Conversion of o-nitrothiophenols into o-aminobenzenesulphonic acids

The mechanism of conversion of o-nitrothiophenols into o-aminobenzenesulphonic acids in aqueous dioxane has been investigated with particular reference to evidence for the rate-determining formation of a cyclic intermediate. This intermediate is susceptible to interception by both external and internal nucleophilic functions, and the sulphonic acid is shown by isotope labelling experiments to take two oxygen atoms from H₂¹⁸O in the reaction mixture, under conditions in which neither starting materials nor products exchange with H₂¹⁸O. A tentative pathway is advanced.