A kinetic and thermodynamic investigation on the antioxidant activity of 2,2′-methylenebis(6-tertbutyl-4-methylphenol) (2), 2,2′-ethylidenebis(4,6-di-tert-butylphenol) (3), and 4,4′-methylenebis(2,6di-tert-butylphenol) (4) are reported. EPR studies of the equilibration between 3 or 4 and a reference phenol, and the corresponding phenoxyl radicals, allowed us to determine the O-H bond dissociation enthalpy (BDE) of the O-H bond as 81.2 and 81.1 kcal/mol in 3 and 4, respectively. Despite this similarity, the absolute rate constants for the reaction with peroxyl radicals, determined by autoxidation studies under controlled conditions, indicate that the o-bisphenols 2 and 3 behave as excellent antioxidants while the p-bisphenol 4 is less effective by a factor of 64 and 22, respectively. FT-IR spectroscopy and product studies suggest that the very good antioxidant activity of the o-bisphenols largely arises from both the reduced steric crowding about the hydroxyl group and the stabilization of the aroxyl radical due to the formation of an intramolecular hydrogen bond between the residual OH and the oxygen radical center.
Amorati R., Lucarini M., Mugnaini V., Pedulli G.F. (2003). Antioxidant activity of o-bisphenols: The role of intramolecular hydrogen bonding. JOURNAL OF ORGANIC CHEMISTRY, 68(13), 5198-5204 [10.1021/jo0342931].
Antioxidant activity of o-bisphenols: The role of intramolecular hydrogen bonding
Amorati R.;Lucarini M.;
2003
Abstract
A kinetic and thermodynamic investigation on the antioxidant activity of 2,2′-methylenebis(6-tertbutyl-4-methylphenol) (2), 2,2′-ethylidenebis(4,6-di-tert-butylphenol) (3), and 4,4′-methylenebis(2,6di-tert-butylphenol) (4) are reported. EPR studies of the equilibration between 3 or 4 and a reference phenol, and the corresponding phenoxyl radicals, allowed us to determine the O-H bond dissociation enthalpy (BDE) of the O-H bond as 81.2 and 81.1 kcal/mol in 3 and 4, respectively. Despite this similarity, the absolute rate constants for the reaction with peroxyl radicals, determined by autoxidation studies under controlled conditions, indicate that the o-bisphenols 2 and 3 behave as excellent antioxidants while the p-bisphenol 4 is less effective by a factor of 64 and 22, respectively. FT-IR spectroscopy and product studies suggest that the very good antioxidant activity of the o-bisphenols largely arises from both the reduced steric crowding about the hydroxyl group and the stabilization of the aroxyl radical due to the formation of an intramolecular hydrogen bond between the residual OH and the oxygen radical center.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
