Kinetics and Mechanism of HgCl₂ Catalysed Oxidation of Organic Sulfides by Permanganate in Non-Aqueous Medium

The rates of mercuric 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 (HgCl₂) is formed before oxidation of the sulfur-containing compounds occurs. The function of the mercuric chloride is to enhance the reactivity of permanganate. A Hammett ρ value of -0.60 ±0.04 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-mercuric 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.