A hydrogen-atom transfer mechanism in the oxidation of alcohols by [FeO₄]^2- in aqueous solution

The ferrate(vi) ion, [FeO₄]^2-, has attracted much interest in recent years because of its potential use as a green oxidant in organic synthesis and water treatment. Although there have been several reports on the use of ferrate(vi) for the oxidation of alcohols to the corresponding carbonyl compounds, the mechanism remains unclear. In this work, the kinetics of the oxidation of a series of alcohols with α-C-H bond dissociation energies ranging from 81 to 95 kcal mol^-1 have been studied by UV/Vis spectrophotometry. The reactions are first-order in both [FeO₄]^2- and [alcohol]. The deuterium isotope effects for the oxidation of methanol/d₄-methanol, ethanol/d₆-ethanol and benzyl alcohol/d₇-benzyl alcohol are 18.0 ± 0.1, 4.1 ± 0.1 and 11.2 ± 0.1, respectively. A linear correlation is found between the second-order rate constants and the α-C-H bond dissociation energies (BDEs) of the alcohols, consistent with a hydrogen atom transfer (HAT) mechanism. The proposed HAT mechanism is supported by DFT calculations.