Cl-INITIATED PHOTOOXIDATION OF CF3C(O)OCH2CF3
The other reaction option for the CF3C(O)OCH(Oꢄ)CF3 radical is
decomposition via C—C or C—O bond cleavage to form (i) the
formyl trifluoromethyl anhydride (CF3C(O)OCH(O)) and CF3
radicals and/or (ii) trifluoroacetaldehyde (CF3CH(O)) and CF3
and CO2, whereby cleavage of the weaker C—C bond is more
favored. It is not possible to confirm whether CF3C(O)OCH(O)
and/or CF3CH(O) are being formed in the system since, in case
they were formed, their absorption bands will be overlapped by
those of CF3C(O)OC(O)CF3. Further reactions of CF3 with O2 will
result in the formation of CF2O observed. Subsequent reaction of
either CF3C(O)OCH(O) and/or CF3CH(O) with Cl could lead to the
CO formation identified. Formation of high yields of CO has been
found in the Cl-atom initiated oxidation of fluorinated aldehydes
by Solignac et al.[24] They suggest that the CO is formed via the
decomposition channel of the perfluoroacyl radicals (CnF2nþ1CO)
which competes with the combination channel with molecular
oxygen. Such a decomposition is possible in the reactions of Cl
with both CF3C(O)OCH(O) and CF3CH(O). The further reactions of
CF3 with O2, which can be formed in both possible CF3C(O)O-
CH(Oꢄ)CF3 decomposition channels described, will result in the
formation of CF2O observed.
The relative importance of the different channels for the
reactions of a series of fluoroacetates, in agreement with the
product yields obtained, can be correlated with the structure of
the molecule regarding the nature and position of the group
substituents (e.g., H atoms by CH3, CF3, or F groups). The
importance of the a-ester channel to produce CF3C(O)OH can be
explained by the identity of the different substituents connected
to the carbon containing the hydrogen atom involved in the
rearrangement. Figure 3 shows the a-ester rearrangement and
the experimentally trifluoro acetic acid yields obtained for the
reactions of Cl with CF3C(O)OCH2CF3 and for CF3C(O)OCH3 and
CF3C(O)OCH2CH3 for comparative purposes. The measured acid
yields for the reactions of Cl with CF3C(O)OCH2CF3, CF3C(O)CH3
and CF3C(O)OCH2CH3 are 0, 23 and 78 %, respectively.
rearrangement compared to the high yield from
CF3C(O)OCH2CH3 which contains an electron donating CH3
group.
The atmospheric lifetime of CF3C(O)OCH2CF3, around 4
months,[13] is determined mainly by the reaction with OH
radicals, even though in the early morning hours in marine
environments, Cl-atom initiated reactions could compete with
the OH reaction. The CF3C(O)OCH2CF3 survive long enough to
become well dispersed from the source origin, with regional scale
transport being likely; the urban emission of this compound is,
therefore, unlikely to contribute to local ozone formation. Product
studies on esters and fluorinated esters have shown that the
oxidation of fluoroacetates leads mainly to the formation of
fluorinated acetic acid and the corresponding anhydrides, as well
as CF2O and its hydrolysis products, CO2 and HF, which will be
removed mostly by heterogeneous processes. In particular,
anhydrides with lower molecular weight are highly soluble
compounds and may be rapidly incorporated into cloud droplets,
contributing to the acidity of precipitation.
Further experiments carried out to study the product
distribution of the fluoroacetate reactions over the range of
temperatures, NOx, and pressures of oxygen typically prevailing
in the troposphere would be desirable to obtain a comprehensive
description of the atmospheric implications associated with
the widespread use of fluoroethers on air quality and their
contribution to TFA formation and acid precipitation in general.
Such data would allow a more accurate representation of the
tropospheric chemistry of fluorinated esters in the various types
of CT models used for designing environmental abatement
strategies and making atmospheric policy decisions.
Acknowledgements
We acknowledge DAAD-PROALAR (Germany), the Deutsche For-
schungsgemeinschaft (DFG), the EU project EUROCHAMP, SECYT
(Argentina), CONICET (Argentina), ANPCyT (FONCYT, Argentina),
This trend can be explained by the identity of the different
substituents connected to the carbon containing the hydrogen
atom involved in the rearrangement. Substituents that donate
electron density, by a positive inductive effect, to the carbon such
as —CH3 will produce a weaker C—H bond and thus facilitate the
rearrangement. By contrast, substituents that withdraw electron
density such as CF3 will strengthen the C—H bond, which will
increase the activation energy for the rearrangement and result
in a lower yield of acid. These effects are clearly evident in the
measured acid yields for the fluoroacetates. The electro-
n-withdrawing effect of the —CF3 group on the alkoxy side of
the ester in CF3C(O)OCH2CF3 considerably reduces the a-ester
´
´
SECyT-UNC (Cordoba, Argentina), Fundacion Antorchas (Argen-
tina), TWAS (Italy), and RSC (UK) for financial support of this
research. We also thank J. Orlando and G. Tyndall for a copy of
their manuscript prior to publication.
REFERENCES
[1] J. H. Seinfeld, S. N. Pandis, Atmospheric Chemistry and Physics, J Wiley,
NY, 1998.
[2] T. J. Wallington, W. F. Schneider, J. Sehested, M. Bilde, J. Platz,
O. J. Nielsen, L. K. Christensen, M. J. Molina, L. T. Molina, P. W.
Wooldridge, J. Phys. Chem. A 1997, 101(44), 8264.
[3] L. K. Christensen, J. Sehested, O. J. Nielsen, M. Bilde, T. J. Wallington,
A. Guschin, L. T. Molina, M. J. Molina, J. Phys. Chem. A 1998, 102, 4839.
[4] L. K. Christensen, T. J. Wallington, A. Guschin, M. D. Hurley, J. Phys.
Chem. A 1999, 103(21), 4202.
[5] Y. Ninomiya, M. Kawasaki, A. Guschin, L. T. Molina, M. J. Molina,
T. J. Wallington, Environ. Sci. Technol. 2000, 34(14), 2973.
[6] L. Chen, S. Kutsuna, K. Nohara, K. Takeuchi, T. Ibusuki, J. Phys. Chem. A
2001, 105, 10854.
[7] K. Nohara, M. Toma, S. Kutsuna, K. Takeuchi, T. Ibusuki, Environ. Sci.
Technol. 2001, 35(1), 114.
[8] N. Oyaro, S. R. Sellevag, C. J. Nielsen, Environ. Sci. Technol. 2004, 38,
5567.
[9] M. P. Sulbaek Andersen, O. J. Nielsen, T. J. Wallington, M. D. Hurley,
G. W. DeMoore, J. Phys. Chem. A 2005, 109(17), 3926.
[10] L. Chen, K. Kutsuna, K. Tokuhashi, A. Sekiya, R. Tamai, Y. Hibino, J. Phys.
Chem. A 2005, 109, 4766.
Figure 3. a-ester rearrangements and the corresponding trifluoroacetic
acid yields obtained for the reactions of Cl with CF3C(O)OCH2CF3 from this
[22]
work and for CF3C(O)OCH3 and CF3C(O)OCH2CH3 from Ref.
parative purposes
for com-
J. Phys. Org. Chem. 2010, 23 950–954
Copyright ß 2010 John Wiley & Sons, Ltd.
View this article online at wileyonlinelibrary.com