Substituted p-hydroquinones as a chain-breaking antioxidant during the oxidation of styrene

The technique based on monitoring oxygen consumption was applied to test 13 substituted p-hydroquinones (QH(2)) as a chain-breaking antioxidant during the oxidation of styrene initiated by 2,2'-azobis(2, 4-dimethylvaleronitrile) at 37degreesC The methodology originally developed to test monophenolic antioxidants was modified to fit it to specific features of oxidative transformation of QH(2). Chain-breaking capability of QH(2) was characterized by two parameters the rate constant, for reaction of QH(2) with the peroxy radical LO2.. (1) QH(2) + LO2 --> QH + LOOH and the stoichiometric factor of inhibition, f, which shows how many kinetic chains may be terminated by one molecule of QH(2) Rate constants, k(1) x 10(5) (in M-1 s(-1)) were found to be 5.54 (nonsubstituted QH(2)), 7.13 (Me-QH(2)), 13.1 (Et-QH(2)), 12.0 (t-Bu-QH(2)); 15.6 (2,6-Me-2-QH(2)); 18.8 (2,3-Me-2-QH(2)), 17.1 (2,5-Me-2-QH(2)), 13.7 (2,6-(MeO)(2)-QH(2)), 4.70 (2,6-Ph-2-QH(2)), 0.90 (2,5-Cl-2-QH(2)); 23.2 (Me-3-QH(2)), 4.40 (2,3-(MeO)(2)-5-Me-QH(2)) Parameter f never exceeds 2 for the majority of QH(2), f was found to be visibly less than 2, decreasing when the rate of initiation decreased Both experimental data and results of computer kinetic simulations suggest that, contrary to monophenolic antioxidants, the main path of QH(.) transformation is the self-disproportionation rather than recombination of QH(.) with LO2. Most likely, f is commonly lower than 2 because of the reaction of QH(.) with molecular oxygen The latter may result in faster depletion of QH2 and the increase in the rate of inhibited oxidation (C) 2002 Wiley Periodicals, Inc.