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Correspondence/Rebuttal
Comment on “Atmospheric degradation of perfluoro-2methyl-3-pentanone: Photolysis, hydrolysis and hydration” Sierra Rayne Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es4009344 • Publication Date (Web): 03 Apr 2013 Downloaded from http://pubs.acs.org on April 17, 2013
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Comment on “Atmospheric degradation of perfluoro-2-methyl-3-pentanone: Photolysis, hydrolysis and hydration” Sierra Rayne Chemologica Research, PO Box 74, 318 Rose Street, Mortlach, Saskatchewan S0H 3EO, Canada.
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In their article, Jackson et al. 1 use density functional theory (DFT) calculations to investigate the potential hydration of a perfluorinated carbonyl compound, perfluoro-2-methyl-3-pentanone (PFMP). The authors state that “[t]he hydrolysis reaction of PFMP can proceed by the wellknown Haloform reaction mechanism.” On the contrary, the Haloform mechanism is where a haloform is produced by the exhaustive halogenation of a methyl ketone in the presence of a base (see, e.g., http://www.organic-chemistry.org/namedreactions/haloform-reaction.shtm and http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch18/ch18-3-2b.html). PFMP is a perfluorinated compound that hydrolyzes via a traditional base-catalyzed mechanism. As there are no α-CH3 groups in PFMP, and since no molecular halogens were present in solution during the hydrolysis in ref. 1, and because no halogenation is occurring during the process, the hydrolysis of PFMP does not proceed via the Haloform reaction.
The authors go on to state that “hydration is not an important environmental fate of PFMP.” Yet, the authors' own data does not appear to support this generalization. Jackson et al. 1 calculate aqueous phase hydration equilibrium constants (log Khyd) for the hydration of PFMP at between -0.43 and -0.85 using the B3LYP/6-311++G(d,p) level of theory in the aqueous phase with the PCM solvent model. The SPARC hydration module (http://archemcalc.com/sparc/; September 2009 release w4.5.1522-s4.5.1522) yields a log Khyd of -0.59, in good agreement with the findings of Jackson et al. 1. Collectively, these results suggest that up to 27% of the total PFMP in aqueous solution (e.g., freshwater and marine systems, groundwaters, precipitation and
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other atmospheric forms of water) will be present in the hydrated form, which is hardly unimportant.
In addition, as has been previously shown,2 geminal diols of perfluoroalkyl compounds are mildly acidic. Using the SPARC pKa module (which was validated and calibrated for geminal diol pKa values in ref. 2), a raw pKa of 7.56 is calculated for the hydrate of PFMP, which is then adjusted to a final value of 7.11 using the regression equation in ref. 2. Consequently, in nearneutral and higher pH waters, the acidic PFMP hydrate will dissociate to the conjugate base, shifting the equilibrium of the initial hydration reaction (i.e., Le Chatelier's principle) to produce more of the undissociated hydrate, and thereby bringing more PFMP into solution as either the undissociated or dissociated hydrate form.
As such, the data presented in ref. 1 do not appear to support the authors' own claims of hydration not being a significant component of the enviromental fate of PFMP.
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AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected]. Tel.: + (1) 306 690 0573. Fax: + (1) 306 690 0573.
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REFERENCES (1)
Jackson, D.A., Young, C.J., Hurley, M.D., Wallington, T.J., Mabury, S.A. Atmospheric
degradation of perfluoro-2-methyl-3-pentanone: Photolysis, hydrolysis and hydration. Environ. Sci. Technol., 2011, 45, 8030-8036. (2)
Rayne, S., Forest, K. Aqueous phase hydration and hydrate acidity of perfluoroalkyl and
n:2 fluorotelomer aldehydes. Nature Precedings, 2010, doi: 10101/npre.2010.4520.1.
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