Aromatic sulphonation. Part 59. Sulphonation of aniline in concentrated aqueous and fuming sulphuric acid

Abstract

From kinetic and product studies it has been concluded that the sulphonation of anilinium sulphate in a large excess of 99.99–102.3% H₂SO₄ at 25 °C proceeds by direct electrophilic attack of the sulphonating entity H₃S₂O₇⁺ on the aromatic ring of the anilinium cation. The main products are m-aminobenzenesulphonic and sulphanilic acid, the amount of o-aminobenzenesulphonic acid being <2%.

In an excess of concentrated aqueous sulphuric acid at elevated temperatures anilinium sulphate is converted into o-aminobenzenesulphonic and sulphanilic acid as the main products. Phenylsulphamic acid was previously thought to bean intermediate in the sulphonation of aniline in ca. 97% H₂SO₄ at 100 °C. Phenylsulphamic acid itself does not yield any m-aminobenzenesulphonic acid. Accordingly, the formation of 5–15% of m-aminobenzenesulphonic acid from anilinium sulphate in 96.8–99.9% H₂SO₄ at 60–100 °C indicates that the reaction under the given conditions proceeds at least partially by direct ring sulphonation of the anilinium ion.