Aromatic sulphonation. Part 94. Sulphonation and sulphation of the three 2- and 4-hydroxy- and 2- and 4-methoxy-benzenesulphonic acids in concentrated sulphuric acid

Abstract

The sulphonation of 2-methoxy-(1) and 4-methoxy-benzene sulphonate (2) and 1-(3), 3-(4), and 4-hydroxybenzenesulphonate (5) in 86.9–98.5% H₂SO₄ at 25 °C has been studied. Substrates (1)–(3) and (5) all yield the corresponding 2,4-disulphonic acid as the exclusive product, whereas (4) yields a mixture of 2-hydroxybenzene-1,4- and 4-hydroxybenzene-1,2-disulphonic acid. 3- and 4-Hydroxybenzenesulphonic acids in sulphuric acid (>87% H₂SO₄) are present in part as their hydrogen sulphates and the sulphuric acid concentration of 50% sulphation is 97.5 and 100.5 ± 0.2% H₂SO₄. The sulphonation is first order in the aromatic substrate. From the rate dependence on the sulphuric acid concentration it is concluded that the substrate species undergoing ring sulphonation is the ^–O₃SC₆H₄OR (R = H; Me) species with H₂S₂O₇ as the sulphonating entity. From the observed proportionality between [^–O₃SC₆H₄OSO₃H]/[^–O₃SC₆H₄OH] and a_SO3 it is concluded that the aromatic species undergoing sulphation is again ^–O₃SC₆H₄OH.