Kinetics and mechanism of condensation reactions of thiobenzamides and N-substituted thioureas

The condensation reaction of thiobenzamides and of N-substituted thioureas in dimethyl sulfoxide or in methanol, in the presence of the mixture DMSO–H+–X− (X = Cl, Br) produces 1,2,4-thiadiazole derivatives. Kinetic investigation emphasizes the importance of the presence of dimethyl sulfoxide, of halide ions and of an acidic catalyst. For reactions of thiobenzamide, the bromide ion increases the reaction rate 150 times more than the chloride ion. The presence of electron-donating groups on the starting thioamidic group enhances the reactivity. Reported data indicate that the mixture DMSO–H+–X− produces a positive halogen species. The proposed mechanism involves the formation of the N-halogenated thioamides (or N-substituted thioureas) in the rate-determining step. The reactivity of thionicotinamide S-oxide is also reported and discussed. © 2002 The Royal Society of Chemistry.