Self‐Reaction of Benzylperoxy Radicals in the Oxidation of Toluene. A Chemiluminescence and Product Study

Abstract

AbstractThe CL study of the oxidation of toluene initiated by AIBN has shown that both triplet and singlet benzaldehyde is formed (T/S ≈︁ 10.5). The lifetimes in toluene are: τ5 ≈︁ 4 · 10−9 s and τT ≈︁ 1.9 · 10−7 s. – In the presence of DPA the kinetic curve of the CL light shows a maximum at [DPA] > 10−4 M. The CL spectrum of the oxidation of toluene lacks both the fluorescence and phosphorescence peaks of the excited benzaldehyde (due to the very low efficiency of photon emission and very strong quenching by O2 and toluene). – Analysis of product distribution by HPLC revealed the formation of benzaldehyde, benzylalcohol, benzylhydroperoxide and benzoic acid. A small amount of H2O2 was also detected by spectrophotometry. Since the ratio of aldehyde/alcohol is always higher than unity detailed study of all circumstances led us to the conclusion that benzylperoxy radicals terminate both by the Russell‐mechanism and by the transition state containing two five‐membered rings suggested by Bennett and Summers. – The activation energy of hydrogen abstraction from toluene by benzylperoxy radicals is 53 kJ/mol and the rate constant of this reaction does not differ essentially from the same reaction of ethylbenzene. The kinetic chain length of oxidation calculated from the analytical data is equal to 2±0.2 (75–90°C).