Products of the gas-phase reactions of the OH radical with p -xylene and 1,2,3- and 1,2,4-trimethylbenzene have been measured by gas chromatography in the presence of varying concentrations of NO 2 . Our product analyses show that the ring-cleavage products 2,3-butanedione (from 1,2,3- and 1,2,4-trimethylbenzene) and 3-hexene-2,5-dione (from p -xylene and 1,2,4-trimethylbenzene) exhibit a dependence of their formation yields on the NO 2 concentration, with higher yields from the reactions of the OH−aromatic adducts with O 2 than from their reactions with NO 2 . Furthermore, our data show that these ring-cleavage products are primary products of the OH−aromatic adduct reactions. Formation yields extrapolated to zero NO 2 concentration should be applicable to ambient atmospheric conditions (provided that there is sufficient NO that peroxy radicals react dominantly with NO), and are from p -xylene, p -tolualdehyde, 0.0706 ± 0.0042 (independent of NO 2 concentration), 2,5-dimethylphenol, 0.138 ± 0.016 (independent of NO 2 concentration), and 3-hexene-2,5-dione, 0.32 (extrapolated); from 1,2,3-trimethylbenzene, 2,3-butanedione, 0.52 (extrapolated); and from 1,2,4-trimethylbenzene, 2,3-butanedione, 0.10 (extrapolated) and 3-hexene-2,5-dione, 0.31 (extrapolated). Our formation yields of 3-hexene-2,5-dione from p -xylene and 1,2,4-trimethylbenzene are similar to those reported for glyoxal from p -xylene and of methylglyoxal from 1,2,4-trimethylbenzene and therefore suggest that these are coproducts, as expected from reaction schemes presented here.
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Bethel et al. (2000) studied this question.