Mechanism of the permanganate oxidation of unsaturated compounds. Part 7. Kinetics of the oxidation of propiolic and phenylpropiolic acids

Abstract

The permanganate oxidation of propiolic acid (PA) and phenylpropiolic acid (PPA) has been studied in aqueous perchloric acid. Short-lived intermediates of the reactions are manganese(III) and the corresponding dioxo-compounds. According to stopped-flow kinetic measurements, the reactions are first order with respect to both MnO₄^– and the acids, and the second-order rate constant is pH dependent, owing to the different reactivities of the acids and anions. The resolved rate constants at 25 °C are (dm³ mol^–1 s^–1) 183 and 43.2 for PA and its anion, and 38.8 and 11.6 for PPA and its anion, respectively. The relative reactivities of several acetylenic species are discussed in terms of a near-concerted nucleophilic attack of MnO₄^– on the triple bond.