7.6.2 Nitration

Nitration of benzene using a mixture of HNO₃ and H₂SO₄ gives the target product rapidly at lower temperatures (Scheme 1).

Scheme 1

Sulfuric acid protonates the hydroxyl group of nitric acid, allowing it to leave as water and form a nitronium ion (⁺NO₂), a powerful electrophile. The nitronium ion reacts with benzene to form a sigma complex. Loss of a proton from the sigma complex gives nitrobenzene (Scheme 2).

Scheme 2

7.6.3 Sulfonation

Aryl sulfonic acid can be easily synthesized by an electrophilic aromatic substitution using sulfur trioxide (SO₃) as the electrophile (Scheme 3).

Scheme 3

Sulfur trioxide is the anhydride of sulfuric acid. Although sulfur trioxide is uncharged, it is a strong electrophile where three sulfonyl (S=O) bonds drawing electron density away from the sulfur atom. Benzene attacks sulfur trioxide, forming a sigma complex. Loss of a proton on the tetrahedral carbon and reprotonation on oxygen gives benzenesulfonic acid (Scheme 4).

Scheme 4

Sulfonation is reversible reaction, and a sulfonic acid group can be removed from an aromatic ring by heating in dilute sulfuric acid. Excess water removes SO₃ from the equilibrium by hydrating it to sulfuric acid (Scheme 5).

Scheme 5