13.3 Isomerism in Organic Compounds

Structural isomerism

The compounds with the same molecular formula but different structure are termed as structural isomers, and the phenomenon is called structural isomerism. There are 5 types of structural isomers:

1. Chain or nuclear, or skeletal isomerism
2. Position isomerism
3. Functional isomerism
4. Tautomerism
5. Metamerism

1. Chain or nuclear, or skeletal isomerism

If two or more compounds have the same molecular formula but have different carbon chains or skeletons, then they are called chain isomers and the phenomenon is called structural isomerism.

For example: C₄H₁₀. The two isomers of C₄H₁₀ differ only in the length of the carbon chain.

Similarly, butyl alcohol and isobutyl alcohol are two chain isomers.

2. Position isomerism

The compounds having the same molecular formula but different positions of functional groups are termed as position isomers, and the phenomenon is known as positional isomerism.

For example, the two isomers of the compound having molecular formula C₃H₈O are as follows:

3. Functional isomerism

The compounds with the same molecular formula but different functional groups are termed as functional isomers, and the phenomenon is known as functional isomerism.

For example,

1. Aldehydes and ketones, e.g. CH₃CH₂CHO and CH₃COCH₃
2. Alcohol and ether, e.g. CH₃CH₂OH and CH₃OCH₃

3. Carboxylic acid and esters, e.g.

4. Tautomerism

It is a special type in which the isomers are interconvertible due to the change in the position of the hydrogen atom within the molecule. For example:

5. Metamerism

The compounds of the same family having the same molecular formula, which differ in the arrangement of carbon atoms on either side of the functional group, are termed metamers, and the phenomenon is called metamerism. Ketones, Ethers, Esters, etc., can exhibit metamerism.

Compounds having same molecular formula and structural formula but different spatial arrangements of the atoms or groups of atoms in a molecule are called stereo isomers. The phenomenon of existence of such compounds is called stereoisomerism.

Stereoisomerism can be further subdivided into geometrical and optical isomerism.

Geometrical Isomerism (cis, trans isomerism)

When the same molecular formula represents two compounds which differ in the spatial arrangement of atoms or groups around carbon-carbon double bond, these are geometrical isomers. Such isomerism arises due to the inability to rotate around a double bond.

e.g, But-2-ene has two geometrical isomers, i.e, cis-but-2-ene and trans-but-2-ene.

1,2-dichloroethene can be taken as another example of such isomerism.

CHCl=CHCl

Optical isomerism

• Optical isomerism can be defined as the phenomenon by which isomers are obtained due to different arrangement of atoms or groups in the three-dimensional space.
• Such compounds resemble in their chemical and most physical properties but differ in optical rotation.
• Optical rotation is a phenomenon where a plane of the plane polarized light is rotated either in a clockwise or anticlockwise direction.
• The optical rotation is measured by an instrument called a polarimeter.
• The optical isomers which are mirror images to each other are called enantiomers.
• The enantiomer which rotates the plane of the polarized light to clockwise direction (right) and the anti-clockwise direction are called dextrorotatory and laevorotatory, respectively and designated as + or d and – or l, respectively.

For a compound to exhibit enantiomerism there should be presence of chiral carbon atom. A carbon atom that is bonded with 4 different groups is called a chiral carbon. In above four examples it can be seen that the central atom is chiral.