Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 22, Issue 23, Pages 7924-7934Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201504492
Keywords
antioxidants; kinetics; proton inventory; radical reactions; reaction mechanisms
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Funding
- Italian MIUR (PRIN) [2010PFLRJR]
- University of Bologna (FARB project) [FFBO123154]
- COST action [CM1201]
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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 3,5di-tert-butylcatechol. The role of pH was investigated: the alue 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.
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