The action of sulphur monochloride on aromatic primary amines: the Herz reaction

1,3,2-Benzothiazathiolium chlorides are obtained by the action of sulphur monochloride on a primary aromatic amine, its salts, or N-acyl derivatives. Four reactions are involved, (i) the formation of the heterocyclic ring, (ii) the chlorination of the aromatic ring, (iii) replacement of electron-attracting groups in the ortho- and para-positions by chlorine, and (iv) the oxidation of the sulphur atom in position 2. A likely sequence of reactions is considered, and experimental evidence is given in support of each stage.