Electrophilic aromatic substitution – Sulfonation of benzene

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Benzene reacts slowly with sulfuric acid alone to give benzenesulfonic acid. The reaction starts with the protonation of one molecule of sulfuric acid by another, followed by the loss of a molecule of water. The cation produced is very reactive and combines with benzene via the slow addition to the aromatic π
system, followed by rapid loss of a proton to regenerate the aromaticity. The product contains the sulfonic acid functional group -SO₂OH.

G. A. Olah, Acc. Chem. Res., 1971, 4, 240–248.