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Environmental fate & pathways

Phototransformation in air

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Description of key information

The estimated half-life of CAS# 297730-93-9 by indirect phototransformation is 1.5 years.  The expected products are hydrofluoric acid (HF), trifluoroacetic acid (TFA), perfluorobutyric acid (PFBA) and carbon dioxide (CO2)

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The chemical stability of CAS# 297730-93-9 with respect to phototransformation in air was addressed in two published reports regarding degradation by indirect phototransformation using hydroxyl radical and/or atomic chlorine.  The key study (Goto et al., 2002) reports and expands upon unpublished tests.  In this study, the authors examined the rate of degradation of CAS# 297730-93-9 relative to reference substances with known degradation rate constants, thus establishing a rate constant for the test substance.   Direct phototransformation was not observed.  The rate constant for reaction with hydroxyl radical was (2.6 ± 0.6) x 10-14 cm3 molecule-1 s-1.  By comparison with accepted atmospheric half-life of 1,1,1-trichloroethane, which has a rate constant of 1.0 x 10-14 cm3 molecule-1 s-1 for reaction with hydroxyl radical, CAS# 297730-93-9 has an estimated half-life in the atmosphere of 1.5 years.  A rate constant for reaction of CAS# 297730-93-9 with chlorine radical was also measured, with a value of (2.3 ± 0.7) x 10-12 cm3 molecule-1 s-1.

The supporting study (Diaz-de-Mera et al., 2009) examined temperature dependence of the reaction of CAS# 297730-93-9 with chlorine radical.  The second order rate constant measured at 298 K had nearly perfect correspondence with the rate constant for the chlorine reaction as measured in the key study, at (2.2 ± 0.6) x 10-12 cm3 molecule-1 s-1.  Fit of the kinetic data to the log-transformed Arrhenius equation gave a good fit (R2= 0.995), parameterized as follows:

 

k = (1.1 ± 0.6) x 10-10 * e(-1190 ± 270 /T).

 

The relative importance of the reaction with chlorine or hydroxyl radical depends on photooxidant concentration and on temperature.  The temperature dependence term (Ea/R) for the chlorine reaction has a value of 1190 (±270) K from the above equation.  Temperature dependence for the reaction with hydroxyl radicals is not known, but by comparison to other segregated fluoroethers (1 -4) (Table A1), the term Ea/R is approximately 2500 K.  At temperatures below 298 K, reaction with chlorine radical becomes increasingly important and degradation of CAS# 297730-93-9 proceeds more rapidly (Table A2).

 

Mechanism: 

Using infrared spectroscopy and reactivity characteristics, Goto et al. 2002 proposed a degradative pathway, based on reaction with either hydroxyl or chlorine radical.  The first step is oxidation of the non-fluorinated ethyl ether group to either an acetate ester (major product) or formate ester (minor product).  The proposed pathway is as follows:

 

n-C3F7CF(OCH2CH3)CF(CF3)2 + OH ─> n-C3F7CF(OCH*CH3)CF(CF3)2 + H2O (1)

n-C3F7CF(OCH2CH3)CF(CF3)2 + OH ─> n-C3F7CF(OCH2CH2*)CF(CF3)2 + H2O (2)

n-C3F7CF(OCH*CH3)CF(CF3)2 + O2 + M ─> n-C3F7CF(OCHOO*CH3)CF(CF3)2 + M (3)

n-C3F7CF(OCH2CH2*)CF(CF3)2 + O2+ M ─> n-C3F7CF(OCH2CH2OO*)CF(CF3)2 + M (4)

 

The peroxyl radicals so formed may react as follows (only reaction of the terminal radical (2) are shown):

 

 

n-C3F7CF(OC2H4OO*)CF(CF3)2 + NO ─> n-C3F7CF(OC2H4O*)CF(CF3)2 + NO2 (5)

n-C3F7CF(OC2H4OO*)CF(CF3)2 + NO + M ─> n-C3F7CF(OC2H4ONO2)CF(CF3)2 + M (6)

n-C3F7CF (OC2H4OO*)CF(CF3)2 + NO2 + M ─> n-C3F7CF(OC2H4O2NO2)CF(CF3)2 + M (7)

C3F7CF(OC2H4OO*)CF(CF3)2 + HO2 ─> products (8)

C3F7CF(OC2HOO*)CF(CF3)2 + R'O2 ─> products (9)

 

The predominant fate is reaction with oxygen and subsequent formation of alkoxy radicals by self- and cross-reaction:

 

n-C3F7CF(OCHO2*CH3)CF(CF3)2 + RO2 ─> n-C3F7CF(OCHO*CH3)CF(CF3)2 + RO + O2 (10)

n-C3F7CF(OCH2CH2O2*)CF(CF3)2 + RO2 ─> n-C3F7CF(OCH2CH2O*)CF(CF3)2 + RO + O2 (11)

 

There are several possible fates of these alkoxy radicals. For n-C3F7CF(OCHO*CH3)CF(CF3)2radicals, the possibilities are:

 

n-C3F7CF(OCHO*CH3)CF(CF3)2 + M ─> n-C3F7CF(OC(O)H)CF(CF3)2 + CH3* + M (12)

n-C3F7CF(OCHO*CH3)CF(CF3)2 + M  ─> n-C3F7CF(OC(O)CH3)CF(CF3)2 +H* + M (13)

n-C3F7CF(OCHO*CH3)CF(CF3)2 + O2  ─> n-C3F7CF(OC(O)CH3)CF(CF3)2 + HO2* (14)

 

while for n-C3F7CF(OCH2CH2O*)CF(CF3)2 radicals the possibilities are:

 

n-C3F7CF(OCH2CH2O*)CF(CF3)2 + M  ─> n-C3F7CF(OCH2*)CF(CF3)2 + HCHO + M (15)

n-C3F7CF(OCH2CH2O*)CF(CF3)2 + M  ─> n-C3F7CF(OCH2C(O)H)CF(CF3)2 + H* + M (16)

n-C3F7CF(OCH2CH2O*)CF(CF3)2 + O2  ─> n-C3F7CF(OCH2C(O)H)CF(CF3)2 + HO2* (17)

 

This transformation was oxygen-dependant, with the acetate being the major product.  The proposed esters were subjected to further indirect phototransformation.  No reaction of the acetate was observed under these experimental conditions, while the formate reacted more slowly than CAS# 297730-93-9.

 

Fate of phototransformation intermediates:

1. Photochemical transformations:  In the process of their examination of CAS# 297730-93-9 photolysis, Goto et al. (2002) examined indirect photolysis of putative formate and acetate esters using chlorine radical.  The formate ester had a rate constant of 9.7 x 10-15 cm3 molecule-1 s-1 (cf. 2.3 x 10-12 cm3 molecule-1 s-1 for CAS# 297730-93-9 itself).  The acetate was not observed to degrade, leading to a limiting rate constant of <6 x 10-17 cm3 molecule-1 s-1.  Other workers have confirmed slow phototransformation of perfluoroalkyl formates and acetates.  Chen et al.(2004) examined photolysis of authentic samples of perfluoroethyl and perfluoropropyl formate with hydroxyl radical.(5)  Both formate esters had rate constants at 272K of ca. 10-14 cm3 molecule-1 s-1.  By comparison with the reference chemical 1,1,1-trichloroethane, the two formates had estimated atmospheric lifetimes of 3.6 and 2.6 years, respectively.(5)

 

Chen et al. (2004) further noted direct photolysis of the ester group, but did not provide sufficient details of their experimental equipment to extrapolate to direct photolysis in the atmosphere.(5)

 

2. Hydrolytic transformations:  The hydrolytic instability of perfluoroalkyl esters due to inductive effects of the fluorine substituent has long been known.  Pavlik and Toren (1970) examined the effect of fluorine substituents on alkaline hydrolysis rates of acetates and trifluoroacetates.(6)  Hydrolysis rates were measured in a 1:1 mixture of acetone and water at pH 11 and at 5 °C and 25 °C.  First order rate constants of the three trifluoroacetates CF3C=O(OR) were too fast to be measured under experimental conditions.  Hydrolysis of 2,2,2-trifluoroethyl acetate ((CF3CH2O)C=OCH3) was 6 times faster than non-fluorinated ethyl acetate.  Hydrolysis of perfluoro-t-butyl acetate, with a half-life of approximately 4 minutes at 25 °C, was 80 times faster than ethyl acetate hydrolysis.  Uchimara et al. (2003) performed ab initio calculations on fluorinated and non-fluorinated methyl acetate compounds.(7)  They concluded that inductive effects of fluorine substitution would stabilize the tetrahedral hydrolysis intermediate and increase the reaction rate.  They speculated that neutral hydrolysis may also be potentiated by induction.(7)  Therefore, hydrolysis of dissolved ester is likely to occur on a timescale of minutes.  As a first approximation, the half-life of perfluoro-t-butyl acetate at pH 11 may be used directly, by assuming a 4-order of magnitude decrease in hydrolysis rate at near-neutral pH, coupled with a similar increase in the inductive fate for the C7 versus tertiary C4 perfluorocarbonyl.  However, the expected low water solubility limits the relevance of hydrolytic degradation.

 

Kutsuna and coworkers (2004) reported an experimental Henry’s Law constant of 175 m3 Pa/mol for 2,2,2-trifluoroethyl acetate.(8) Given a hydrolytic rate constant of approximately 5 x 10-5 s-1, and the measured HLC noted above, Kutsuna estimated an atmospheric half-life of >49 years.(8) Similar conclusions were made for 2,2,2-trifluoroethyl formate.(9)  The size, insolubility and Henry’s Law constant of CAS# 297730-93-9 (HLC 4.7 x 107 m3∙Pa/mol) and related highly fluorinated C7 compounds, suggest it is unlikely that the perfluoro-C7 acetate or formate would partition into atmospheric water.  This lack of partitioning into water results in a long hydrolytic half-life in the atmosphere.

 

3.  Relative importance:  Regarding the relative importance of photolytic and hydrolytic transformation, Kutsuna et al.(2005) measured hydrolysis and estimated the HLC of perfluoroethyl formate in water.(9)  Using these two numbers and the phototransformation rate determined by Chen et al.,(5) they determined that  hydrolytic half-life in the atmosphere is at least 20 times longer in cloud water than it is by photolysis.(9)  It is expected that hydrolysis of CAS# 297730-93-9-derived esters is less important, and that phototransformation is the predominant fate.  Based on the available data, the esters appear to be longer lived in the atmosphere than the parent compound.

 

Ultimate phototransformation products: 

Under the proposed degradation pathway, the predominant degradation products are perfluorobutyric acid (PFBA), hydrofluoric acid (HF) and trifluoroacetic acid (TFA). Short-chain perfluorocarboxylic acids react slowly with hydroxyl radicals (kOH ≈ 10-13 cm3∙molecule-1∙s-1) and are soluble in water.  Removal of acids is expected to be through depositional pathways (atmospheric half-life ca. 7 days) rather than photolysis (lifetime ca. 90 days).(10)

 

No test guideline is available for phototransformation in air.   The photooxidant studies were conducted using reliable, scientifically sound techniques commonly reported in the peer-reviewed literature.  The results of the key study were published in a leading peer-reviewed journal of environmental science and bears directly on reaction kinetics of CAS# 297730-93-9.  As academic studies, full details in depicted spectra, quantified results, and analytical methodology were lacking.  Therefore, the studies were classified as reliable with restrictions.  The supporting study provided summary and graphical data only, and the reliability of the study cannot be assessed at this time.

 

Table A1, Activation energy terms of segregated fluoroethers

Fluoroether

Ea/R (K)

Cl

OH

CF3OCH3

N/A

1750(3)

C3F7CH2OH

24(1)

1460\(4)

C3F7OCH3

445(1)

2130(4)

C4F9OCH3

225(2)

2200(4)

C4F9OCH2CH3

852(2)

2030(4)

C7F15OCH2CH3

1190

N/A

All values except CF3OCH3were determined using a consistent experimental approach

Table A2, Estimated kinetic data for CAS# 2967730-93-9 with Cl and HO∙ (based on data in Diaz-de-Mera et al., 2009

Temperature

(K)

2nd order reaction rate

(cm3 molecule-1 s-1)

1st order reaction rate1 (s-1)

Rate ratio

kCl

kOH

k'Cl

k'OH

(k'Cl/k'OH)

253

1.1 x 10-12

5.9 x 10-15

5.4 x 10-9

3.0 x 10-9

1.8

273

1.5 x 10-12

1.2 x 10-14

7.7 x 10-9

6.1 x 10-9

1.3

298

2.2 x 10-12

2.6 x 10-14

1.1 x 10-8

1.3 x 10-8

0.85

307

2.4 x 10-12

3.3 x 10-14

1.2 x 10-8

1.7 x 10-8

0.75

324

2.9 x 10-12

5.1 x 10-14

1.5 x 10-8

2.5 x 10-8

0.6

333

3.3 x 10-12

6.2 x 10-14

1.7 x 10-8

3.1 x 10-8

0.54

343

3.9 x 10-12

7.7 x 10-14

1.9 x 10-8

3.9 x 10-8

0.48

1, First-order rate constants estimated assuming global average concentrations of 5E+05 cm-3 and 5000 cm-3 for hydroxyl and chlorine radicals, respectively.

 

References:

1) Y. Díaz-de-Mera, A. Aranda, I. Bravo, D. Rodríguez, A. Rodríguez, E. Moreno. 2008.  Atmospheric chemistry of HFE-7000 (CF(3)CF(2)CF(2)OCH(3)) and 2,2,3,3,4,4,4-heptafluoro-1-butanol (CF(3)CF(2)CF(2)CH(2)OH): kinetic rate coefficients and temperature dependence of reactions with chlorine atoms.  Environ. Sci. Pollut. Res. Int. Vol. 15, No. 7, pp. 584-91.

 

2) A. Aranda, Y. Díaz-de-Mera, I. Bravo, D. Rodríguez, A. Rodríguez, E. Martinez. 2006.  Atmospheric HFEs Degradation in the Gas Phase: Reactions of HFE-7100 and HFE-7200 with Cl Atoms at Low Temperatures.  Environ. Sci. Technol. Vol. 40, pp. 5971-5976.

 

3) L. Chen, S. Kutsuna, K. Nohara, K. Takeuchi, T. Ibusuki.  2001.  Kinetics and Mechanisms for the Reactions of CF3OCH3 and CF3OC(O)H with OH Radicals Using an Environmental Reaction Chamber.  J. Phys. Chem. A  Vol. 105, pp. 10854-10859

 

4) I. Bravo, Y. Díaz-de-Mera, A. Aranda, K. Smith, K. P. Shine, G. Marston.  2010.  Atmospheric chemistry of C4F9OC2H5 (HFE-7200), C4F9OCH3 (HFE-7100), C3F7OCH3 (HFE-7000) and C3F7CH2OH: temperature dependence of the kinetics of their reactions with OH radicals, atmospheric lifetimes and global warming potentials.  Phys. Chem. Chem. Phys.  Vol. 12, pp. 5115 – 5125

 

5) L. Chen, S. Kutsuna, K. Tokuhashi, A. Sekiya.  2004.  Kinetics study of the gas-phase reactions of C2F5OC(O)H and n-C3F7OC(O)H with OH radicals at 253–328 K.  Chem. Phys. Lett.  Vol. 400, pp. 563-568.

 

6) J. Pavlik and  P. E. Toren.  1970.  Perfluoro-t-butyl Alcohol and its Esters.  J. Org. Chem.  Vol. 35, No. 6, pp. 2054-2056.

 

7) Uchimaru,  S. Kutsuna, A. K. Chandra,  Masaaki Sugie, Akira Sekiya.  2003.  Effect of fluorine substitution on the rate for ester hydrolysis: estimation of the hydrolysis rate of perfluoroalkyl esters.  J. Mol. Structure (Theochem) Vol. 635, pp.  83-89.

 

8) S. Kutsuna, L. Chen, K. Ohno, K. Tokuhashi, A. Sekiya.  2004.  Henry’s law constants and hydrolysis rate constants of 2,2,2-trifluoroethyl acetate and methyl trifluoroacetate.  Atm. Env. Vol. 38 pp. 725-732.

 

9) S. Kutsuna, L. Chen, T. Abe, J. Mizukado, T. Uchimaru, K. Tokuhashi, A. Sekiya.  2005.  Henry’s law constants of 2,2,2-trifluoroethyl formate, ethyl trifluoroacetate,and non-fluorinated analogous esters.  Atm. Env. Vol. 39 pp. 5884–5892.

 

10) M. D. Hurley, M. P. Sulbaek Andersen, T. J. Wallington, D. A. Ellis, J. W. Martin, S. A. Mabury.  2004.  Atmospheric chemistry of Perfluorinated Carboxylic Acids: Reaction with OH Radicals and Atmospheric Lifetimes.  J. Phys. Chem. A Vol. 108, pp. 615-620.