Nature of the transient species formed in the pulse radiolysis of 4‐hydroxybenzyl alcohol in aqueous solutions: observation of equilibrium in the reaction of OH‐adducts with HPO ions

Abstract

AbstractReactions of .OH/O.− radicals, H‐atoms as well as specific oxidants such as N and Cl radicals with 4‐hydroxybenzyl alcohol (4‐HBA) in aqueous solutions have been investigated at various pH values using the pulse radiolysis technique. At pH 6.8, .OH radicals were found to react with 4‐HBA (k = 6 × 109 dm3 mol−1 s−1) mainly by contributing to the phenyl moiety and to a minor extent by H‐abstraction from the CH2OH group. .OH radical adduct species of 4‐HBA, i.e., .OH‐(4‐HBA) formed in the addition reaction were found to undergo dehydration to give phenoxyl radicals of 4‐HBA. Decay rate of the adduct species was found to vary with pH. At pH 6.8, decay was very much dependent on phosphate buffer ion concentrations. Formation rate of phenoxyl radicals was found to increase with phosphate buffer ion concentration and reached a plateau value of 1.6 × 105 s−1 at a concentration of 0.04 mol dm−3 of each buffering ion. It was also seen that .OH‐(4‐HBA) adduct species react with HPO ions with a rate constant of 3.7 × 107 dm3 mol−1 s−1 and there was no such reaction with H2PO ions. However, the rate of reaction of .OH‐(4‐HBA) adduct species with HPO ions decreased on adding KH2PO4 to the solution containing a fixed concentration of Na2HPO4 which indicated an equilibrium in the H+ removal from .OH‐(4‐HBA) adduct species in the presence of phosphate ions. In the acidic region, the .OH‐(4‐HBA) adduct species were found to react with H+ ions with a rate constant of 2.5 × 107 dm3 mol−1 s−1. At pH 1, in the reaction of .OH radicals with 4‐HBA (k = 8.8 × 109 dm3 mol−1 s−1), the spectrum of the transient species formed was similar to that of phenoxyl radicals formed in the reaction of Cl radicals with 4‐HBA at pH 1 (k = 2.3 × 108 dm3 mol−1 s−1) showing that .OH radicals quantitatively bring about one electron oxidation of 4‐HBA. Reaction of .OH/O.− radicals with 4‐HBA by H‐abstraction mechanism at neutral and alkaline pH values gave reducing radicals and the proportion of the same was determined by following the extent of electron transfer to methyl viologen. H‐atom abstraction is the major pathway in the reaction of O.− radicals with 4‐HBA compared to the reaction of .OH radicals with 4‐HBA. At pH 1, transient species formed in the reactions of H‐atoms with 4‐HBA (k = 2.1 × 109 dm3 mol−1 s−1) were found to transfer electrons to methyl viologen quantitatively. Copyright © 2009 John Wiley & Sons, Ltd.