangement of atoms or groups within the molecule, without any reference to space, these are known as structural isomers and the phenomenon as structural isomerism. Thus, the structural isomers have the same molecular formula, but possess different structural formulae.
Structural isomerism may again be of several types:
(a) Chain, nuclear or skeleton isomerism:
This type of isomerism is due to the difference in the nature of the carbon chain (i.e. straight or branched) which forms the nucleus of the molecule, e.g.,
(b) Position isomerism:
It is due to the difference in the position of the side chain atom or group or an unsaturated linkage in the same carbon chain.
(c) Functional isomerism:
This type of isomerism is due to difference in the nature of functional group present in the isomers, e.g.
Tautomerism may be defined as the phenomenon in which a single compound exists in two readily inter-convertible structures that differ markedly in the relative position of at least one atomic nucleus, generally hydrogen. The two different structures are known as tautomers of each other. There are several types of tautomerism of which keto-enol tautomerism is the most important. In this type, one form (tautomer) exists as a ketone while the other exists as an enol. The two simplest examples are of acetone and phenol.
Type # 2. Stereo Isomerism:
When isomers have the same structural formula but differ in relative arrangement of atoms or groups in space within the molecule, these are known as stereoisomers and the phenomenon as stereoisomerism. The spatial arrangement of atoms or groups is also referred to as configuration of the molecule and thus we can say that the stereoisomers have the same structural formula but different configuration.
The stereoisomerism is of two types:
(a) Geometrical isomerism:
The isomers which possess the same structural formula but differ in the spatial arrangement of the groups around the double bond are known as geometrical isomers and the phenomenon is known as geometrical isomerism. This isomerism is shown by alkenes or their derivatives. In cis-isomer similar groups lie on the same side, while the similar groups when lie on opposite sides, the isomer is known as trans.
(b) Optical isomerism:
This type of isomerism arises from different arrangements of atoms or groups in space, resulting in two isomers which are mirror image of each other. Optical isomers contain an asymmetric (chiral) carbon atom (a carbon atom attached to four different atoms or groups) in their molecules. Optical isomers have similar chemical and physical properties and differ only in their behavior towards plane polarized light. The isomer which rotates the plane polarized light to left is known as laevo (1) while that rotates the plane polarized light to the right is known as dextro (d).
Enantiomers are d-and l -forms of a compound, which are non-super-imposable mirror image of each other. Racemic modification is an equimolecular mixture of d and l forms of the same compound. The process of converting d- or l- form of an optically active compound into dl- (racemic) form is known as racemisation.
Since the rotation of d is cancelled by equal but opposite rotation of l, racemic mixture (r) is always optically inactive. Separation of dl-mixture of a compound into d and l isomers is known as resolution.
This can be done by several ways, viz.: