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Chemosphere 92, 2013, 1580 .pdf


Original filename: Chemosphere 92, 2013, 1580.pdf
Title: Comment on “Photolysis of Enrofloxacin in aqueous systems under simulated sunlight irradiation: Kinetics, mechanism and toxicity of photolysis products [Li et al., Chemosphere 85 (2011) 892–897]”
Author: Sierra Rayne

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Chemosphere 92 (2013) 1580

Contents lists available at SciVerse ScienceDirect

Chemosphere
journal homepage: www.elsevier.com/locate/chemosphere

Discussion

Comment on ‘‘Photolysis of Enrofloxacin in aqueous systems under
simulated sunlight irradiation: Kinetics, mechanism and toxicity
of photolysis products [Li et al., Chemosphere 85 (2011) 892–897]’’
Sierra Rayne ⇑
Chemologica Research, PO Box 74, 318 Rose Street, Mortlach, Saskatchewan, Canada S0H 3E0

In their article, Li et al. (2011) investigate the photolysis of the
antibiotic enrofloxacin and propose a mechanism for its photodegradation in pure water (see ‘‘Fig. 6.: Proposed transformation
mechanism for photolysis of Enro in pure water’’ in Li et al.
(2011)). There appear to be a number of problems with this
proposed mechanism. A mechanism must represent the actual
speciation(s) of the compound(s) under consideration in the experimental conditions being investigated. The pKa value of the carboxylic acid group in the 3-position on enrofloxacin is 5.94 ± 0.04, and
the pKa value of the basic piperazinyl group in the 7-position is
8.70 ± 0.44 (Lizondo et al., 1997). Thus, in pure water, the carboxylic acid group will either be effectively entirely dissociated to the
carboxylate anion, or in some intermediate state of dissociation
with significant populations of both the acid and anion forms present, and the piperazinyl moiety will be protonated to the cationic
amine form. In contrast, Li et al. (2011) only show the neutral
forms of each functional group in their mechanism.
Of substantial concern is the authors’ claim that enrofloxacin
will somehow undergo defluoridation (i.e., ‘‘-F-’’) to yield their
product 3. The authors suggest that the loss of a fluoride ion leads
to a subsequently unsubstituted benzene position in the resulting
product. If an anionic fluoride ion comes off in the mechanism, it
must be replaced by a hydride ion to yield the product 3 that Li
et al. (2011) claim to observe. What is the source of hydride ions
in pure water at room temperature? The authors’ mechanism to
give product 3 also shows a simultaneous loss of a cyclopropyl
group to yield cyclopropane, which must occur with both hydrogen atom addition to the cyclopropyl group (to give cyclopropane)
and to the decyclopropylated amine group to yield product 3?
What is the source of the hydrogen atoms for this reaction? Overall, this mechanism seems implausible.

⇑ Tel./fax: +1 306 690 0573.
E-mail address: sierra.rayne@live.co.uk
0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.chemosphere.2013.04.052

Li et al. (2011) claim photochemical loss of ‘‘HCOO-’’ (do the
authors mean COOH instead?) from enrofloxacin to give product
1. This is almost certainly not the actual mechanism, particularly
since the starting material carboxylic acid group would be deprotonated in pure water. A more likely mechanism yielding product
1 from enrofloxacin is a photodecarboxylation from the deprotonated enrofloxacin anion (i.e., from the form of enrofloxacin with
the carboxylate anion), yielding carbon dioxide and the corresponding decarboxylated enrofloxacin carbanion (this carbanion
would be partially stabilized by the a-carbonyl group), which is
subsequently protonated in pure water to give product 1.
Product 5 is highly unusual. The authors are proposing that the
cyclopropyl ring on enrofloxacin loses a –CH2– group to become an
ethyl substituent, while the piperazinyl group fragments to leave
behind just a methyl group. This seems a most unlikely photoproduct/mechanism, and may indicate an error in the structural assignment for this compound. The photochemical loss of an ethyl radical
from the piperazinyl group to give product 2 requires addition of a
hydrogen atom in order to balance the reaction (leading to questions regarding the source of the hydrogen atoms in pure water).
Product 4 warrants further discussion. What is this compound? It
appears to have an oxygen atom double bonded to nothing. The
authors are in need of clarifying what product 4 is.
References
Li, Y., Niu, J., Wang, W., 2011. Photolysis of Enrofloxacin in aqueous systems under
simulated sunlight irradiation: kinetics, mechanism and toxicity of photolysis
products. Chemosphere 85, 892–897.
Lizondo, M., Pons, M., Gallardo, M., Estelrich, J., 1997. Physicochemical properties of
enrofloxacin. J. Pharmaceut. Biomed. 15, 1845–1849.


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