The atmospheric oxidation of ethyl formate and ethyl acetate over a range of temperatures and oxygen partial pressures

The Cl-atom-initiated oxidation of two esters, ethyl formate [HC(O)OCH ₂CH₃] and ethyl acetate [CH₃C(O)OCH ₂CH₃], has been studied at pressures close to 1 atm as a function of temperature (249-325 K) and O₂ partial pressure (50-700 Torr), using an environmental chamber technique. In both cases, Cl-atom attack at the CH₂ group is most important, leading in part to the formation of radicals of the type RC(O)OCH(O•)CH₃ [R = H, CH₃]. The atmospheric fate of these radicals involves competition between reaction with O₂ to produce an anhydride compound, RC(O)OC(O)CH₃, and the so-called α-ester rearrangement that produces an organic acid, RC(O)OH, and an acetyl radical, CH₃C(O). For both species studied, the α-ester rearrangement is found to dominate in air at 1 atm and 298 K. Barriers to the rearrangement of 7.7 ± 1.5 and 8.4 ± 1.5 kcal/mole are estimated for CH₃C(O)OCH(O•)CH₃ and HC(O)OCH(O•)CH₃, respectively, leading to increased occurrence of the O₂ reaction at reduced temperature. The data are combined with those obtained from similar studies of other simple esters to provide a correlation between the rate of occurrence of the α-ester rearrangement and the structure of the reacting radical.