An Introductory Idea to Structural Isomerism

• Isomerism is a phenomenon of molecules that have same number of atoms of the same elements (i.e. same molecular formula) but different arrangements of atoms (i.e. different structural formula).

 

• Thus isomers differ in physical and chemical

A. CONSTITUTIONAL/ STRUCTURAL ISOMERISM:

Structural Isomerism is a phenomenon where molecules have the same molecular formula but have different atom connectivity.

For example:

It can be of SIX types.

1. CHAIN ISOMERISM: Same molecular formula but different chain length i.e. straight chain, branched chain e.g. n-pentane, isopentane and neopentane all three have molecular formula C₅H₁₂ but have different chain length.

2. POSITIONAL ISOMERISM: Same molecular formula but the positions of the same functional group are different, e.g. Pent-1-ol, Pent-2-ol and Pent-3-ol all three have molecular formula C₅H₁₂O but the position of –OH groups are different in each molecule.

3. FUNCTIONAL ISOMERISM: Same molecular formula but the functional groups are different, e.g. all the below three molecule have same molecular formula C₄H₈O, but 1^st one has keto functionality, 2^nd one has alcohol functionality and the last one has ether functionality.

4. RING-CHAIN ISOMERISM: Same molecular formula but having chain or ring structures, e.g. Propene and cyclopropane both have molecular formula C₃H₆ but propene is acyclic or chain and cyclopropane is cyclic.

5. TAUTOMERISM: is a special type of functional group isomerism in which the isomers are readily interchangeable and maintain a dynamic equilibrium with each other. A reversible reaction occurs between the two molecules having the same molecular formula.

6. METAMERISM: Same molecular formula but the number of carbon atoms on different sides of the functional group is different, e.g. in methylpropyl ether there are three C atoms on the left of ether group and one C atom on right of the ether group but in diethyl ether there are two C atoms on each side of the ether group though methylpropyl ether and diethyl ether both have same molecular formula.

In GAMSAT, the following type of vignette-based question can be asked from Tautomerism.

Tautomerism is a special type of functional isomerism in which the isomers are readily interchangeable and maintain a dynamic equilibrium with each other.

One of the main kinds of tautomerism is Keto-enol tautomerism: Keto = aldehyde or ketone group, enol = -OH group (hydroxyl group).

The form containing keto group is called keto, while that having alkene and –ol functional groups is called an enol form. Conversion of keto form into enol form is known as enolisation. It is catalysed by acid or base. The less stable form is called the labile form. The probability of an enol undergoing tautomerism decreases with the increasing side groups attached to the parent carbon atom.

Besides Keto-enol tautomerism, there are other two kinds of tautomerism:

1. Nitro-acinitro tautomerism
2. Nitroso-oximino tautomerism

To show tautomerism there should be at least one α- H with respect to the tautomeric group

Q. Keto- enol tautomerism will be shown by

Select one:

Approach: Option A is the correct answer.

Because only in option A there is α-H present with respect to the keto group. But all other options lacks α-H with respect to the keto functionality.