Abstract
The kinetics of oxidation of phenol and a few ring-substituted phenols by heteropoly 11-tungstophosphovanadate(V), [PVVW11O40]4− (HPA) have been studied spectrophotometrically in aqueous acidic medium containing perchloric acid and also in acetate buffers of several pH values at 25 °C. EPR and optical studies show that HPA is reduced to the one-electron reduced heteropoly blue (HPB) [PVIVW11O40]5−. In acetate buffers, the build up and decay of the intermediate biphenoquinone show the generation of phenoxyl radical (ArO·) in the rate-determining step. At constant pH, the reaction shows simple second-order kinetics with first-order dependence of rate on both [ArOH] and [HPA]. At constant [ArOH], the rate of the reaction increases with increase in pH. The plot of apparent second-order rate constant, k 2, versus 1/[H+] is linear with finite intercept. This shows that both the undissociated phenol (ArOH) and the phenoxide ion (ArO−) are the reactive species. The ArO−–HPA reaction is the dominant pathway in acetate buffer and it proceeds through the OH− ion triggered sequential proton transfer followed by electron transfer (PT-ET) mechanism. The rate constant for ArO−–HPA reaction, calculated using Marcus theory, agrees fairly well with the experimental value. The reactivity of substituted phenoxide ions correlates with the Hammett σ+ constants, and ρ value was found to be −4.8. In acidic medium, ArOH is the reactive species. Retardation of rate for the oxidation of C6H5OD in D2O indicates breaking of the O–H bond in the rate-limiting step. The results of kinetic studies show that the HPA-ArOH reaction proceeds through a concerted proton-coupled electron transfer mechanism in which water acts as proton acceptor (separated-CPET).
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Acknowledgments
The authors P. S and K. R thank University Grants Commission, New Delhi, India, for the award of major research project and financial assistance. The authors thank Professor D. M. Stanbury, Auburn University, USA, for helpful discussions during the preparation of the manuscript. We also thank Sophisticated Analytical Instrumentation Facility, Indian Institute of Technology, Bombay, for EPR facilities and Managing Board, Virudhunagar Hindu Nadars’ Senthikumara Nadar College, Virudhunagar, for infrastructural facilities.
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Vairalakshmi, M., Raj, V., Sami, P. et al. Studies on electron transfer reactions: oxidation of phenol and ring-substituted phenols by heteropoly 11-tungstophosphovanadate(V) in aqueous acidic medium. Transition Met Chem 36, 875–882 (2011). https://doi.org/10.1007/s11243-011-9544-5
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DOI: https://doi.org/10.1007/s11243-011-9544-5