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.