OXIDATION OF TOLUENE IN NO-CHI FREE-AIR - PRODUCT DISTRIBUTION AND MECHANISM

Product distributions resulting from the oxidation of toluene in air i nitiated by chlorine atoms. by toluene photolysis, and by hydroxyl rad icals were studied at atmospheric pressure in the absence of nitrogen oxides. In the first case only benzaldehyde and benzylalcohol with a r atio of 7.8 +/- 1.7 were observed, from which a branching ratio alpha = 0.76 +/- 0.05 was derived for the radical propagating pathway of the self-reaction of benzylperoxy radicals. in the second and third case the apparent branching ratio decreased to alpha = 0.56 +/- 0.07 and 0. 37 +/- 0.08, respectively, indicating the occurrence of additional rea ctions between benzylperoxy and other peroxy radicals. The major ring retaining products resulting from the reaction of OH with toluene were o-cresol, p-cresol, and methyl-p-benzo-quinone (57.9 +/- 12.5% reacte d carbon, corrected for secondary losses), in addition to 7.7 +/- 1.6% benzaldehyde and benzylalcohol arising from methyl hydrogen abstracti on. Formaldehyde, acetaldehyde, glyoxal, methylglyoxal, carbon monoxid e, and carbon dioxide were observed as ring cleavage products (25.0 +/ - 7.2%). Except for CO2, which appears to be an important ring cleavag e product only under NOx-free conditions, the distribution of ring cle avage products was similar to that found by Gery et al. (1985) in the presence of NOx. Comparison of experimental data and results from comp uter simulations show that self-reactions of the hydroxy-methyl-cyclo- hexadienylperoxy radicals are negligible compared to internal rearrang ement reactions leading to ring stabilization or ring cleavage. The me chanism of CO2 production appears to require the formation of a Criege e intermediate. (C) 1996 John Wiley & Sons. Inc.