5.3 Carbon-Sulfur Bond Formation
5.3.1 Sulfonation
As in nitration, the sulfonation of both strongly activated and strongly deactivated nuclei can be performed at varied reaction conditions. The electrophile is SO3, which is present in concentrated H2SO4 as the result of the equilibrium. The mechanism of sulfonation is similar to that of other electrophilic substitutions.
However, the process is reversible which led to employ sulfonic acid as protecting group for regioselective substitutions. For example, sulfonation of acetanilide give p -sulfonic acid which could be nitrated at ortho position and then, the sulfonic acid group can be eliminated by treatment with dil. H2SO4 .
5.3.2 Chlorosulfonation
Aromatic compounds that are either deactivated or unstable towards acids react with chlorosulfonic acid to give arylsulfonyl chloride.
5.3.3 Sulfonylation
Benzenesulfonyl chloride reacts with benzene in the presence of Lewis acid to give diphenyl sulfone. The reaction is similar to that of Friedel-Crafts acylation and has the same limitations as chlorosulfonation.
