Atmospheric Chemical Mechanism of the Photooxidation of Fluoroacetates: A Source of Fluoroacetic Acids

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

Florianopolis , Brasil

Conferencia; 10th Latin American Conference on Physical Organic Chemistry (CLAFQO),; 2009

Since Fluoroesters (FESs) are known products of the atmospheric oxidation of some hydrofluorinated ethers (HFEs)1, it is necessary to obtain a full understanding of the atmospheric chemistry of FESs in order to evaluate the environmental suitability of HFEs compared to other potential replacement compounds of CFCs, HFCs and HCFCs. The gas-phase removal processes of FESs include OH, NO3, Cl reactions and other process like dissolution in clouds and seawater2,3. In recent years the oxidations of volatile organic compounds (VOCs) by the highly reactive chlorine atom has gained much attention mainly in the marine troposphere where significant chlorine atom concentration may be present4. In this work we report a product study for the reactions of Cl atoms with methyl trifluoroacetate (CF3C(O)OCH3), ethyl trifluoroacetate (CF3C(O)OCH2CH3), methyl difluoroacetate (CF2HC(O)OCH3) and 2,2,2-trifluoroethyl trifluoroacetate (CF3C(O)OCH2CF3). The experiments were conducted using a 1080 liters quartz-glass environmental chamber at (298 ± 2) K in one atmosphere of synthetic air using in situ FTIR spectroscopy to monitor the organics. Product identification and quantification under atmospheric conditions were performed for the first time for the reactions cited above. The product distribution suggests that the atmospheric oxidation of fluoroacetates by Cl atoms proceeds via H-atom abstraction from –CH3 or –CH2 groups. Under atmospheric conditions the alkyl radicals formed react with molecular oxygen to give peroxy radicals. After that, peroxy radicals react with NO, NO2, HO2 and other peroxy radicals in the atmosphere forming alcoxy radicals. The atmospheric fate of the alcoxy radicals formed includes: a) reaction with O2 to lead the corresponding anhydrides via a H- abstraction or b) a ester rearrangement reaction via a five member ring intermediate where an intramolecular H- abstraction reaction takes place between the O atom of the ester and the adjacent H of the alkyl chain to form the corresponding fluorinated acetic acid. Atmospheric implications of the reactions studied are assessed taking into account the calculated lifetimes of the fluoracetates studied towards different tropospheric oxidants as well as photolysis or physical loss processes. This work is a part of an ongoing plan in our laboratory to study the kinetics and product distribution of the CFCs replacements compounds and their impact on air quality. 5-7.