Kinetics and mechanism of the oxidation of ethyl acetoacetate and diethyl malonate by dodecatungstocobaltate(III).

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The reactions of dodecatungstocobaltate(III) with ethyl acetoacetate (EAA) and diethyl malonate (DEM) have been studied in aqueous acidic medium at 60°C. A first-order dependence of rate on both the concentrations of oxidant and reductant is obtained and the reactions are catalyzed by alkali metal ions. The general rate law for the reactions is expressed as kobs=2(k0+[M+])[R] where k0 and k denote the spontaneous and catalyzed paths respectively and [R], the reductant concentration. For EAA oxidation the values of k0=(1.82±0.10)×10−4 dm3 mol−1 s−1 and kLi+=(2.23±0.09)×10−3dm6 mol−2 s−1, kNa+=(1.19±0.07)×10−3 dm6 mol−2 s−1, kk+=(1.01×0.02)×10−3 dm6 mol−2 s−1. The corresponding values for DEM are k0=(2.63±0.07)×10−5 dm3 mol−1 s−1 and kLi+=(2.11±0.07)×10−4 dm6 mol−2 s−1, kNa+=(9.13±0.25)×10−5 dm6 mol−2 s−1, kK+=(8.25±0.06)×10−5 dm6 mol−2 s−1. The less resonance stabilization of the protonated free radical species compared to the free radical obtained from the ester itself in the rate-determining step would cause a lowering in rate and this seems to explain the [H+]−1 dependence for the ester oxidations. A plausible mechanism considering an outer-sphere association between the complex and reductant has been suggested. The phenomenon of catalysis is explained by assuming an intermediate bridge formation via alkali metal ions between the reactants.
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