Mechanism of the permanganate oxidation of unsaturated compounds. Part VI. Kinetic investigation of the oxidation of methylmaleic acid, methylfumaric acid, and dimethylmaleic acid

Abstract

The kinetics of the permanganate oxidation of methylmaleic (MMA), methylfumaric (MFA), and dimethylmaleic (DMA) acids have been studied by the stopped-flow technique in aqueous solution between pH 0.0 and 5.0. Manganese(III) has been detected as a short-lived intermediate in all cases. The reactions are first order with respect to both MnO₄^– and the substrate. The second-order rate constant depends strongly on the pH. The rate of interconversions of the acid, the mono-, and the di-anion is much higher than the rate of oxidation. The rate constants are, in the above order (at 25°C, in dm³ mol^–1 s^–1) : 150, 660, and 310 (MMA); 1 240, 370, and 98 (MFA); 240, 90, and 54 (DMA). Low activation enthalpies and high negative activation entropies are observed in all cases. The rate-determining step is the cis-attack of permanganate on the double bond, resulting in the formation of a short-lived, undetectable cyclic hypomanganate (MnO₄^3–) ester. The relative reactivities reflect the influence of steric factors.