In order to study the detailed mechanism of the combination of tertiary peroxy radicals in the liquid phase the oxidation of 2,2′-azobis[2-methylpropanenitrile] (AIBN) has been investigated by using chemiluminescence (CL) spectroscopy, complemented by detailed product analyses using FT-IR and GC-MS methods. It has been established that the intensity of CL did not decrease during the oxidation as expected provided that the source of the light emission was the simple termination of the cyanopropylperoxy radicals formed. The application of sensitizers 9,10-diphenylanthracene and 9,10-dibromoanthracene has shown that the source of CL was basically the light emission by excited ketone or aldehyde molecules (with a lifetime of ∼7 × 10-7 s) and the ratio of the triplet and singlet excitation yields was (Φ*T/Φ*S) ∼22. Experiments carried out in the presence of 2,6-di-tert-butyl-p-cresol inhibitor (DTBC) have indicated that at least three different elementary processes, both radical and molecular, are accompanied by CL. It was assumed the cyanopropylperoxy radicals combine into alkoxy radicals (with a rate constant of 9 × 104 M-1 s-1 at 80°C), the latter undergoing β-scission in two directions: (i) yielding CH3COCH3 and ĊN (with a rate constant of 5.7 × 104 s-1 at 80°C and ΔH° = 183 kJ mol-1) or (ii) yielding CH3COCN and ĊH3 (with a rate constant of 1.65 × 105 s-1 at 80°C and ΔH° = 21.2 kJ mol-1). Methyl radicals are transformed in a very fast process into CH3O2• and the interaction of the latter with rO2• is the main termination resulting in a fast initial increase of the CL. Both peroxy radicals rO2• and CH3O2• and alkoxy radicals rO• can abstract hydrogen from the solvent yielding rOOH with a rate constant of 1.5 × 10-2 M-1 s-1 and CH3OOH with a rate constant of 1.0 × 10-2 M-1 s-1 as well as acetone cyanohydrine with a rate constant of 4.2 × 103 M-1 s-1 at 80°C. The monomolecular decomposition of CH3OOH (k = 8 × 10-4 s-1 at 80°C) leads to the anomalous behavior of the CL observed experimentally.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry