Aromatic sulphonation. Part 58. Protonation and sulphonation of methanesulphonanilide in aqueous sulphuric acid

Abstract

Kinetic and protonation studies in aqueous sulphuric acid at 25 °C show that sulphonation of methanesulphonanilide is preceded by a protonation equilibrium step. Half-protonation of the anilide occurs at 84% H₂SO₄. The substrate entity undergoing sulphonation is the unprotonated methanesulphonanilide. At relatively low sulphuric acid concentrations it is sulphonated by the entity H₃SO₄⁺ and above 90% H₂SO₄ by the entity H₂S₂O₇. The decrease in the ortho : para sulphonation ratio with increasing sulphuric acid concentration in the region 80–99.8% H₂SO₄ is ascribed to a change in substrate solvation.