Kinetics and Mechanism of ZnCl2 Catalyzed Oxidation of Organic Sulfides by Permanganate in Non-Aqueous Medium

The rates of zinc chloride (Lewis acid) catalysed oxidation of thioanisole and several para-substituted thioanisoles by permanganate have been studied in anhydrous acetone solutions. The kinetic rate law obtained indicates that a complex between permanganate and the Lewis acid (ZnCl2) is formed before oxidation of the sulfur-containing compounds occurs. The function of the zinc chloride is to enhance the reactivity of permanganate. A Hammett ρ value of -1.09±0.05 is obtained for the oxidation of sulfides at 23.0 C. A negative ρ value means that the sulfur is electron deficient in the transition state. Values for the HOMO energies of sulfides and the LUMO energies for permanganate ion and the permanganate-zinc chloride complex have been calculated. The results indicate that electron donating substituents on the ring increase the rate of the reaction and electron withdrawing groups slow down the reaction.