Electrophilic Aromatic Substitution: Sulfonation of Benzene

Sulfonation of benzene is a type of electrophilic aromatic substitution, where benzene reacts with fuming sulfuric acid or oleum, a mixture of sulfur trioxide and concentrated sulfuric acid, to form benzenesulfonic acid.

Depending on the reaction conditions, the reactive electrophile is either neutral sulfur trioxide or a protonated sulfur trioxide ion.

When the neutral sulfur trioxide is attacked by the π electron cloud of nucleophilic benzene, it forms a resonance-stabilized intermediate complex.

Deprotonation of the intermediate complex restores aromaticity.

Finally, proton transfer from the solvent generates benzenesulfonic acid as the product.

Alternatively, the proton transfer from sulfuric acid to sulfur trioxide forms a protonated sulfur trioxide ion.

The protonated electrophile is attacked by the π bond of benzene to form an arenium ion.

Finally, deprotonation of the arenium ion restores aromaticity, giving benzenesulfonic acid and regenerating the catalyst.

Notably, the sulfonation reaction can be reversed by passing dilute sulfuric acid in the presence of steam.