Isomerism

What is isomerism?

The word isomers is derived from Greek: the prefix iso- means “same,” while -meros means “parts or units. Isomers are molecules with the same molecular formulae but different structures due to the different arrangements of atoms in space.  The phenomenon of the possibility of isomers is called isomerism.

There are two main types of isomerism:

1. Structural Isomerism
2. Stereoisomerism

Structural Isomerism

When isomerism is due to a difference in the arrangement of atoms within the molecule, without any kind of reference to the spatial arrangement, the phenomenon is called Structural Isomerism. Structural isomers are compounds that have the same molecular formula but different structural formulas. Structural isomerism can be exhibited in five different ways. These are :

1. Chain isomerism
2. Position isomerism
3. Functional group isomerism
4. Metamerism
5. Tautomerism

Chain isomerism

Chain isomers have the same molecular formula but differ in the nature of the carbon chain order. This type of isomerism arises due to the difference in the order in which the carbon atoms are bonded to each other. For example, for butane (C₄H₁₀), the following arrangements are possible.

Position isomerism

In this type of isomerism, isomers have the same molecular formula but are different in the position of a functional group on the carbon chain. The arrangement of carbon atoms remains the same. For example, Bromopropane can have two positional isomers given below.

Functional group isomerism

In functional group isomerism, the compounds have the same molecular formula but different functional groups. For example, C₂H₆O has the same molecular formula but different functional groups (ether and alcohol) due to different arrangements of atoms.

Metamerism

In this type of isomerism, compounds have the same molecular formula but different alkyl groups on either side of functional groups due to the unequal distribution of carbon atoms. Metamers are isomers having the same molecular formula but different alkyl groups on either side of functional groups. For example, 3-pentanone and 2-pentanone are metamers.

For an ether compound having the molecular formula C₄H₁₀O, both diethyl ether and methyl n-propyl ether are possible metamers.
               

Stereoisomerism