Aromatic sulphonation. Part 60. Sulphonation in the reactions of aromatic compounds with chlorosulphuric acid in nitromethane and in dichloromethane

Abstract

Kinetic and product studies have been carried out on the reactions of benzene, toluene, and chlorobenzene with chlorosulphuric acid in nitromethane and in dichloromethane. The initial and main products are arenesulphonic acids. The arenesulphonyl chlorides and diaryl sulphones are secondary products; with an initial chlorosulphuric acid to aromatic substrate ratio of less than 1 : 1 they are formed only in small amounts On the basis of kinetic evidence it is proposed that the sulphonation in nitromethane in the initial stages proceeds by electrophilic attack of MeNO₂,SO₃H⁺ on the aromatic substrate. From the substituent effect on the rate of substitution the reaction constant for this type of sulphonation was estimated (ρ=–10 ± 1). In dichloromethane, the sulphonating species in the reaction of chlorobenzene is ClS₂O₆H. The sulphonation of benzene, in contrast to that of chlorobenzene, exhibits an induction period. The sulphonating entity for the main reaction of benzene is ArS₂O₆H,ClSO₃H, whereas that for the initial reaction is probably the same as for chlorobenzene, viz, ClS₂O₆H. The behaviour of chlorosulphuric acid in both nitromethane and dichloromethane was studied u.v.spectroscopically with the aid of aromatic indicators (In). Evidence was obtained for ion pair formation in nitromethane according to the equation: In +2 ClSO₃H ⇌ InH⁺, ClSO₃(ClSO₃H)^–. Apparently the chlorosulphate anion requires two molecules of acid for (self-conjugative) stabilization.