Studies on electron transfer reactions: oxidation of phenol and ring-substituted phenols by heteropoly 11-tungstophosphovanadate(V) in aqueous acidic medium

Abstract

The kinetics of oxidation of phenol and a few ring-substituted phenols by heteropoly 11-tungstophosphovanadate(V), [PV V W 11 O 40 ] 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) [PV IV W 11 O 40 ] 5-. In acetate buffers, the build up and decay of the intermediate biphenoquinone show the generation of phenoxyl radical (ArO Á ) 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 r ? constants, and q value was found to be -4.8. In acidic medium, ArOH is the reactive species. Retardation of rate for the oxidation of C 6 H 5 OD in D 2 O 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).

Electrochemical study of H3PMo12 retention on Vulcan carbon grafted with NH2 and OH groups

“…The oxidation of phenolic groups could generate phenoxyl radical, and the presence of radical coupling should not be ruled out [68]. The intrinsic oxidative nature of POM could also be responsible for some covalent interaction with the functional group present in the matrix (OH in this case).…”

“…This behavior of phosphomolybdic acid could be related to the mechanism of its incorporation into different carbon matrices. Specifically for the VC-OH matrix (Scheme 3), we could propose the interaction of phenolic OH group with water molecules by the transfer of proton to water showing an equilibrium between these species [68]. In addition, because of the presence of fully dissociated POM in water, the matrix could present protonated -OH groups, allowing electrostatic interactions between this positively charged species and anionic phosphomolybdate.…”

Chitosan-Polyoxometalate Nanocomposites: Synthesis, Characterization and Application as Antimicrobial Agents

“…At pH = 6, HQ can be oxidized (E 0 = 0.7 V vs NHE) by the POMs to its corresponding dicarbonyl product, p-benzoquinone (BQ) [74,75], whereby dioxygen is responsible for catalyst re-oxidation [76]. Because of its higher reduction potential, only Mo 10 V 2 (E 0 = 0.7 V vs NHE) converts HQ into BQ (27 % yield in 15 h), upon reduction of V(V) ions to V(IV) [77,78].…”

En Route to Intermolecular Cross-Dehydrogenative Coupling Reactions

“…Due to phenol's electron-rich property and its presence of a hydroxy group, depending on the reaction conditions, formal single-electron oxidation of phenolic arenes could be accomplished either through HAA or stepwise proton and electron transfer from the hydroxy group. 68 various nucleophilic functionalizations of phenols, and one type of valuable product is biphenol. Biphenol structures are found in various natural products and pharmaceutical molecules and are also important ligands in organic synthesis.…”