Mechanistic and spectral investigation of the oxidation of 4-hydroxycoumarin by aqueous alkaline permanganate using the stopped flow technique

Abstract

Abstract The kinetics of the oxidation of 4-hydroxycoumarin (HDC) by permanganate in aqueous alkaline medium at a constant ionic strength of 0.40 mol dm−3 was studied spectrophotometrically using a rapid kinetic accessory. The reaction between permanganate and HDC in alkaline medium exhibits 6:1 stoichiometry (permanganate:HDC). The reaction is of first order in [permanganate ion] and has less than unit order in both [HDC] and [alkali]. A decrease in the dielectric constant of the medium increases the rate of reaction. The effect of added products and ionic strength of the reaction medium have been investigated. The oxidation reaction in alkaline medium has been shown to proceed via a permanganate–HDC complex, which decomposes slowly in a rate determining step followed by a fast reaction between a free radical of HDC and five molecules of permanganate species to give the products. The main products were identified by spot test, IR and NMR. A suitable mechanism is proposed. The reaction constants involved in the different steps of the mechanism are calculated. The activation parameters with respect to the slow step of the mechanism are computed and discussed and thermodynamic quantities are determined.