- 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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- Temperature and Pressure Effects on the Reversible Reaction of a Parent Phenol (GH) Corresponding to Galvinoxyl with Aliphatic Amines
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The effects of temperature and pressure on the reversible reaction between GH and aliphatic amines in acetonitrile have been studied spectrophotometrically.The formation of the free ions rather than that of ion pairs has been confirmed by the modified Benesi-Hildebrand (B-H) plot.From the plot the equilibrium constants were estimated.From the temperature dependence of the equilibrium constant (K), the heat and entropy of reactions were estimated to be: (ΔH0)av=-44.5 kJ mol-1, and (ΔS0)av=-193 J K-1 mol-1.From the pressure dependence of K the reaction volume (ΔV0)av was estimated to be -40 dm3 mol-1.When unbranched primary amines were used, a side reaction took place for which an explanation has been given.
- Nishimura, Norio,Iga, Siroh,Satoh, Masahiro,Yamamoto, Shunzo
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p. 2437 - 2441
(2007/10/02)
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- SPECTRA AND STRUCTURE OF SCREENED PHENOLS, THEIR RADICALS AND IONS
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The vibrational spectra of screened phenols, stable phenoxyl radicals and ions obtained by addition (loss) of the proton to the phenol molecule, and one-electron reduction (oxidation) of the phenoxyl radical, were studied and interpreted.The proton phototransfer between phenol molecules of the same type in frozen aliphatic hydrocarbon solutions was revealed by luminescence.
- Kotorlenko, L. A.,Aleksandrova, V. S.,Yankovich, V. N.
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p. 501 - 504
(2007/10/02)
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