Aromatic sulphonation. Part 63. Sulphonation of m-aminobenzenesulphonic acid in fuming sulphuric acid. Formation of an overcrowded tetrasulphonic acid

Abstract

m-Aminobenzenesulphonic acid upon sulphonation in fuming sulphuric acid at elevated temperatures yields a mixture of anilinium-2,5-di-, -2,4,5-tri-, and -2.3,4,6-tetra-sulphonic acids. The degree of polysubstitution increases with increasing (fuming) sulphuric acid concentration. The ratio of di-, tri-, and tetra-sulphonic acids is thermodynamically controlled. Substitution takes place ortho and/or para to the ammonium substituent. It is proposed that the reaction proceeds via a rate-limiting conversion of the ammonium group into a sulphamic acid group. The resulting sulphamic acid is then sulphonated in its N-unprotonated form. In aqueous sulphuric acid at room temperature the overcrowded aniline- 2,3,4,6-tetrasulphonate is readily desulphonated (with release of steric strain) to yield the 2.4,5-trisulphonate. From the orientation it is concluded that the reaction takes place via a species in which the amino-group is unprotonated. In highly concentrated sulphuric acid the steric compression in the tetrasulphonic acid is decreased by intramolecular anhydride formation.