Atmospheric Oxidation Mechanism of Benzene. Fates of Alkoxy Radical Intermediates and Revised Mechanism

The fate of alkoxy radicals formed in the atmospheric oxidation of benzene initiated by OH radical is investigated by using quantum chemistry and kinetics calculations. The two alkoxy radicals (R2 and R3), formed from the commonly accepted bicyclic radical intermediates, are found to undergo ring-closure preferentially, in addition to the ring-breakage, as suggested in previous studies. The ratio between the ring-closure and ring-breakage is ∼2:1. The ring-closure route will lead to equal amounts of glyoxal and 2,3-epoxybutandial, while the ring-breakage route leads to glyoxal and butenedial. Overall, the new mechanism suggests the yield of glyoxal to be three times that of butenedial, consistent with the previous experimental measurements. The new mechanism calls for the search of the newly proposed product 2,3-epoxybutandial.