CHAIN-BREAKING PHENOLIC ANTIOXIDANTS - STERIC AND ELECTRONIC EFFECTS IN POLYALKYLCHROMANOLS, TOCOPHEROL ANALOGS, HYDROQUINONES, AND SUPERIOR ANTIOXIDANTS OF THE POLYALKYLBENZOCHROMANOL AND NAPHTHOFURAN CLASS

Antioxidant activities of four classes of phenols were measured by the inhibition of thermally initiated autoxidation of styrene at 30-degrees-C. Class I, 6-hydroxypolyalkylchromans (model compounds) showed the same inhibition rate constants (k(inh) as their alpha-tocopherol analogs, in the range 1.5 X 10(6) M-1 s-1 to 3.8 x 10(6) M-1 s-1 for two methyl or ethyl groups ortho to the phenolic hydroxyl group. Bulky ortho groups reduce the k(inh), two isopropyl groups by 3-fold and 5-isopropyl-6-tert-butyl by 5-fold, compared to that of alpha-tocopherol, due to steric hindrance to the approach of peroxyl radicals. Class II, the a-naphthol derivatives, 6-hydroxy-2,5-dimethyl-2-phytyl-7,8-benzochroman (9a) and the corresponding chromene (9b), exhibit higher k(inh) values, 4 times that of alpha-tocopherol. A new synthetic antioxidant, 2,3-dihydro-5-hydroxy-2,2,4-trimethylnaphtho[1,2-b]furan, exhibits k(inh) = 2.87 x 10(7) M-1 s-1, 10 times that of a-tocopherol or any model compound, and is the most active phenolic antioxidant known. Class III, hydroquinones including 2,3,6-trimethylhydroquinone and the alpha-, beta-, and gamma-tocopherylhydroquinones, are 2 to 4 times less active as antioxidants than a-tocopherol, and class IV, the ubiquinones, are 8-10 times less active than a-tocopherol. The stoichiometric factors, n, for peroxyl radical trapping are 1.5-2.0 for phenols of classes I and II, 1.0-1.9 for ubiquinones but less than 0.5 for hydroquinones of class III due to ''wasting'' oxidation reactions of the latter. Antioxidant activities in solution are interpreted in terms of steric hindrance, retarding hydrogen transfer to peroxyl radicals, and electronic effects which increase k(inh) by stabilization of the ArO. radicals. Results in styrene solution are in contrast with other data in aqueous lipid membranes where H-bonding by water on the antioxidants is a significant effect on k(inh).