Mechanism of the permanganate oxidation of unsaturated compounds. Part IV. Kinetic investigation of the oxidation of maleic and fumaric acid

Abstract

The kinetics of the permanganate oxidation of maleic and fumaric acids to formyl(hydroxy)acetic acid have been studied by the stopped-flow technique in aqueous solution between pH 0·5 and 5·5. The reactions are first order with respect to both MnO₄^– and the substrates. The second-order rate constant depends strongly on the pH. The rate of interconversion of the acid, the mono-, and the di-anion is much higher than the rate of oxidation. The resolved rate constants are, in the above order (at 25°C): 1080, 1880, and 1000 dm³ mol^–1s^–1(maleic acid and 15,000, 3000, and 1180 dm³ mol^–1 s^–1(fumaric acid). Low activation enthalpies and high negative activation entropies are observed for both substrates. The rate-determining step is the cis-attack of permanganate on the double bond, resulting in the formation of a short-lived cyclic intermediate which contains manganese(V). The 14-fold difference between the reactivities of fumaric and maleic acids points to steric hindrance in the latter against forcing the cis-carboxy-groups towards each other.