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Kinetic and mechanistic studies of cerium(III)-catalysed oxidation of 4-oxo acids by bromate in acid medium: nonlinear Hammett plots

The kinetic and mechanistic aspects of cerium(III)catalysed oxidation of 4-oxo acids by bromate (uncontaminated with bromine) in an acid medium have been studied. The reaction exhibited first-order each in [bromate] and [acid], fractional order each in [oxo acid] and [cerium(III)], and also showed solvent isotope effect of 0.55 (k(H2O)/k (D2O)). The reaction did not induce polymerization and the induction period was not observed. The influence of ionic strength on the rate was negligible, while increase in reaction rate was found by lowering the dielectric constant of the medium. The reaction rate was affected by the presence of substituents in the reaction centre. Increase in the reaction rate by electron releasing substituents and decrease in the rate by the presence of electron-withdrawing substituents were observed. The Hammett plots were characterized by smooth curvature; however, linearity was observed using exalted σ values. A negative value for the reaction constant was observed. The isokinetic relationship was evaluated from the Arrhenius and Hammett plots. The formation of a ternary complex between oxidant, substrate and catalyst has been proposed in the mechanism. Based on the proposed mechanism, the rate law has been derived. The formation of a π-complex between Ce(III) and -ene of the enol form of oxo acid has been proposed. This reaction finds application in the synthesis of substituted benzoic acids and involves the phenomena of intramolecular catalysis and neighbouring group participation.