Aromatic sulphonation. Part 56. The rearrangement of phenylsulphamic acid to aniliniumsulphonic acids in concentrated sulphuric acid: evidence for an intermolecular reaction pathway

Abstract

Phenylsulphamic acid in a large excess of 96–100% H₂SO₄ at 25° yields both orthanilic and sulphanilic acid intermolecularly by initial C-sulphonation and subsequent N-desulphonation. This conclusion is based on (i) the u.v. and n.m.r. observation in ca. 100% H₂SO₄ of o- and p-sulphophenylsulphamic acid as reaction intermediates and (ii) the dependence of both the rate of sulphonation of phenylsulphamic acid and of the isomer distribution of the aniliniumsulphonic acid products on the sulphuric acid concentration. The reacting and the major substrates were identified by kinetic, product, and protonation studies to be PhNHSO₃H and PhNH₂SO₃^– respectively. The ionization PhNH₃SO₃^–⇌ PhNHSO₃^–+ H⁺ has an apparent pK_a value of –2.03, based on the H_o acidity function.