Atmospheric Photooxidation of Fluoroacetates as a Source of Fluorocarboxylic Acids

A 1080 L environmental chamber with in situ FTIR spectroscopy detection was used to study the product distribution and the mechanism of the Cl-initiated photooxidation of a series of fluoroacetates. The gas-phase reactions of Cl atoms with ethyl trifluoroacetate (CF3C(O)OCH2CH3), methyl trifluoroacetate (CF3C(O)OCH3), and methyl difluoroacetate (CF2HC(O)OCH3) were investigated at 296 +/- 2 K and atmospheric pressure (similar to 760 Torr) of synthetic air. The fate of the fluoroalkoxy radicals formed in the reaction with Cl atoms mainly occurs through (i) an H-atom abstraction by reaction with O-2, to produce the corresponding fluoroanhydride and (ii) an alpha-ester rearrangement via a five-membered ring intermediate to give the corresponding fluoroacetic acid. The yields of fluoroacids (CF2XC(O)OH, with X = H, F) obtained were as follows: 78 +/- 5, 23 +/- 2, and 30 +/- 5% for CF3C(O)OCH2CH3, CF3C(O)OCH3, and CF2HC(O)OCH3, respectively. Yields of <= 20, <= 80, and <= 55% have been estimated for the anhydride formation from CF3C(O)OCH2CH3, CF3C(O)OCH3, and CF2HC(O)OCH3, respectively. Formation of CF2O, with yield of 13 +/- 2% has been observed for the reaction of Cl with CF2HC(O)OCH3. The measured yields are rationalized in terms of mechanisms consisting of competitive reaction channels for the radicals formed in the oxidation, that is, reaction with O-2, alpha-ester rearrangement and a decomposition pathway. The stability of the five-membered transition state of the alpha-ester rearrangement is correlated with the acid yields observed for the different fluoroacetates. Atmospheric implications, especially with regard to the fluorocarboxylic acid formation, are discussed.