Aquachlororuthenium(III) complex catalysis in the oxidation of malonic and methylmalonic acids by bromate in perchloric acid medium. Study of induction period and evaluation of individual kinetic parameters

Abstract

The kinetics and mechanism of an aquachlororuthenium(III) complex catalyzed oxidation of malonic and methylmalonic acids by bromate in aqueous acetic acid medium containing perchloric acid and mercury(II) is reported here. The reaction shows an induction period (τ0), whose duration is inversely proportional to [substrate]0 and [catalyst]0, and temperature; however, independent of [bromate]0. After induction, the reaction exhibits first order each in [bromate] and [catalyst], less than unit order in [substrate] and an inverse fractional order in [HClO4]. The reaction also shows an inverse solvent isotope effect \({(k_{D_{2}O}/k_{H_{2}O} = 0.72 \pm 0.05.)}\) The observed rate law is interpreted by a mechanism involving the oxidation of RuIII to RuV by BrV (Kf), followed by complex formation (Kc) between the RuV and enol form (Ken) of the substrate in 1:1 ratio, which decomposes (kd) into products in the rate-determining step with regeneration of RuIII. The observed induction period is interpreted as the time-lag in building-up of the reactive intermediate(s). The reaction constants (Kf, Kc, Ken and kd) involved in the mechanism proposed have been evaluated and τ0 has been correlated in terms of initial concentrations of substrate and catalyst as: 1/τ0 = Q(X-1)[Sa]0(x-1) = Q′(Z-1)[Ru(III)]0(z-1).