Hydrogen atom transfer in the oxidation of alkylbenzenesulfonates by ferrate(VI) in aqueous solutions

Ferrate(vi), [FeO4]2-, is a very powerful oxidant that can oxidize a wide variety of inorganic and organic compounds. However, the mechanisms of many of these oxidation reactions have not been studied in detail. In this work, we have investigated the kinetics and mechanism of the oxidation of 4-alkylbenzenesulfonates by ferrate in aqueous solutions at pH 7.45-9.63 by UV/Vis spectrophotometry. The reactions are first order with respect to both [ferrate] and [4-alkylbenzenesulfonate]. The second-order rate constants for the oxidation of 4-isopropylbenzenesulfonate by ferrate at 25 °C and I = 0.3 M are found to be (5.86 ± 0.08) × 10-1 M-1 s-1 and (4.11 ± 1.50) × 10-3 M-1 s-1 for [Fe(O)3(OH)]- and [FeO4]2-, respectively, indicating that [Fe(O)3(OH)]- is two orders of magnitude more reactive than [FeO4]2- and is the predominant oxidant in neutral and slightly alkaline solutions. This is further supported by the effect of the ionic strength on the rate constant. No solvent kinetic isotope effect (KIE) was found but a moderate primary KIE = 1.6 ± 0.1 was observed in the oxidation of 4-ethylbenzenesulfonate and 4-ethylbenzenesulfonate-d9. Alkyl radicals were trapped by CBrCl3 in the oxidation of alkylarenes by ferrate. Combined with DFT calculations, a hydrogen atom transfer (HAT) mechanism was proposed for the reactions between [Fe(O)3(OH)]- and 4-alkylbenzenesulfonates.