Peroxy radical reactions in the photo-oxidation of CH3CHO

Abstract

The behaviour of reactants, products and radical intermediates during the photo-oxidation of acetaldehyde at room temperature and 700 Torr pressure has been studied using a novel apparatus. The apparatus consists of a ‘double multi-path’ spectrometer, combining both i.r. and u.v. absorption spectrometry with the additional capability of modulated photolysis for transient detection. Photo-oxidation of CH₃CHO gave CO, CO₂, H₂O, CH₂O, HCOOH, CH₃OH, CH₃OOH, CH₃COOH and CH₃COOOH as identifiable products by long-path F.t.i.r. spectrometry. The effect of variation of the O₂ and CH₃CHO concentration upon the different photolysis products was studied, and computer simulation of experimental data was used to test a proposed mechanism containing 50 elementary reactions. Absorption vs. time profiles at selected wavelengths in the 210–275 nm region were recorded in the modulated photolysis of CH₃CHO–air mixtures. Transient absorptions were assigned to peroxy radicals, and kinetic analysis of the data assisted by computer simulation resulted in a value k₁₀= 4.8 × 10^–12 cm³ molecule^–1s^–1for the reaction HO₂+ CH₃O₂→ products. This value is somewhat higher than the value k_10a= 3.5 × 10^–12 cm³ molecule^–1s^–1 for the reaction HO₂+ CH₃O₂→ CH₃OOH + O₂ obtained from the computer simulations of the methyl hydroperoxide formation, indicating that an additional product channel may occur.