Publication
637
J. Am. Chem. Soc. 130, 15817-15819, 2008.
DOI:
110.1021/ja8064914
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Evidence for concerted proton-electron tranfer in the electrochemical oxidation of phenols with water as proton acceptor. Tri-t-butyphenol. |
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Cyrille Costentin, Cyril Louault, Marc Robert, and Jean-Michel Savéant
Contribution from the Laboratoire d'Electrochimie Moléculaire, Unité Mixte de Recherche Université - CNRS No 7591, Université Paris - Diderot, Bätiment Lavoisier, 15 rue Jean de Baïf, 75205 Paris Cedex 13, France
Proton-coupled electron transfer oxidation of phenols play a prominent role in several natural processes, and one may wonder if their high efficiency is related to the possibility that the electron and proton transfer steps are concerted. The cyclic voltammetric observation of the electrochemical oxidation and reverse reaction has allowed, with the example of 2,4,6-tri- tert -butylphenol in nonbuffered aqueous media, the clear identification of a pathway in which a phenol is directly and reversibly converted into the phenoxyl radical while the generated proton is accepted by a water molecule in a concerted manner. In very basic media, a stepwise mechanism takes place in which the phenol is deprotonated by OH - and the resulting phenoxide ion rapidly oxidized into the phenoxyl radical. As the pH decreases, this pathway progressively shuts down to the advantage of the concerted pathway. The latter assignment is confirmed by the observation of a substantial H/D kinetic isotope effect. At moderately basic pH 10.5, the contributions of the two pathways are about equal and the occurrence of the two competing routes is directly visualized in the cyclic voltammetry response. |