Reaction of acetaldehyde and acetyl radical with atomic and molecular oxygen

The reaction of acetaldehyde with atomic oxygen, O(³P), was studied by the discharge-flow-photoionization mass spectrometry method at room temperature (295 ± 4 K) for the conditions of excess atomic oxygen. The rate constant for reaction 1, CH₃CHO + O → CH₃CO + OH, was (3.9 ± 0.3) × 10^-13 cm³ molecule^-1 s^-1. Branching fractions for the subsequent reactions 2a, CH₃CO + O → CH₂CO + OH, and 2b, CH₃CO + O → CH₃ + CO₂, were determined to be 22 ± 5% and 76 ± 24%, respectively. The experiments in the presence of molecular oxygen show that the ratio of rate constants for reaction 3, CH₃CO + O₂ + M → CH₃CO₃ + M, to reaction 2 is (6.3 ± 0.5) × 10^-3 [total pressure 3.6 Torr (He)]. From the rate constant for reaction 3 obtained by McDade et al., the rate constant for reaction 2 was calculated: k₂ = (3.2 ± 0.7) × 10^-10 cm³ molecule^-1 s^-1. When the concentration of molecular oxygen increased, the yields of both CH₃ and CH₂CO decreased. From this decrease, the branching fractions for reactions 4a, CH₃CO₃ + O → CH₂CO + OH + O₂, and 4b, CH₃CO₃ + O → CH₃ + CO₂ + O₂, were determined: < 10% and 27 ± 25%, respectively. A discussion of the other products from CH₃CO₃ + O is presented.