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
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 aSO3 it is concluded that the aromatic species undergoing sulphation is again –O3SC6H4OH.