Oxidation of toluene in NOx free air: product distribution and mechanism

Article abstract of DOI:10.1002/(SICI)1097-4601(1996)28:5<315::AID-KIN1>3.0.CO;2-Y

Product distributuion resulting from the oxidation in air initiated by chlorine atoms, by toluene photolysis, and by hydroxyl radicals were studied at atmospheric pressure in the absence of nitrogen oxides.In the first case only benzaldehyde and benzylalcohol with a ratio of 7.8 +/- 1.7 were observed, from wich a branching ratio α = 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 α = 0.56 +/- 0.07 and 0.37 +/- 0.08, respectively, indicating the occurence of additional reactions 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 +/- 12.5percent reacted carbon, corrected for secondary losses), in addition to 7.7 +/- 1.6percent benzaldehyde and benzylalcohol arising from methyl hydrogen abstraction.Formaldehyde, acetaldehyde, glyoxal, methylglyoxal, carbon monoxide, and carbon dioxide were observed as ring cleavage products (25.0 +/- 7.2percent).Except for CO2, which appears to be an important ring cleavage product only under NO_x-free conditions, the distribution of ring cleavage products was similar to that found by Gery et al. (1985) in the presence of NO_x.Comparison of experimental data and results from computer simulations show that self-reactions of the hydroxy-methyl-cyclo-hexadienylperoxy radicals are negligible compared to internal rearrangement reactions leading to ring stabilization or ring cleavage.The mechanisms of CO2 production appears to require the formation of a Criegee intermediate.