Question

Describe the mechanism of Nitration of benzene.

Answer 1

It is prepared by the action of a mixture of con. HNO₃ and con. H₂SO₄ (nitrating mixture) on benzene maintaining the temperature below 333 K.

Nitration:

Sulphuric acid generates the electrophile – NO₂⁺, nitronium ion-from nitric acid. This is an example of aromatic electrophilic substitution reaction.

The generation of nitronium ion

To the nitronium ion (being an electron-deficient species) the π bond of benzene, donates a pair of electrons forming a 6-bond. A species with a + ve charge is formed as an intermediate. This is called ‘arenium ion’ and is stabilised by Resonance.

In the last step, the hydrogen atom attached to the carbon carrying the nitro group is pulled out as a proton, by the Lewis base HSO₄^–, so that stable aromatic system is formed.