BOND-DISSOCIATION ENTHALPY OF ALPHA-TOCOPHEROL AND OTHER PHENOLIC ANTIOXIDANTS

The equilibrium constants, K-1, 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, K-1 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 K-1 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 alpha-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-methoxyphenol (77.61 kcal mol(-1)).