Publication 653

PNAS 106, 18143–18148, 2009. DOI: 10.1073/pnas.0910065106
The electrochemical approach to concerted proton—electron transfers in the oxidation of phenols in water

Laboratoire d'Electrochimie Moléculaire, Université Paris Diderot, UMR CNRS 7591, 15, rue Jean-Antoine de Baïf, 75205 Paris Cedex 13, France

Establishing mechanisms and intrinsic reactivity in the oxidation of phenol with water as the proton acceptor is a fundamental task relevant to many reactions occurring in natural systems. Thanks to the easy measure of the reaction kinetics by the current and the setting of the driving force by the electrode potential, the electrochemical approach is particularly suited to this endeavor. Despite challenging difficulties related to self-inhibition blocking the electrode surface, experimental conditions were established that allowed a reliable analysis of the thermodynamics and mechanisms of the proton-coupled electron-transfer oxidation of phenol to be carried out by means of cyclic voltammetry. The thermodynamic characterization was conducted in buffer media whereas the mechanisms were revealed in unbuffered water. Unambiguous evidence of a concerted proton–electron transfer mechanism, with water as proton acceptor, was thus gathered by simulation of the experimental data with appropriately derived theoretical relationships, leading to the determination of a remarkably large intrinsic rate constant. The same strategy also allowed the quantitative analysis of the competition between the concerted proton–electron transfer pathway and an OH^--triggered stepwise pathway (proton transfer followed by electron transfer) at high pHs. Investigation of the passage between unbuffered and buffered media with the example of the PO₄H₂^-/PO₄H^2- couple revealed the prevalence of a mechanism involving a proton transfer preceding an electron transfer over a PO₄H^2--triggered concerted process.