CHEMILUMINESCENCE SPECTROMETRY OF RADICAL REACTIONS .3. CHEMILUMINESCENCE IN THE COMBINATION REACTION OF BENZYLPEROXY RADICAL

It has been established that the decomposition of dibenzyl-diazene (omega, omega-azotoluene) in inert solvent and oxygen atmosphere is unsuitable for studying the termination of benzylperoxy radicals by chemiluminescence (CL) since it is transformed into benzal-benzylhydrazone due to proton catalysis by the benzoic acid formed. The CL study of the oxidation of toluene initiated by AIBN has shown that both triplet and singlet benzaldehyde is formed (T/S approximately 10.5). The lifetimes in toluene are: tau(S) approximately 4 . 10(-9) s and tau(T) approximately 1.9 . 10(-7) s. During the initiated oxidation of toluene H2O2 is also formed and 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. (Possibly 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. Since the aldehyde/alcohol ratio is always higher than 1, a detailed study of all circumstances led us to the conclusion that benzylperoxy radicals terminate both by the Russell-mechanism and by the two five-membered rings containing a transition state suggested by Bennett and Summers. The activation energy of hydrogen abstraction from toluene by benzylperoxy radicals is approximately 53 kJ/mol and the rate constant of this reaction does not differ essentially from that of the same reaction of ethylbenzene.