Sulfonation of benzene is a reaction wherein benzene is treated with fuming sulfuric acid at room temperature to produce benzenesulfonic acid. Fuming sulfuric acid is a mixture of sulfur trioxide and concentrated sulfuric acid.

The active electrophile is either neutral sulfur trioxide or protonated sulfur trioxide ion, depending on the reaction conditions. One of the mechanisms involves neutral sulfur trioxide acting as an electrophile, which reacts with the π electron cloud of nucleophilic benzene, forming a resonance stabilized intermediate. Further, the loss of a proton from the intermediate restores its aromaticity. Finally, proton transfer from the solvent leads to the formation of benzenesulfonic acid.

The other mechanism involves protonated sulfur trioxide ion as the electrophile, generated upon activation of sulfur trioxide through proton transfer from sulfuric acid.

As an electrophile, the protonated ion attacks with the π electron cloud of benzene, forming an arenium ion.

Finally, deprotonation of the arenium ion restores aromaticity, providing the benzenesulfonic acid as the final product and regenerating the acid catalyst.

Dilute sulfuric acid and steam can reverse the sulfonation reaction.

Aromatic sulfonation is widely used in the synthesis of detergents, dyes, and sulfa drugs.