Sulfonation

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Sulfonation requires sulfur trioxide, which is protonated by sulfuric acid and thus "activated", becoming a stronger electrophile.

Sulfonation of benzene with fuming sulfuric acid (H₂SO₄ with 8% SO₃): benzenesulfonic acid

Sulfonation mechanism: SO₃ protonated by H₂SO₄ is attacked by benzene; the benzenonium ion loses a proton to give benzenesulfonic acid

Sulfonation is reversible. The sulfonic group can be introduced and, in a subsequent step, removed. This allows the temporary blocking of a position of the benzene ring in a synthetic sequence.

Sulfonation is reversible: with H₂SO₄/SO₃ at room temperature the SO₃H group is introduced, and with dilute H₂SO₄ at high temperature it is removed