18.7:
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.
18.7:
Electrophilic Aromatic Substitution: Nitration of Benzene
The nitration of benzene is an example of an electrophilic aromatic substitution reaction. It involves the formation of a very powerful electrophile, the nitronium ion, which is linear in shape. The reaction occurs through the interaction of two strong acids, sulfuric and nitric acid.
Sulfuric acid is stronger and protonates the nitric acid on the hydroxyl group, followed by loss of water molecule, generating the nitronium ion.
The nitronium ion acts as an electrophile that reacts with benzene to form a resonance-stabilized arenium ion. The arenium ion then loses its proton to a Lewis base forming nitrobenzene.
The resulting nitro group can be reduced to a primary amino group. Reduction is achieved either by hydrogenation with a transition metal catalyst such as nickel, palladium, or platinum under mild conditions or upon treatment with metals in aqueous acid. Iron, zinc, and tin in dilute HCl are widely used reducing agents. However, ammonium ion is obtained as a salt under acidic conditions, which is then treated with a base such as sodium hydroxide to liberate the free amine.