Aromatic sulfonation. Part XIX: Two mechanisms of sulfonation in aqueous sulfuric acid†

The dependence of the sulfonation rates and isomer distributions of several aromatic compounds on the sulfuric acid concentration is explained in terms of two reaction mechanisms. One involves H2S2O7 as the sulfonating entity and predominates at high sulfuric acid concentrations; the other, with H3SO4+ as the sulfonating entity, predominates at lower acid concentrations. The latter mechanism is more selective than the former.