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

Abstract

AbstractThe technique based on monitoring oxygen consumption was applied to test 13 substituted p‐hydroquinones (QH2) as a chain‐breaking antioxidant during the oxidation of styrene initiated by 2,2′‐azobis(2,4‐dimethylvaleronitrile) at 37°C. The methodology originally developed to test monophenolic antioxidants was modified to fit it to specific features of oxidative transformation of QH2. Chain‐breaking capability of QH2 was characterized by two parameters: the rate constant k1 for reaction of QH2 with the peroxy radical LO2·: (1) QH2 + LO2· → QH· + LOOH and the stoichiometric factor of inhibition, f, which shows how many kinetic chains may be terminated by one molecule of QH2. Rate constants k1 × 105 (in M−1 s−1) were found to be 5.54 (nonsubstituted QH2); 7.13 (Me‐QH2); 13.1 (Et‐QH2); 12.0 (t‐Bu‐QH2); 15.6 (2,6‐Me2‐QH2); 18.8 (2,3‐Me2‐QH2); 17.1 (2,5‐Me2‐QH2); 13.7 (2,6‐(MeO)2‐QH2); 4.70 (2,6‐Ph2‐QH2); 0.90 (2,5‐Cl2‐QH2); 23.2 (Me3‐QH2); 4.40 (2,3‐(MeO)2‐5‐Me‐QH2). Parameter f never exceeds 2; for the majority of QH2, 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. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 162–171, 2002; DOI 10.1002/kin.10041