Oxidation mechanisms for ethyl chloride and ethyl bromide under atmospheric conditions

The Cl-atom initiated oxidation of ethyl chloride and ethyl bromide has been investigated as a function of temperature (220-298 K) in an environmental chamber equipped with an FTIR spectrometer. In the absence of NO_x, reaction with O₂ (CH₃CHClO + O₂ → CH₃C(O)Cl + HO₂, reaction 9) and decompositon via HClelimination (CH₃CHClO → CH₃CO + HCl, reaction 8a) are shown to be competing fates of the CH₃CHClO radical generated in the ethyl chloride system, with k₉/k_8a = 3.2 × 10^-24 exp(2240/T) cm³ molecule^-1. The CH₃CHClO radical is also shown to be subject to a chemical activation effect; when produced in the exothermic reaction of CH₃CHClO₂ with NO, about 50% of the nascent CH₃CHClO radicals decompose promptly via HCl elimination, before collisional deactivation can occur. The reaction of Cl with ethyl bromide occurs 70-80% via abstraction from the -CH₂Br group and 20-30% via abstraction from the -CH₃ group. The observation of ethene in this system indicates that the BrCH₂CH₂ radical (generated via abstraction from the -CH₃ group) undergoes Br-atom elimination (BrCH₂CH₂ → CH₂=CH₂ + Br, reaction 37) in competition with its reaction with O₂ (BrCH₂CH₂ + O₂ → BrCH₂CH₂O₂, reaction 34) At atmospheric pressure, this competition is governed by the rate coefficient ratio, k₃₇/k₃₄ = 1.4 × 10²³ exp(-2800/T) molecule cm^-3. The chemistry of the CH₃CHBrO radical (generated from abstraction at the -CH₂Br site) is dominated by Bratom elimination; no evidence for its reaction with O₂ could be found at any temperature.