Kinetics and mechanism of oxidation of S-phenylthioacetic acids by Ce(IV)

Article abstract of DOI:10.1016/S0040-4020(01)85685-4

The kinetics of oxidation of several S-phenylthioacetic acids by ceric ammonium nitrate (CAN) in presence of perchloric acid has been studied spectrophotometrically in 50% (v/v) aqueous acetic acid. The order with respect to Ce(IV) is one and the order with respect to S-phenylthioacetic acid is found to be 0.8. A linear plot of k(obs)^-1 vs [substrate]^-1 with an intercept on the rate axis suggests the formation of an equilibrium complex between the reactants prior to the rate determining step. The added acrylonitrile retards the reaction rate considerably suggesting that the oxidation process may involve a free radical mechanism. Electron-releasing substituents generally accelerate the rate, while electron-withdrawing groups retard the rate. A good correlation is found to exist between log k_1.8 and Hammet σ contants. Rate = k₁[Ce(IV)(T)] / 1/K[S] + K(h)[H₂O]/K[S][H⁺] + 1 where 'S' is the substrate K₁, K(h) are equilibrium constants and k₁ is the rate constant for the rate limiting step.