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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 4 2− ions
Author(s) -
Dhiman Surajdevprakash B.,
Naik Devidas B.
Publication year - 2010
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.1583
Subject(s) - chemistry , radiolysis , radical , adduct , aqueous solution , reaction rate constant , moiety , ion , alcohol , photochemistry , inorganic chemistry , medicinal chemistry , kinetics , organic chemistry , physics , quantum mechanics
Reactions of . OH/O .− radicals, H‐atoms as well as specific oxidants such as N 3 .and Cl 2 .−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 × 10 9 dm 3 mol −1 s −1 ) mainly by contributing to the phenyl moiety and to a minor extent by H‐abstraction from the CH 2 OH 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 × 10 5 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 4 2−ions with a rate constant of 3.7 × 10 7 dm 3 mol −1 s −1 and there was no such reaction with H 2 PO 4 −ions. However, the rate of reaction of . OH‐(4‐HBA) adduct species with HPO 4 2−ions decreased on adding KH 2 PO 4 to the solution containing a fixed concentration of Na 2 HPO 4 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 × 10 7 dm 3 mol −1 s −1 . At pH 1, in the reaction of . OH radicals with 4‐HBA ( k = 8.8 × 10 9 dm 3 mol −1 s −1 ), the spectrum of the transient species formed was similar to that of phenoxyl radicals formed in the reaction of Cl 2 .−radicals with 4‐HBA at pH 1 ( k = 2.3 × 10 8 dm 3 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 × 10 9 dm 3 mol −1 s −1 ) were found to transfer electrons to methyl viologen quantitatively. Copyright © 2009 John Wiley & Sons, Ltd.