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-degre es-C. Class I, 6-hydroxypolyalkylchromans (model compounds) showed the same inhibition rate constants (k(inh) as their alpha-tocopherol anal ogs, in the range 1.5 X 10(6) M-1 s-1 to 3.8 x 10(6) M-1 s-1 for two m ethyl or ethyl groups ortho to the phenolic hydroxyl group. Bulky orth o groups reduce the k(inh), two isopropyl groups by 3-fold and 5-isopr opyl-6-tert-butyl by 5-fold, compared to that of alpha-tocopherol, due to steric hindrance to the approach of peroxyl radicals. Class II, th e a-naphthol derivatives, 6-hydroxy-2,5-dimethyl-2-phytyl-7,8-benzochr oman (9a) and the corresponding chromene (9b), exhibit higher k(inh) v alues, 4 times that of alpha-tocopherol. A new synthetic antioxidant, ydro-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 compou nd, and is the most active phenolic antioxidant known. Class III, hydr oquinones including 2,3,6-trimethylhydroquinone and the alpha-, beta-, and gamma-tocopherylhydroquinones, are 2 to 4 times less active as an tioxidants than a-tocopherol, and class IV, the ubiquinones, are 8-10 times less active than a-tocopherol. The stoichiometric factors, n, fo r 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 effect s 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 significan t effect on k(inh).