Correspondence/Rebuttal pubs.acs.org/est
Response to Comment on “Atmospheric Degradation of Perfluoro-2methyl-3-pentanone: Photolysis, Hydrolysis, and Hydration”
R
ayne has recently raised three concerns regarding our paper on the environmental degradation pathways of perfluoro-2-methyl-3-pentanone (PFMP).1 First, Rayne has suggested that the hydrolysis of PFMP does not proceed by the Haloform reaction since no molecular halogens nor α-CH3 groups were present. While this technically is true, the hydrolysis of PFMP is completely analogous to the second step of the Haloform reaction mechanism where a halogenated ketone is capable of base-catalyzed hydrolysis since the carbanion leaving group is stabilized by electron withdrawing groups. As we mentioned in our paper, “the overall mechanism of the reaction [hydrolysis of PFMP] is BAC2 which consists of separate addition and elimination steps via a tetrahedral intermediate”.1 Additionally, Saloutina et al., who first reported the basic hydrolysis of PFMP, reported the reaction as a “haloform cleavage”.2 Since ketones do not normally undergo hydrolysis reactions, our reason for reporting the hydrolysis of PFMP akin to a Haloform reaction was simply to assist the general reader by making a connection with a wellknown process. Rayne has suggested that hydration (gem-diol formation) is an important environmental fate of PFMP since the predicted Khyd of 0.26 suggests up to 27% of PFMP in aqueous solution at equilibrium will be hydrated. However, this statement assumes no environmental PFMP is present in the gas phase and that hydrate formation represents an irreversible sink for PFMP. Since PFMP has a high predicted air/water partition constant (KAW = 5300),3 very little PFMP in the environment will be present in the aqueous phase. In our original paper,1 we examined the Khyd for PFMP using computational methods to see whether the reaction would be irreversible in the environment, as is the case for hexafluoroacetone (log Khyd = 6.08).4 If PFMP had a similarly large Khyd value, then photolysis would likely be hindered as an environmental sink since the gem-diol form of PFMP does not absorb actinic radiation. However, since PFMP has an approximate Khyd of 0.26, it will be in equilibrium in the aqueous phase with its hydrate and therefore, can either undergo direct photolysis in the aqueous phase or readily partition back into the gas phase due to its high KAW. Therefore, direct photolysis will be the ultimate environmental sink for PFMP, as shown in calculations detailed in the Supporting Information for our paper.1 Lastly, Rayne predicted the pKa of the PFMP-diol to be 7.11 using a computational method. Rayne suggested that deprotonation of the PFMP-diol in the aqueous phase will bring more PFMP into solution by Le Chatelier’s principle, further increasing the importance of hydration. Unfortunately, this prediction does not take into account the large KAW for PFMP (5300)3, as well as the average pH of atmospheric water (5.6).5 The following three equilibrium reactions can be formulated which describe the partitioning of PFMP between the gas and aqueous phases along with hydrate formation and hydrate ionization. © 2013 American Chemical Society
PFMP(g) ⇌ PFMP(aq)
(1)
PFMP(aq) ⇌ PFMP‐diol(aq)
(2)
PFMP‐diol(aq) ⇌ PFMP‐diol‐anion(aq)
(3) −4
Using the equilibrium constants K1 = 1.89 × 10 (1/KAW), K2 = 0.26 (approximate Khyd for PFMP), and K3 = 0.03 (calculated for pKa = 7.11 at pH 5.6), we predict that 99.98% of PFMP will be in the gas phase with the remainder being aqueous, most of which is nonhydrated PFMP. In an extremecase scenario involving PFMP partitioning between the atmosphere and the ocean where the pH is 8.1, K3 = 9.8. Using these values, we demonstrate that while the deprotonation of PFMP does assist its migration into the aqueous phase as Rayne predicts, 99.93% of PFMP will still be gaseous with the remainder being aqueous. These calculations do not yet take into account the very low fraction of liquid water in the atmosphere, which further enhances the importance of photolysis, as detailed in the SI of our paper.1 Therefore, we still conclude that hydration is not a significant environmental sink for PFMP and that direct photolysis in the gas phase will dominate over hydrolysis in the aqueous phase, mostly because of the large KAW for PFMP and its relatively small Khyd.
Derek A. Jackson† Cora J. Young†,§ Michael D. Hurley‡ Timothy J. Wallington‡ Scott A. Mabury*,† †
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Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario, Canada M5S 3H6 ‡ Ford Motor Company, Mail Drop SRL-3083, Dearborn, Michigan, USA 48121
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: 416-978-1780. Present Address §
Department of Chemistry, Memorial University of Newfoundland, St. John’s, NL, Canada A1B 3X7 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank John Ginder for helpful discussions. While this article is believed to contain correct information, Ford Motor Company (Ford) does not expressly or impliedly warrant, nor assume any responsibility, for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, nor represent that its use would not infringe the Published: April 3, 2013 4954
dx.doi.org/10.1021/es4012965 | Environ. Sci. Technol. 2013, 47, 4954−4955
Environmental Science & Technology
Correspondence/Rebuttal
rights of third parties. Reference to any commercial product or process does not constitute its endorsement. This article does not provide financial, safety, medical, consumer product, or public policy advice or recommendation. Readers should independently replicate all experiments, calculations, and results. The views and opinions expressed are of the authors and do not necessarily reflect those of Ford. This disclaimer may not be removed, altered, superseded, or modified without prior Ford permission.
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REFERENCES
(1) Jackson, D. A.; Mabury, S. A. Atmospheric degradation of perfluoro-2-methyl-3-pentanone: Photolysis, hydrolysis, and hydration. Environ. Sci. Technol. 2011, 45, 8030−8036. (2) Saloutina, L. V.; Filyakova, T. I.; Zapevalov, A. Y.; Kodess, M. I.; Kolenko, I. P. Synthesis and some reactions of 2-X-perfluoro-2-methyl3-pentanones. Russ. Chem. Bull. 1982, 8, 1893−1896. (3) Cahill, T.; Mackay, D. Assessment of the Atmospheric Fate of Novec 1230, A report prepared for 3M in support of the registration of Novec 1230 in Canada; Canadian Environmental Modelling Centre, Trent University: Peterborough, ON, Canada. 2002. (4) Guthrie, J. P. Carbonyl addition reactions: factors affecting the hydrate−hemiacetal and hemiacetal−acetal equilibrium constants. Can. J. Chem. 1975, 53, 898−906. (5) De Bruyn, W. J.; Shorter, J. A.; Davidovits, P.; Worsnop, D. R.; Zahniser, M. S.; Kolb, C. E. Uptake of haloacetyl and carbonyl halides by water surfaces. Environ. Sci. Technol. 1995, 29, 1179−1185.
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