Reaction mechanism of desulphonation of benzene ring using dil. #"HCl"# and, #"H"_2"SO"_4//150^@c# "?"

The sulfonation of benzene is a reversible reaction.

Thus, we can remove a sulfonic acid group by heating the compound with an acid.

The reaction with steam alone is often sufficient for desulfonation.

The mechanism of desulfonation

Step 1. Attack by an electrophile

A hydronium ion protonates the aromatic ring and generates a resonance-stabilized carbocation intermediate.

Passo 2. Desulfonation

A base deprotonates the sulfonic acid, and the sulfur trioxide leaves, regenerating the aromatic ring.