Oxidation of 4-methylanisole by aqueous cerium(IV) in a two-phase immiscible liquid/liquid system: Interfacial versus homogeneous control

The kinetics of the oxidation of 4-methylanisole (MA) by Ce(IV) have been measured in both two-phase (MA/aqueous sulfuric acid) and single-phase (aqueous sulfuric acid) arrangements by using microelectrochemical measurements at expanding droplets (MEMED) and UV-visible spectrophotometry, respectively. In the two-phase arrangement, the electron transfer (ET) reaction appeared to occur interfacially, rather than by dissolution of MA followed by reaction in solution as previously considered. The effective first-order rate constant for the consumption of Ce(IV) at the interface is 2.0 x 10(-4) cm s(-1). The homogeneous reaction is first-order in both Ce(IV) and MA, with a rate constant for the loss of Ce(IV) of 1.0 mol(-1) dm(3) s(-1). The lifetime of the 4-methylanisole radical cation in aqueous sulfuric acid, formed in the first step of the oxidation process, has been measured independently as similar to10 ins using steady-state ultramicroelectrode voltammetry. The factors leading to the different energetics of the one phase and interfacial reaction are discussed, and the implications of the observed kinetics on scale-up are highlighted.