Homogeneous catalysis of manganese(II) in the acid bromate oxidation of malonic acid in the presence of bromocomplexing metal ions: Unusual kinetic behaviour of malonic acid

Abstract

Manganese(II)-catalysed pure bromate (unmixed with Br₂) oxidation of malonic acid (MA) in aqueous acetic acid media containing sulphuric acid and Hg(II)/Tl(III), bromocomplexing metal ion, is free from induction period and exhibits 1.5 order in [bromate] and 0.75 order in [Mn(II)]. The reaction rate increases with increasing [H₂SO₄] and the rate constant follows the ho function. The kinetic behaviour of MA is unusual, being 0.15 order in the lower [MA] range and tending to 0.32 order at higher [MA]. Redox potential for the Mn(II)/Mn(III)-malonic acid couple is determined as 0.89 V (vs SCE) and 1.15 V (vs NHE). These findings are accounted quantitatively and the catalytic effect of Mn(II) ion is displayed by its complex forming ability. The proposed mechanism involves oxidation of the formed Mn(II)-malonic acid binary complex to an intermediate by acid bromate which subsequently decomposes to the products. Reaction constants involved in the proposed mechanism have been evaluated and discussed.