Atmospheric sink of methyl chlorodifluoroacetate and ethyl chlorodifluoroacetate: temperature dependent rate coefficients, product distribution of their reactions with Cl atoms and CF2ClC(O)OH formation

MARÍA B. BLANCO, IAN BARNES, PETER WIESEN AND MARIANO A. TERUEL

RSC Advances

RSC

Año: 2016 vol. 6 p. 51834 - 51844

Rate coefficients in function of temperature have been measured for the first time for the gas-phase reactions of chlorine atoms with methyl chlorodifluoracetate (k1) and ethyl chlorodifluoroacetate (k2) using the relative rate technique. The experiments were carried out in a 1080 L photoreactor over the temperature range 287?313 K at a total pressure of 1000  10 mbar of synthetic air using in situ FTIR spectroscopy to monitor reactants and products. The following Arrhenius expressions were obtained: k(MCDFA+Cl) = (9.6 ± 5.1)  10-12 exp [-(1363 ± 79)/T] and k(ECDFA+Cl) = (64.4 ± 29.7) 10-12 exp [-(1110 ± 68)/T]. The kinetic results are compared with previous experimental and theoretical studies.In addition, a product study of the reactions of Cl with methyl chlorodifluoracetate and ethyl chlorodifluoroacetate is reported. The results indicate that in the absence of NOx the main fate of the alkoxy radicals formed after H-atom abstraction by Cl from the ?CH3 group in methyl chlorodifluoroacetate is reaction of the radical with O2 to form the mixed anhydride CF2ClC(O)OC(O)H. In the case of ethyl chlorodifluoroacetate the main fate of the alkoxy fromed via H-atom abstraction by Cl from the ?CH2- entity in the ethyl group is an α-ester rearrangement to produce chlorodifluoroacetic acid and .the corresponding radical.The yields of chlorofluoracetic acid (CF2ClC(O)OH) obtained were as follows: (34 ± 5) % and (86 ± 8) % for the reactions of Cl with CF2ClC(O)OCH3 and CF2ClC(O)OCH2CH3, respectively. The measured yields are rationalized in terms of mechanisms consisting of competitive reaction channels for the formed in the oxidation, i.e. reaction with O2, -ester rearrangement and a decomposition pathway.Atmospheric implications are discussed according to the rate coefficients obtained in function of the temperature and the altitude, and regarding to the formation of chlorofluorocarboxylic acid.