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).
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