- Bond Dissociation Enthalpy of α-Tocopherol and Other Phenolic Antioxidants
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The equilibrium constants, K1, for the reaction between galvinoxyl and a series of phenolic antioxidants have been determined by means of EPR spectroscopy.With aroxyl radicals decaying at appreciable rates, K1 was obtained by performing kinetic analyses of the time dependence of the concentrations of the equilibrating radicals after mixing the reactants.In two cases the temperature dependence of K1 was also studied and the entropy change for the equilibration reaction was determined.Bond dissociation enthalpies, DH, of the ArO-H bond of the examined phenols were calculated by comparison with the known value of 2,4,6-tri-tert-butylphenol (81.24 kcal mol-1).A larger than expected DH value was found for probucol (81.03 kcal mol-1) and an explanation of this behavior was given in terms of the preferred conformation adopted by the para alkylthio group.The DH value of α-tocopherol (78.93 kcal mol-1) was found to be very close to that of the phenolic precursor of galvinoxyl (78.80 kcal mol-1) and somewhat larger than that of 2,6-di-tert-butyl-4-methoxyphenyl (77.61 kcal mol-1).
- Lucarini, Marco,Pedulli, Gian Franco,Cipollone, Marta
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p. 5063 - 5070
(2007/10/02)
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