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Environmental fate & pathways

Phototransformation in air

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Description of key information

Key result for CAS# 756-13-8:
The expected atmospheric half-life of CAS# 756-13-8 due to direct photolysis is approximately one week. Ultimate reaction products are expected to be a mixture of carbonyl difluoride, hydrofluoric acid and trifluoroacetic acid (TFA, CAS# 76-05-1).
Key result for PFPA (degradation product), CAS# 422-64-0:
Half-life in air: 61.71 days using the estimated (AOPWINv1.92) rate constant of 5.2E-13cm^3/molecule-sec and ECHA accepted 5E+05 hydroxyl radicals per cubic centimetre.

Key value for chemical safety assessment

Additional information

Discussion for CAS# 756-13-8:

The phototransformation in air of CAS# 756-13-8 was assessed in six experimental studies.  The key experimental study examined direct photolysis of CAS# 756-13-8 relative to a reference chemical of known photoactivity (acetaldehyde, CAS# 75-07-0).  The half-life of CAS# 756-13-8 was in the range of 2.9-5.8 days.  The first supporting study (Gushin et al., 1999) addressed indirect photolysis of CAS# 756-13-8.  The photooxidant in this experiment was hydroxyl radical produced by photolysis of ozone in the presence of water vapor.  In control experiments, ~12% of CAS# 756-13-8 degraded on UV irradiation absent ozone, and on addition of ozone the rate of decline did not significantly increase.  Based on comparison of the UV absorbance spectrum of CAS# 756-13-8 and acetaldehyde and the atmospheric fate of acetaldehyde, an atmospheric lifetime of 3-5 days (equivalent to a half-life of 2-3.5 days) was estimated for CAS# 756-13-8.  The second supporting study (Taniguchi et al., 2003) is an extension of the first supporting study by an enlarged research team.  The Taniguchi study confirms that CAS# 756-13-8 does not react appreciably with photooxidants (hydroxyl radical, atomic chlorine, ozone), but does react by direct UV photolysis.  The study demonstrated formation of 2,2,2-trifluoroacetyl fluoride (TFA-F) and carbonyl difluoride by IR spectroscopy, and claimed formation of trifluoromethanol and bis(trifluoromethyl) trioxide (CF3O3CF3).  Based on photolysis of the reference substance (13C-labeled acetaldehyde), the authors estimated a tropospheric lifetime of 1-2 weeks for CAS# 756-13-8 (equivalent to a half-life of 5-10 days).  TFA-F is a reactive chemical that hydrolyzes to TFA and hydrofluoric acid on contact with water.  The third supporting experimental study (D’anna et al., 2005) was performed under natural sunlight in a high-volume outdoor reactor.  A quantum efficiency of 0.043 and a removal rate after correction for non-reactive losses of 6.4 ± 0.3 x 10-6s-1were calculated for CAS# 756-13-8, giving an estimated practical tropospheric half-life of 4.8 days.   Dark reactions were not assessed in this study.  Based on the available evidence, the expected atmospheric half-life of CAS# 756-13-8 due to direct photolysis is approximately one week.  A fourth supporting study (Cahill and Mackay, 2002) suggested that photolysis dominates the atmospheric fate of CAS# 756-13-8 and that hydrolysis (although rapid) does not contribute significantly to atmospheric transformation.

 

The direct photolysis pathway is presumed to begin with absorbance around 300 nm, followed by scission between the carbonyl and alpha carbons on the branched side of the molecule.  The products of scission are the linear perfluoropropionyl radical and the secondary perfluoropropyl radical (rather than the perfluoroisobutanoyl and perfluoroethyl radicals).  Previous research on perfluoropropionaldehyde photolysis has suggested that perfluoropropionic acid would be formed in significant amounts from perfluoropropionyl radical.  However, the fifth supporting study (Jackson et al., 2011) demonstrated that perfluoropropionic acid is not produced appreciably even under low NOx-conditions (0.6% of theoretical yield).  This result is in accord with the literature on hexafluoroacetone photolysis, which shows that trifluoroacetyl radicals formed by hydrogen abstraction from trifluoroacetaldehyde are significantly more stable than those formed by photoinduced scission of hexafluoroacetone.  In the former case, reaction products expected from trifluoroacetyl radical are routinely detected (for example, reaction products with bromine) while in the latter case the trifluoroacetyl reaction products are seldom detected.  It is presumed that excess energy remaining with the acetyl radical after direct photolysis results in spontaneous decarbonylation, forming a perfluoroalkyl radical (McIntosh and Porter, 1968).

 

Ultimate reaction products are expected to be a mixture of carbonyl difluoride, hydrofluoric acid and trifluoroacetic acid (TFA, CAS# 76-05-1). In the experimental study, the rate constant obtained for indirect photolysis of TFA was approximately 1 x 10-13cm3molecule-1s-1(Hurley et al., 2004).  Similarly, direct and indirect photolytic reactions of TFA are not expected.  Therefore, TFA is expected to be removed efficiently from the atmosphere by rainout.

 

Reference:

J. S. E McIntosh and G. B. Porter.  1968.  Stability of trifluoroacetyl radical.  Trans. Faraday Soc. Vol. 64, pp. 119-123.

Discussion for PFPA (degradation product), CAS# 422-64-0:

The indirect phototransformation in air of PFPA was assessed in one QSAR study (key) and one experimental study (supporting). The QSAR result was obtained using the AOPWINv1.92 submodule within EPISuite v4.00. The QSAR indirect photolysis rate constant is deemed the key result for PFPA based on the applicability domain of this QSAR. The supporting experimental study (Hurley et al., 2004) examined indirect photolysis of PFPA relative to two reference chemicals of known photolytic activity (ethylene and acetylene). The experimental rate constant is deemed a supporting result due to interferences between the measuring system and test substance. The QSAR estimated indirect photolysis rate constant of 5.2E-13 cm^3/molecule-sec is in reasonable agreement with the experimental rate constant for PFPA reaction with hydroxyl radicals of 1.69E-13 cm^3/molecule-sec. Direct photolysis of PFPA is not known (Sulbaek Andersen, et al., 2003). Based on the estimated rate constant (k) from the QSAR study, the atmospheric half-life is 61.71 days using the EChA accepted hydroxyl radical concentration of 5E+05 radicals per cubic centimetre and 12-hr daylight. Based on the experimental rate constant (k), an atmospheric half-life of approximately 190 days can be calculated for PFPA.

Reference:

M.P. Sulbaek Andersen, M.D. Hurley, T.J. Wallington, J.C. Ball, J.W. Martin, D.A. Ellis and S.A. Mabury. 2003. Atmospheric chemistry of C2F5CHO: mechanism of the C2F5C(O)O2+HO2 reaction. Chem. Phys. Lett. 381: 14-21.