Atmospheric Photooxidation of Fluoroacetates as a Source of Fluorocarboxylic Acids

Abstract

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 (CF(3)C(O)OCH(2)CH(3)), methyl trifluoroacetate (CF(3)C(O)OCH(3)), and methyl difluoroacetate (CF(2)HC(O)OCH(3)) were investigated at 296 +/- 2 K and atmospheric pressure (approximately 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 (CF(2)XC(O)OH, with X = H, F) obtained were as follows: 78 +/- 5, 23 +/- 2, and 30 +/- 5% for CF(3)C(O)OCH(2)CH(3), CF(3)C(O)OCH(3), and CF(2)HC(O)OCH(3,), respectively. Yields of <or=20, <or= 80, and <or=55% have been estimated for the anhydride formation from CF(3)C(O)OCH(2)CH(3), CF(3)C(O)OCH(3), and CF(2)HC(O)OCH(3), respectively. Formation of CF(2)O, with yield of 13 +/- 2% has been observed for the reaction of Cl with CF(2)HC(O)OCH(3). 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.

Theoretical investigation on mechanism and kinetics of the Cl-initiated hydrogen abstraction reactions of ethyl trifluoroacetate at 298 K

“…The calculated k Cl value obtained from MPWB1K results is 2.15×10 −12 cm 3 molecule −1 s −1 , which is also in reasonably good agreement with the G2(MP2) and experimental values at 298 K. The calculated branching ratio values show that the -CH 2 site has greater contribution (90 %) to the overall rate constant, which indicates clearly the dominance of H-abstraction from the -CH 2 group of ETFA. Hence our calculation predicts reaction channel (2) as the major reaction channel, which is in accordance with the experimental observation made by Blanco et al [22].…”

“…The barrier height values show that hydrogen abstraction by Cl atoms from the -CH 2 group is easier than that from the -CH 3 group. This finding is in accord with the mechanism proposed by Blanco et al [22], which is based on his experimental observation that reaction of CF 3 C(O)OCH 2 CH 3 +Cl proceeds mainly from the -CH 2 site. …”

“…Therefore, it is necessary to understand the chemistry of fluorinated esters in the atmosphere in order to assess the environmental acceptability of HFEs as a plausible replacement for chlorofluorocarbons. The reaction mechanisms of FESs with atmospheric oxidants have been studied extensively both experimentally [20][21][22][23][24][25] and theoretically [26][27][28][29][30][31][32] and continue to receive considerable attention. FESs absorb strongly in the earthly infrared (IR) radiation region of 800-1,200 cm −1 .…”

Theoretical study on mechanism, kinetics, and thermochemistry of the gas phase reaction of 2,2,2-trifluoroethyl butyrate with OH radicals at 298 K

“…It is well established that the fluorinated esters (FESs) are the primary products of the atmospheric oxidation of HFEs. [4][5][6] Like HFEs, FESs also undergo photochemical oxidation in troposphere with atmospheric oxidants, OH radicals, or Cl atoms. The degradation of FESs produces an environmentally burdening product like trifluoroacetic acid (TFA) and COF 2 .…”

Computational Study on OH and Cl Initiated Oxidation of 2,2,2-Trifluoroethyl Trifluoroacetate (CF3C(O)OCH2CF3)

“…31 The total partition function of reactants [CF 3 C(O)OCH 2 CF 3 ] and transition states (TS-OH and TS-Cl) were corrected for hindered rotor as given by the following expression (8) where Q HO is the harmonic oscillator partition function, the internal rotation partition function and Q v=i is the partition function of normal mode vibrations corresponding to internal rotation.…”