Ring-cleavage Reactions of Aromatic Hydrocarbons Studied by FT–IR Spectroscopy. I. Photooxidation of Toluene and Benzene in the NO<i>x</i>–Air System

Abstract

Abstract The products of the atmospheric photooxidation reactions of toluene and benzene were analyzed quantitatively using long-path FT–IR spectroscopy. Methylglyoxal (only in the case of toluene), glyoxal and maleic anhydride were observed as aromatic ring-cleavage products. In the toluene photooxidation, yields of methylglyoxal, glyoxal, and maleic anhydride were 14±4, 15±4, and 4.0±0.4% (mole/mole), respectively. The formation of small molecules, formaldehyde and formic acid, was observed. It is suggested that the precursor of the ring-cleavage reaction is OH–aromatics–O2 adducts, and that the α-dicarbonyls and unsaturated γ-dicarbonyls are produced by ring cleavage after cyclization of the OH–aromatics–O2 adduct. The observation of maleic anhydride is considered as an evidence of the aromatic ring cleavage into C2 and C4 fragments. The fraction of the ring cleavage process in the total reaction is 29 and 15–20% in the cases of toluene and benzene, respectively.