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Peroxyl Radical Reactions in Water Solution: A Gym for Proton‐Coupled Electron‐Transfer Theories
Author(s) -
Amorati Riccardo,
Baschieri Andrea,
Morroni Gloria,
Gambino Rossana,
Valgimigli Luca
Publication year - 2016
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201504492
Subject(s) - chemistry , trolox , radical , autoxidation , electron transfer , photochemistry , phenols , radiolysis , catechol , solvent , hydroquinone , reaction rate constant , proton , proton coupled electron transfer , acetic acid , organic chemistry , antioxidant , kinetics , physics , quantum mechanics , dpph
The reactions of alkylperoxyl radicals with phenols have remained difficult to investigate in water. We describe herein a simple and reliable method based on the inhibited autoxidation of water/THF mixtures, which we calibrated against pulse radiolysis. With this method we measured the rate constants k inh for the reactions of 2‐tetrahydrofuranylperoxyl radicals with reference compounds: urate, ascorbate, ferrocenes, 2,2,5,7,8‐pentamethyl‐6‐chromanol, Trolox, 6‐hydroxy‐2,5,7,8‐tetramethylchroman‐2‐acetic acid, 2,6‐di‐ tert ‐butyl‐4‐methoxyphenol, 4‐methoxyphenol, catechol and 3,5‐di‐ tert ‐butylcatechol. The role of pH was investigated: the value of k inh for Trolox and 4‐methoxyphenol increased 11‐ and 50‐fold from pH 2.1 to 12, respectively, which indicate the occurrence of a SPLET‐like mechanism. H(D) kinetic isotope effects combined with pH and solvent effects suggest that different types of proton‐coupled electron transfer (PCET) mechanisms are involved in water: less electron‐rich phenols react at low pH by concerted electron‐proton transfer (EPT) to the peroxyl radical, whereas more electron‐rich phenols and phenoxide anions react by multi‐site EPT in which water acts as proton relay.