Aromatic sulphonation. Part XL. Rates of sulphonation ortho, meta, and para to a sulphonic acid group in methyl substituted benzenesulphonic acids

Abstract

Rates and isomer distributions for the sulphonation of benzenesulphonic acid and sixteen (poly)methyl derivatives in fuming sulphuric acid have been determined. With 3-methyl- and 3,5-dimethyl-benzenesulphonic acid sulphonation partly ortho to the SO₃H group was observed. The isomer distributions are dependent on the sulphuric acid concentration, but approach a constant ratio at acid concentrations below 104 and above 115% H₂SO₄. It is suggested that the substrate species undergoing sulphonation in 115% H₂SO₄ is ArSO₃H₂⁺. Positional relative rates for sulphonation in weak oleum at 25·0° have been calculated. The observed rate order is explained in terms of steric and electronic effects of the methyl substituents on the rates of formation and decomposition of the inter-mediate σ complex. The low reactivity of the higher methylated sulphonic acids is ascribed to buttressing of the substituent ortho to the reaction centre by the function ortho to that group, i.e. meta to the reaction centre.