21.0 Steric inhibition of resonance

  General Organic Chemistry

The structural feature which influences the chemical reactions due to bulky substituents in the molecule is called steric effect. When the bulky group hinders the reaction, it is called steric hindrance which may be due to,

(a) the sheer bulk of the substituents causing the approach of the reagent more difficult or
(b) electronic factor i.e. promoting or inhibiting electron availability at a particular site

Note: Resonance ability of an atom is lost if it loses planarity with the other part of the system due to steric crowding by the bulky group in adjacent positions.

Example:

The above two compounds (A) and (B) have everything identical expect position of the two methyl group. It is expected that compound (A) should be a stronger base than compound (B) due to the closeness of two electrons donating a methyl group to $-NH_2$. The fact is opposite to this.

In compound (B), $-NO_2$ is surrounded by two bulky methyl group and they sterically repel the $-NO_2$ group. In order to minimize the steric repulsion by two adjacent methyl group, the nitro group loses planarity with the benzene ring.

So, now $-NO_2$ due to lack of planarity with the ring is not able to resonate. This is known as steric inhibition of resonance.

Thus in compound (B), $-NO_2$ is not decreasing the basic strength by resonance. In compound (A), $-NO_2$ lies in the plane of the ring. As it is in resonance with the ring, it decreases the basic strength of $-NH_2$ by resonance. Hence the compound (A) becomes a weaker base.

Example:

Compound (C) is stronger acid in spite of closeness of two electron donating methyl group to $-COOH$.