Treating benzene with fuming sulfuric acid yields benzenesulfonic acid, which is formed by the following reaction:

The SO₃ that participates in the reaction above is formed by the reaction of sulfuric acid molecules:

The reaction of benzene and SO₃ to give benzenesulfonic acid involves the following elementary processes. SO₃ acts as an electrophile in a reaction with benzene, forming an arenium ion. A proton is then transferred from the arenium ion to HSO₄^–, forming the benzenesulfonate ion, C₆H₅SO₃^–, and H₂SO₄. Finally, C₆H₅SO₃^– is protonated by H₃O⁺ to give benzenesulfonic acid and a water molecule. Write balanced chemical equations for these elementary processes, using curved arrows to show the movement of electrons.

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+ SO3 conc. H₂SO4 SO₂H Benzenesulfonic acid