Studies on electron transfer reactions: oxidation of phenol and ring-substituted phenols by heteropoly 11-tungstophosphovanadate(V) in aqueous acidic medium |
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Authors: | Murugesan Vairalakshmi Veerian Raj Ponnusamy Sami Kasi Rajasekaran |
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Institution: | (1) Postgraduate Department of Chemistry, VHNSN College, Virudhunagar, 626001, India; |
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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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