Mechanism of the permanganate oxidation of unsaturated compounds. Part V. Intermediates and kinetics of the oxidation of substituted propynes

Abstract

The permanganate oxidation of propargyl alcohol (PA), but-2-yne-1,4-diol (BD), and propargyl chloride (PC) and bromide (PB) has been studied in aqueous perchloric acid. The stoicheiometry for PA and BD depends on the substrate: MnO₄^– mole ratio, three limiting cases being observed. Manganese(III) has been detected as a shortlived intermediate by the stopped-flow technique. The organic substrates yield αβ-dioxo-intermediates, which are oxidised by manganese(III) to the corresponding acids. In the presence of pyrophosphate, the dioxo-compounds are stable products. Stopped-flow kinetic measurements under conditions eliminating interference by the Guyard reaction show that the reactions are first order with respect to both MnO₄^– and the substrates, and independent of pH between 0·98 and 4·9. The second-order rate constants at 25°C are: 10·0 (PA), 11·5 (PC and PB), and 12·5 (BD) dm³ mol^–1 s^–1, with the activation parameters ΔH^‡= 26·4 (PA) and 22·0 (BD) kJ mol^–1; ΔS^‡=–138 (PA) and –150 (BD) J mol^–1 K^–1. The rate-determining step is assumed to be concerted attack of MnO₄^– on the triple bond, resulting in the formation of a short-lived cyclic intermediate containing manganese(V).