Electrophilic Aromatic Substitution: Nitration of Benzene

Nitration of benzene is achieved via treating a mixture of concentrated nitric acid and sulfuric acid to form nitrobenzene.

The reaction proceeds via an electrophilic aromatic substitution and involves the formation of a nitronium ion, a strong electrophile.

The nitronium ion formation proceeds via a proton transfer from sulfuric acid to the hydroxyl group of nitric acid, forming the conjugate acid of nitric acid.

Further, the loss of water from the conjugate acid produces the nitronium ion.

Subsequently, the nitronium ion reacts with the π electron cloud of nucleophilic benzene, forming a resonance-stabilized arenium ion.

Finally, proton transfer from the arenium ion to a Lewis base gives nitrobenzene. Interestingly, the resulting nitro group can be reduced to a primary amino group either by hydrogenation with a transition metal catalyst under mild conditions or by reduction with metal in acid.

When reduced under acidic conditions, the amine is obtained as a salt, which liberates the free amine upon treatment with a strong base.