Atmospheric sink of methyl chlorodifluoroacetate and ethyl chlorodifluoroacetate: Temperature dependent rate coefficients, product distribution of their reactions with Cl atoms and CF2ClC(O)OH formation

Article abstract of DOI:10.1039/c6ra03454c

Rate coefficients as a function of temperature have been measured for the first time for the gas-phase reactions of chlorine atoms with methyl chlorodifluoracetate (k₁) and ethyl chlorodifluoroacetate (k₂) using the relative rate technique. The experiments were carried out in a 1080 L photoreactor over the temperature range 287-313 K at a total pressure of 1000 ± 10 mbar of synthetic air using in situ FTIR spectroscopy to monitor reactants and products. The following Arrhenius expressions were obtained: k_(MCDFA+Cl) = (9.6 ± 5.1) × 10^-12exp[-(1363 ± 79)/T] and k_(ECDFA+Cl) = (64.4 ± 29.7) × 10^-12exp[-(1110 ± 68)/T]. The kinetic results are compared with previous experimental and theoretical studies. In addition, a product study of the reactions of Cl with methyl chlorodifluoracetate and ethyl chlorodifluoroacetate is reported. The results indicate that in the absence of NO_x the main fate of the alkoxy radicals formed after H-atom abstraction by Cl from the -CH₃ group in methyl chlorodifluoroacetate is reaction of the radical with O₂ to form the mixed anhydride CF₂ClC(O)OC(O)H. In the case of ethyl chlorodifluoroacetate the main fate of the alkoxy formed via H-atom abstraction by Cl from the -CH₂- entity in the ethyl group is α-ester rearrangement to produce chlorodifluoroacetic acid and the corresponding radical. The yields of chlorofluoracetic acid (CF₂ClC(O)OH) obtained were as follows: (34 ± 5)% and (86 ± 8)% for the reactions of Cl with CF₂ClC(O)OCH₃ and CF₂ClC(O)OCH₂CH₃, respectively. The measured yields are rationalized in terms of mechanisms consisting of competitive reaction channels for the formed products in the oxidation, i.e. reaction with O₂, α-ester rearrangement and a decomposition pathway. Atmospheric implications are discussed according to the rate coefficients obtained as a function of temperature and altitude, and regarding the formation of chlorofluorocarboxylic acid.

Full text of DOI:10.1039/c6ra03454c

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Atmospheric sink of methyl chlorodifluoroacetate
and ethyl chlorodifluoroacetate: temperature
dependent rate coefficients, product distribution of
their reactions with Cl atoms and CF2ClC(O)OH
formation†
Cite this: RSC Adv., 2016, 6, 51834
a
b
b
a
*
Marıa B. Blanco, Ian Barnes, Peter Wiesen and Mariano A. Teruel
´
Rate coefficients as a function of temperature have been measured for the first time for the gas-phase
reactions of chlorine atoms with methyl chlorodifluoracetate (k1) and ethyl chlorodifluoroacetate (k2)
using the relative rate technique. The experiments were carried out in a 1080 L photoreactor over the
temperature range 287–313 K at a total pressure of 1000 ꢀ 10 mbar of synthetic air using in situ FTIR
spectroscopy to monitor reactants and products. The following Arrhenius expressions were obtained:
k(MCDFA+Cl) ¼ (9.6 ꢀ 5.1) ꢁ 10ꢂ12 exp[ꢂ(1363 ꢀ 79)/T] and k(ECDFA+Cl) ¼ (64.4 ꢀ 29.7) ꢁ 10ꢂ12 exp
[ꢂ(1110 ꢀ 68)/T]. The kinetic results are compared with previous experimental and theoretical studies. In
addition,
a product study of the reactions of Cl with methyl chlorodifluoracetate and ethyl
chlorodifluoroacetate is reported. The results indicate that in the absence of NOx the main fate of the
alkoxy radicals formed after H-atom abstraction by Cl from the –CH3 group in methyl
chlorodifluoroacetate is reaction of the radical with O2 to form the mixed anhydride CF2ClC(O)OC(O)H.
In the case of ethyl chlorodifluoroacetate the main fate of the alkoxy formed via H-atom abstraction by
Cl from the –CH2– entity in the ethyl group is a-ester rearrangement to produce chlorodifluoroacetic
acid and the corresponding radical. The yields of chlorofluoracetic acid (CF2ClC(O)OH) obtained were as
follows: (34 ꢀ 5)% and (86 ꢀ 8)% for the reactions of Cl with CF2ClC(O)OCH3 and CF2ClC(O)OCH2CH3,
respectively. The measured yields are rationalized in terms of mechanisms consisting of competitive
reaction channels for the formed products in the oxidation, i.e. reaction with O2, a-ester rearrangement
and a decomposition pathway. Atmospheric implications are discussed according to the rate coefficients
Received 5th February 2016
Accepted 17th May 2016
DOI: 10.1039/c6ra03454c
obtained as
a
function of temperature and altitude, and regarding the formation of
www.rsc.org/advances
chlorofluorocarboxylic acid.
replacement materials in many applications such as solvents,
heat-transfer uids in refrigeration systems, cleaning agents in
the electronic industry, foam-blowing, lubricant deposition,
and dry etching agents and as anesthetics.1,2
Introduction
Oxygenated Volatile Organic Compounds (OVOCs) such as
hydrouoroethers (HFEs) and hydrochlorouoroethers (HCFEs)
have an oxygen linkage which makes the hydrogen atoms of the
ether molecule more labile and increases their reactivity toward
OH radicals and Cl atoms in the lower atmosphere compared to
chlorouorocarbons (CFCs) and hydrochlorouorocarbons
(HCFCs). This increased atmospheric photooxidation potential
of HFEs and HCFEs over CFCs and HCFCs makes them good
The hydroxyl radical (OH) is the primary cleaning agent of
the lower atmosphere and is thus the dominant sink for many
greenhouse gases and pollutants. In coastal and marine regions
and in inland areas near to pollutant sources the concentration
of Cl can attain concentration levels which make reactions with
Volatile Organic Compounds (VOCs) competitive with the OH-
radical initiated degradation of these VOCs.3–6 In recent years,
evidence has been presented for Cl-atom chemistry in conti-
nental regions far removed from coastal and marine regions
where observations of Cl-atom precursors, have been made.7–10
Additionally, urban sources of Cl atoms due to industrial
activities can exist, and Cl concentrations higher than the
concentration levels observed in marine regions have been re-
ported, for example, in the vicinity of ceramic industries.11
a
´
CONICET, Instituto de Investigaciones en Fisicoquımica, I.N.F.I.Q.C., Facultad de
´
´
Ciencias Quımicas, Universidad Nacional de Cordoba, Ciudad Universitaria, 5000
´
Cordoba, Argentina. E-mail: mteruel@fcq.unc.edu.ar; Fax: +54-351-4334188
b
¨
¨ ¨
Bergische Universitat Wuppertal, Fakultat fur Mathematik und Naturwissenschaen,
Physikalische & Theoretische Chemie, Gauss Strasse 20, 42119 Wuppertal, Germany
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra03454c
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Chlorinated uoroesters (CFESs) are emitted to the atmo- seals and is closed at both ends by aluminum anges which
sphere mainly by anthropogenic sources. Methyl chlorodi- contain inlet ports for reactants etc. The reactor can be
uoroacetate (MCDFA) is used as a building block in organic temperature regulated within the range 287 to 313 K with
synthesis12 and ethyl chlorodiuoroacetate (ECDFA) is a precision of ꢀ2 K through a regulation system using three
employed extensively in re retardants and also nds use in the PT-100 thermo-elements. The chamber can be evacuated by
production of pesticides, dyestuffs, phosphates and medicine.13 a pumping system consisting of a turbo-molecular pump
MCDFA and ECDFA can be produced directly in the tropo- backed by a double stage rotary fore pump to 10ꢂ3 Torr. The
sphere through the photooxidation of hydrochlorouoroethers photolysis system consists of 32 superactinic uorescent lamps
analogous to the formation of uoroesters from HFEs.14,15 In (Philips TL05 40 W), which are evenly spaced around the reac-
order to evaluate if HCFEs are environmentally acceptable tion vessel. The lamps emit light with a maximum intensity at
replacements for CFCs and HCFCs, rate coefficients and 360 nm. The chamber is equipped with a White type multiple-
product distributions for the reactions of the CFESs with the reection mirror system for sensitive in situ long path absorp-
main tropospheric oxidants are necessary.
tion monitoring of reactants and products in the IR spectral
Hence, as part of a systematic study on the kinetics and range 4000–700 cmꢂ1 and was operated at total optical path
oxidation products of the atmospheric reactions of haloesters, length of 484.7 ꢀ 0.8 m. IR spectra were recorded with a spectral
we report in this work, rate coefficients for the reactions of Cl resolution of 1 cmꢂ1 using a Nicolet Nexus FT-IR spectrometer
atoms with methyl chlorodiuoroacetate and ethyl chlorodi- equipped with a KBr beam splitter and a liquid nitrogen cooled
uoracetate measured over the temperature range 287–313 K mercury–cadmium–telluride (MCT) detector.
using a relative kinetic technique:
ClCF2C(O)OCH3 + Cl / products
ClCF2C(O)OCH2CH3 + Cl / products
The rate coefficients for the reaction of Cl atoms with the
chlorouoroacetates were determined using the relative kinetic
technique. Chlorine atoms were generated by photolysis of
molecular Cl2:
(1)
(2)
Cl2 + hn / 2Cl
(3)
This study extends our previous kinetic database for the title
reactions at 298 K16 through provision of the temperature
dependence for the reactions.
In the presence of Cl atoms, the chlorouoroacetates and the
reference compounds decay through the following reactions:
To the best of our knowledge, the temperature dependence
of the reactions has not been previously reported. Although,
there are many kinetic and mechanistic studies of the reactions
of Cl atoms with a variety of hydrogenated esters, the database
for haloesters is very limited. Furthermore, there is no infor-
mation available on the degradation mechanisms of chloro-
uorinated esters in the atmosphere and the resulting products.
Therefore, the main reaction products of the haloesters have
been identied and their yields quantied. This information
has been used to propose atmospheric degradation mecha-
nisms for the Cl-atom mediated oxidation of the compounds.
Formation of chlorodiuoracetic acid has been observed in
high yields. Field measurements of the acid in precipitation
suggest that it may accumulate in the aquatic enviroment.17
The aim of the present work is to determine the fate of the
chlorouoroalkoxy radical formed in the Cl-initiated photo-
degradation of the chlorouoroacetates studied, and appraise
its possible contribution to the accumulation in the environ-
ment of chlorouorinated compounds such as chlorodiuor-
acetic acid (CDFA).
Cl + chlorofluoroacetate / products, kchlorofluoroacetate
Cl + reference / products, kreference
(4)
(5)
Provided that the reference compound and the reactant are
lost only by reactions (4) and (5), then it can be shown that:
(
)
ꢀ
ꢁ
½chlorofluoroacetateꢃ
kchlorofluoroacetate
kreference
½referenceꢃ
½chlorofluoroacetateꢃ0
½referenceꢃ0
ln
¼
ln
t
t
(I)
where, [chlorouoroacetate]0, [reference]0, [chlorouoroacetate]t
and [reference]t are the concentrations of the halogenated esters
and reference compound at times t ¼ 0 and t, respectively and
kchlorouoroacetate and kreference are the rate coefficients of reactions
(4) and (5), respectively.
The relative rate technique relies on the assumption that
both the chlorouoroacetates and reference organics are
removed solely by reaction with Cl atoms. To verify this
assumption, mixtures of chlorine and both organics (chloro-
uoroacetate and reference hydrocarbon) in air were prepared
and allowed to stand in the dark for 30 minutes the typical
duration of an experiment. In all cases, reaction of the organic
species with the Cl-atom precursor Cl2 in the absence of UV
light was of negligible over this time period. Additionally, to test
for possible photolysis of the reactants mixtures of the chloro-
uoroesters and reference compounds in air in the absence of
molecular chlorine were irradiated using the output from all the
uorescent lamps surroundings the chamber for 30 min.
Photolysis was not observed for any of the reactants.
Experimental
The experiments were performed in a 1080 L photoreactor
a detailed description of which can be found elsewhere thus
only a brief general overall description is given here.18
A
diagram of the reactor is presented below (Scheme 1). The
reactor has a total length of 6.2 m and an internal diameter of
0.47 m, it consists of two quartz glass cylinders which are
connected by a central enamel ange ring and silicon rubber
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Scheme 1 Schematic diagram of the 1080 L quartz-glass reactor.
For the kinetic investigations chloromethane was used as the atoms with chloromethane. Fig. S1 and S2† show examples of
reference compound and experiments were performed on the kinetic data obtained at three temperatures (287, 303 and
Cl2/chlorouoroester/chloromethane/air mixtures which were 313 K) for both methyl chlorodiuoroacetate and ethyl chlor-
irradiated for periods of 15–20 minutes during the course of odiuoroacetate plotted according to eqn (I). The rate coeffi-
which infrared spectra were recorded with the FTIR spectrom- cient ratios kCFES/kreference obtained from linear least squares
eter. Typically 64 interferograms were co-added per spectrum analyses of such plots are listed in Table 1. The rate coefficient
over a period of approximately 1 minute. Similar experiments ratios are each from a minimum of at least three or four
were performed for the product studies without the reference experiments for each temperature and CFES and the errors are
compound in the reaction mixtures.
the 2s statistical errors from the linear regression analyses.
The rate coefficients for the reaction of Cl atoms with the
chlorouoroacetates listed in Table 1 have been put on an
absolute basis using rate coefficients for Cl with chloromethane
at the respective temperatures calculated from the following
Arrhenius expression (with k in cm3 per molecule per s units):
Results and discussion
Temperature dependence of the reactions of Cl atoms with
MCDFA and ECDFA
Rate coefficients for the reactions of Cl atoms with MCDFA and
ECDFA were determined at atmospheric pressure over the
temperature range of 287–313 K relative to the reaction of Cl
k(Cl + chloromethane) ¼ 3.3 ꢁ 10ꢂ11 eꢂ(1250)/T cm3 per molecule
per s.19
Table 1 Temperature dependent rate coefficient ratios, kCFES/kreference, and rate coefficients, kCFES, and the resulting Arrhenius parameters for
the gas-phase reactions of Cl atoms with methyl chlorodifluoroacetate (MCDFA) and ethyl chlorodifluoroacetate (ECDFA)
kCFES ꢁ 1013
A ꢁ 1012
T (K)
kCFES/kreference
(cm3 per molecule per s)
CFES
Ea/R (K)
(cm3 per molecule per s)
Methyl chlorodiuoroacetate
287
294
298
303
313
287
294
298
303
313
0.19 ꢀ 0.01
0.20 ꢀ 0.02
0.20 ꢀ 0.01
0.20 ꢀ 0.02
0.21 ꢀ 0.02
3.14 ꢀ 0.05
3.08 ꢀ 0.04
3.13 ꢀ 0.04
3.12 ꢀ 0.03
3.06 ꢀ 0.04
0.83 ꢀ 0.36
0.93 ꢀ 0.41
0.98 ꢀ 0.44
1.12 ꢀ 0.49
1.24 ꢀ 0.52
13.53 ꢀ 4.36
14.29 ꢀ 4.38
15.59 ꢀ 4.43
17.10 ꢀ 4.52
18.64 ꢀ 4.69
1363 ꢀ 79
1110 ꢀ 68
9.6 ꢀ 5.1
Ethyl chlorodiuoroacetate
64.4 ꢀ 29.7
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the best of our knowledge there are no prior experimental
The errors given for the rate coefficients are a combination of determinations of the Arrhenius parameters for these reactions,
the 2s statistical errors from the linear regression analyses plus hence, this study represents the rst temperature dependence
an added uncertainty to take into account the error associated study of the reactions of Cl atoms with MCDFA and ECDFA.
with the values of the reference rate coefficients.
The positive Ea/R values of (1363 ꢀ 79) K and (1110 ꢀ 68) K
The values of the rate coefficients for the reactions of Cl for the reaction of Cl atoms with MCDFA and ECDFA, respec-
atoms with methyl chlorodiuoroacetate and ethyl chlorodi- tively, indicates the dominance of a hydrogen abstraction
uoroacetate as a function of temperature are plotted in pathway under the experimental conditions i.e. the rate coeffi-
Arrhenius form in Fig. 1. Linear least-squares analysis of the cients increase slightly with temperature.
plots leads to the following Arrhenius expressions in units of
cm3 per molecule per s:
The Ea/R values of 1363 K and 1110 K reported in this work
for the reactions of MCDFA + Cl and ECDFA + Cl, respectively
are consistent with the available Ea/R literature data for the
reactions of Cl atoms with hydrouoroethers and uoroesters
where Ea/R values of 852, 1190, 2254, 968, 1216, 1637 and 1368 K
have been reported for the reactions of Cl atoms with HFE-7200
(C4F9OC2H5), HFE-7500 (n-C3F7CF(OC2H5)CF(CF3)2), HFE-7100
(C4F9OCH3), HFE-7300 (n-C2F5CF(OCH3)CF(CF3)2), CF3C(O)
OCH3, CF3C(O)OCH2CH3 and CF2HC(O)OCH3, respectively.20–23
The employment of temperature dependent kinetic data in
model calculations increases the accuracy of the models since
the temperature gradient of the troposphere and the seasonal
temperature variations can be taken into consideration.
k(MCDFA+Cl) ¼ (9.6 ꢀ 5.1) ꢁ 10ꢂ12 exp[ꢂ(1363 ꢀ 79)/T]
k(ECDFA+Cl) ¼ (64.4 ꢀ 29.7) ꢁ 10ꢂ12 exp[ꢂ(1110 ꢀ 68)/T]
The errors in the activation term and the pre-exponential
factor are the 2s random statistical errors from ts to the data
presented in Table 1 and plotted in Fig. 1 (panels a and b). To
Comparison with previous kinetic determinations
The following rate coefficients have been obtained at 298 K for
the reaction of Cl with MCDFA and ECDFA:
k1 ¼ (0.98 ꢀ 0.44) ꢁ 10ꢂ13 cm3 per molecule per s
k2 ¼ (15.55 ꢀ 4.43) ꢁ 10ꢂ13 cm3 per molecule per s
The values of k1 and k2 at room temperature are in very good
agreement with our previously reported values16 of k(Cl+MCDFA)
¼
(1.0 ꢀ 0.2) ꢁ 10ꢂ13 and k(Cl+ECDFA) ¼ (15 ꢀ 3) ꢁ 10ꢂ13 cm3 per
molecule per s16 obtained at 298 K and 750 Torr total pressure in
a 80 liter Teon bag using the relative kinetic method with gas
chromatography/ame ionization detection (RR-GC/FID) for the
analyses. These two measurements are represented in Fig. 1 by
lled symbols, for the reaction of Cl with MCDFA (Fig. 1a) and
ECDFA (Fig. 1b), respectively. Mishra et al.24 have calculated the
rate coefficient for H-atom abstraction from CF2ClC(O)OCH3 by
Cl atoms using canonical transition state theory (CTST) and
employing Wigner25 and Eckart26 symmetrical methods which
gave values of 0.96 ꢁ 10ꢂ13 and 1.20 ꢁ 10ꢂ13 cm3 per molecule
per s, respectively, at 298 K for the reaction. These values are in
very good agreement with the experimental value of 0.98 ꢁ
10ꢂ13 cm3 per molecule per s reported in this work and the
previous experimental value reported by our group, using
a different experimental setup, of (1.0 ꢀ 0.2) ꢁ 10ꢂ13 cm3 per
molecule per s.19
In another study Mishra et al.27 have reported rate coeffi-
cients for H-atom abstraction from the ethyl group in CF2ClC(O)
OCH2CH3 by Cl atoms at 298 K. They reported rate coefficients
of 3.88 ꢁ 10ꢂ14 and 1.82 ꢁ 10ꢂ12 cm3 per molecule per s for the
reaction channels CF2ClC(O)OCH2CH3 + Cl / CF2ClC(O)
OCH2CH2 + HCl and CF2ClC(O)OCH2CH3 + Cl / CF2ClC(O)
Fig. 1 Arrhenius plots of the temperature dependent kinetic data
obtained for the reactions of Cl atoms with MCDFA (a) and ECDFA (b)
in the present study. For comparison, the room temperature rate
coefficient from our previous work16 is also shown (filled symbol).
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OCHCH3 + HCl, respectively. Mishra et al.27 reported a value of
1.86 ꢁ 10ꢂ12 cm3 per molecule per s for the overall rate coeffi-
cient of the reaction (i.e. the sum of the rate coefficients for each
reaction channel) which is in very good agreement with our
previous experimental value of (1.5 ꢀ 0.3) ꢁ 10ꢂ13 cm3 per
molecule per s16 and the rate coefficient of (1.55 ꢀ 0.44) ꢁ 10ꢂ13
cm3 per molecule per s reported in this work.
The RC(O)O– group is known to have a deactivating effect
on C–H bonds positioned both a and also b to the group.28 It
is interesting to compare the rate coefficients for Cl with the
two chlorodiuoroesters with those of methyl acetate (MA)
and ethyl acetate (EA). The reactions of Cl with MA and EA
have rate coefficients of 2.2 ꢁ 10ꢂ12 and 1.76 ꢁ 10ꢂ11 cm3 per
molecule per s, respectively, whereby the contribution from
Product distribution studies
Fig. 2 shows typical product spectra recorded in the Cl-atom
initiated oxidation of CF2ClC(O)OCH3. Trace (A) shows
a product spectrum obtained aer irradiation of a Cl2/
chlorouoroester/air mixture and subtraction of residual
MCDFA bands. It can be seen from a comparison of the spec-
trum with reference spectra of chlorodiuoroacetic acid
(trace (B)) and carbon monoxide (trace (C)) that these two
compounds are products formed in the Cl + MCDFA reaction.
Trace (D) is the residual spectrum that is obtained aer
subtraction of known products from the spectrum in trace (A).
Concentration–time proles of MCDFA and the identied
products are shown in Fig. S3.† Plots of the concentration of the
identied products as a function of reacted MCDFA (Fig. S4 in
ESI†) are linear supporting that they are primary products.
From a minimum of 3 experiments averaged molar formation
yields of (34 ꢀ 5)% and (26 ꢀ 3)% were obtained for chlorodi-
uoracetic acid and carbon monoxide, respectively.
In the reaction of Cl atoms with ethyl chlorodiuoroacetate,
chlorodiuoracetic acid and carbon monoxide have been also
identied as products. Fig. S5,† trace (A), shows an IR spectra
acquired aer irradiation of a mixture of ECDFA and Cl2 in air
where residual absorptions due to ECDFA have been subtracted.
Fig. S5,† trace (B), shows a reference spectra of chlorodiuoro-
acetic acid (CF2ClC(O)OH), trace (C) a reference spectra of
carbon monoxide and trace (D) the residual spectrum that is
obtained aer subtraction of known products from the spec-
trum in trace (A).
H-atom abstraction from the acetyl group CH3C(O)O is <1 ꢁ
10ꢂ13
.
The rate coefficients for the reactions of Cl atoms
28
with MCDFA and ECDFA are factors of approximately 22 and
11 slower than those of Cl with MA and EA, respectively,
indicating the strong deactivating effect of the ClF2C– group
on the reactivity of the chlorodiuoroesters toward Cl. It is
also interesting to note when comparing rate coefficients for
halogenated esters23 that k(Cl + ClF2CC(O)OCH3) > k(Cl +
F3CC(O)OCH3) and k(Cl + ClF2CC(O)OCH2CH3) > k(Cl +
F3CC(O)OCH2CH3) supporting an expected stronger negative
inductive effect for F3C– over ClF2C–.
Concentration–time proles of ECDFA and the identied
products chlorodiuoroacetic acid and carbon monoxide are
shown in Fig. S6 in the ESI.† The concentration–time contour of
chlorodiuoroacetic acid and the linearity of the plot of its
concentrations as a function of reacted ECDFA support that it is
a primary product. Carbon monoxide shows the same behavior,
however, as discussed later the CO is thought to result from the
fast reaction of Cl atoms with HCHO. Analysis of the slopes of
the plots shown in Fig. S7† give product yields of (86 ꢀ 8)% for
chlorodiuoroacetic acid and (34 ꢀ 6)% for carbon monoxide in
the reaction of Cl with ethyl chlorodiuoroacetate.
Table 2 lists the yields of the products formed in the reac-
tions of MCDFA and ECDFA with Cl atoms. The errors quoted
are a combination of the 2s statistical errors from the regres-
sion analysis plus a contribution to take into account the
uncertainty in the spectral subtraction procedure.
In the atmosphere the oxidation of the saturated esters is
mainly initiated by reaction with OH radicals and proceeds via
H-atom abstraction from the alkyl groups (–CH–, –CH2–, –CH3)
in the ester forming alkyl radicals. The reactions of the hal-
oesters studied with Cl atoms are less selective than the anal-
ogous OH reactions but the subsequent chemistry is similar.
Thus, although reactions of Cl-atoms with the esters are not so
atmospherically important as the OH reactions, studies of their
mechanisms are still necessary and are a convenient way to
emulate OH-radical initiated chemistry.
Fig. 2 Trace (A) shows the infrared spectrum of a MCDFA/Cl2/air
reaction mixture after irradiation and subtraction of residual methyl
chlorodifluoroacetate. Trace (B) shows a reference spectra of chlor-
odifluoroacetic acid (CF2ClC(O)OH), trace (C) a reference spectrum of
carbon monoxide and trace (D) the residual product spectrum ob-
tained after subtraction of features due to chlorodifluoroacetic acid
and carbon monoxide from the spectrum in trace (A).
Under atmospheric conditions alkyl radicals react with
oxygen to give peroxy radicals. The peroxy radicals formed can
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Table 2 Formation yields of the oxidation products identified in the
Cl-atom initiated oxidation of MCDFA and ECDFA at 298 K and
atmospheric pressure of synthetic air
formed in the reaction of MCDFA + Cl. The observation of the
formation of near equivalent amounts of chlorodiuoracetic
acid and carbon monoxide around 30% (see Table 2) supports
that the CF2ClC(O)OCH2Oc radicals undergoes an a-ester rear-
rangement (Fig. 3, channel A) forming the corresponding acid
and HCO radicals which will react further with O2 to give CO
and HO2 radicals. This is comparable to the acetic acid yield of
around 30% obtained by Christensen et al. in their study on the
reaction of Cl with methyl acetate in the absence of NO31 and
a triuoroacetic acid yield of ꢄ25% obtained by our group in
a previous study on the reaction of Cl with methyl tri-
uoroacetate under similar conditions to this work.32
The possible C–O bond cleavage decomposition channel of
the CF2ClC(O)OCH2Oc radical to form the CF2ClC(O)Oc radical
and HCHO has been omitted in Fig. 3 since formation of
formaldehyde was not observed in the reaction system. The
CF2ClC(O)Oc radical can decompose to form CO2 and the CF2Clc
radical which can react with O2 to form carbonyl uoride
(CF2O).32 Carbonyl uoride was observed in the system but its
formation was clearly from secondary reactions. In addition,
a previous study on the fate of the analogous alkoxy radical
CH3C(O)OCH2Oc formed in the oxidation of methyl acetate has
shown that the decomposition channel was also negligible.31
The reaction of CF2ClC(O)OCH2Oc radicals with O2 will lead
to the formation of the mixed acid anhydride CF2ClC(O)OC(O)H
(Fig. 3, channel B). Since a reference spectrum for CF2ClC(O)
OC(O)H is not commercially available, it was not possible to
directly assign absorptions bands in the product spectrum to
this compound. Preparative methods are known for mixed
anhydrides such as CF2ClC(O)OC(O)H33 unfortunately infrared
spectra for the compounds have not yet been reported and it
was beyond the scope of the present work to synthesize CF2-
ClC(O)OC(O)H. However, anhydrides are known important
products in the Cl-atom mediated oxidation of alkyl acetates
Chlorodiuoroacetate
Products identied
Yield (%)
CF2ClC(O)OCH3
(methyl chlorodiuoroacetate)
CF2ClC(O)OCH2CH3
CF2ClC(O)OH
CO
CF2ClC(O)OH
CO
34 ꢀ 5
26 ꢀ 3
86 ꢀ 8
34 ꢀ 6
(ethyl chlorodiuoroacetate)
react with NO, NO2, HO2, and other peroxy radicals forming, to
a large extent, the corresponding alkoxy radicals.29 In the
present NOx free reaction systems the alkoxy radicals will be
formed by reaction of the peroxy radicals with RO2/HO2 and
those shown in Schemes 2 and 3 for MCDFA and ECDFA,
respectively, are expected to be dominant.
Alkoxy radicals are known to react by three different path-
ways: (i) reaction with oxygen forming a carbonyl compound
and HO2 radicals, (ii) decomposition via C–C bond cleavage
producing a stable product and a radical and, (iii) if possible,
isomerization via a six-membered transition state producing
polyfunctional compounds. In addition in the case of esters, the
alkoxy radical, formed by H-atom abstraction from the carbon
attached to the non-carbonyl oxygen of the ester entity, can
undergo what is known as an a-ester rearrangement30 to form
an acid and a radical. The products identied in the irradiation
of MCDFA/Cl2/air and ECDFA/Cl2/air mixtures allow the eluci-
dation of the fate of the alkoxy radicals.
a
Atmospheric fate of CF2ClC(O)OCH2Oc radicals. Fig. 3
shows the possible fate of the CF2ClC(O)OCH2Oc radicals
Scheme 2 Formation of alkoxy radicals in reaction of Cl + MCDFA.
Fig. 3 Atmospheric fate of the alkoxy radicals formed after H-atom
abstraction by Cl from the –CH3 group in MCDFA. The dashed boxes
Scheme 3 Formation of alkoxy radicals in the reaction of Cl + ECDFA. indicate the identified and quantified products formed.
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and alkyl haloacetates.30,31 The residual product spectrum bares lower than the corresponding values for the –CH3 site indi-
many similarities with that obtained in the study of Cl atoms cating that the formation of CF2ClC(O)OCHO$CH3 radicals is
with CF3C(O)OCH3 where the main absorptions bands were the main fate for the reaction of Cl with ECDFA (see Scheme 3).
assigned to formic triuoroacetic anhydride CF3C(O)OCHO.32
However, they concluded that some H-abstraction from the
The unique structural feature of an anhydride is the pres- –CH3 group in ECDFA could occur and could not be completely
ence of two carbonyl groups linked to each other via an oxygen excluded. Orlando and Tyndall42 have studied the reaction of Cl
atom. This structure gives rise to two carbonyl absorptions in with ethyl acetate and found that Cl atom attack at the –CH2–
the infrared arising from symmetrical and asymmetrical group dominates for this non-halogenated ester.
stretching of the carbonyl groups. The residual product spec-
The experimental evidence in this work, i.e. the high
trum from Cl + MCDFA contains two carbonyl absorptions at formation yield of chlorodiuoroacetic acid, also supports that
1831 and 1805 cmꢂ1 separated by 26 cmꢂ1 whereby the in the reaction of Cl with ECDFA attack of the Cl atom at the
absorption at 1805 cmꢂ1 is stronger than that at 1831 cmꢂ1. The –CH2– group dominates. The CF2ClC(O)OCH$CH3 radical thus
small carbonyl band splitting is expected for anhydrides of formed will add O2 to form a peroxy radical further reactions of
structure RC(O)OCHO.34 In the ngerprint region of the product which with RO2/HO2 will yield to a large extent the alkoxy
spectrum band maxima occur at ꢄ1279, ꢄ1193, ꢄ1115, ꢄ999, radical CF2ClC(O)OCHO$CH3.
A
simplied mechanism
ꢄ966 cmꢂ1 whereby those at 1115 and 966 cmꢂ1 are the showing the further possible reaction pathways of the CF2-
strongest and equally intense. Interpretation of anhydride ClC(O)OCHO$CH3 radical is presented in Fig. 5.
spectra in the ngerprint region is complicated because of
The observation of a high chlorodiuoroacetic acid yield
a large number of C–O and C–C stretchings and various bending (ꢄ85%) supports that the a-ester rearrangement (Fig. 4, channel
modes, many of which are strongly mixed. The strong band at A) is the main reaction channel for the CF2ClC(O)OCHO$CH3
1115 cmꢂ1 is an indication for the presence of an anhydride and radical, assuming that no other source of the acid exists. Blanco
is generally associated with C–O and C–C vibrations. However, et al.32 in their study on the Cl-atom oxidation of uoroacetates
although many anhydrides have strong bands in this region it is report a value of 78 ꢀ 5% for the formation of triuoroacetic
not a unique marker for the presence of an anhydride since acid in the reaction of Cl with ethyl triuoroacetate. Orlando
many other compounds absorb in this region.35,36 The bands at and Tyndall42 have reported a yield of 53% for the formation of
1279 and 1193 cmꢂ1 are consistent with vibrations from C–F acetic acid in the reaction of Cl with ethyl acetate and concluded
and C–Cl in the ClF2C– group: similar bands occur in the from their system analysis that other routes to the formation of
30
´
infrared spectra of formic triuoroacetic anhydride and tri- the acid were very minor. In addition, Tuazon et al. and
uoroacetic anhydride.35–37 The absorption bands at ꢄ999 and Picquet-Varrault et al.43 have reported yields of 96% and 75% for
ꢄ960 cmꢂ1 are most probably also from C–F and C–Cl vibra- the formation of acetic acid in the reaction of OH radicals with
tions, halogenated compounds such as 1,2-dichloro- CH3C(O)OCH2CH3, respectively. All of these studies are
tetrauoroethane show fairly strong bands in this region.
consistent with the main reaction pathway being abstraction of
An infrared spectrum has been simulated for CF2ClC(O) an H-atom from the –CH2– group with subsequent chemistry via
OC(O)H using the using Gaussian 03 ab initio quantum chem- an a-ester rearrangement leading to the corresponding acid
istry package.38 The spectrum was calculated using the B3LYP/6 compound.
31+G level of theory.39–41 The simulated infrared spectrum is
The co-product of CF2ClC(O)OH, the CH3C(O) radical, will
compared in Fig. S8,† panel B, with the residual product spec- add O2 to form the CH3C(O)OO peroxy radical. This radical can
trum (panel A) from the Cl + MCDFA reaction. Although the react with HO2 to form peroxy acetic acid (CH3C(O)OOH), or
computed spectrum captures the main spectral features of the react with other peroxy radicals to form CO2 and CH3 radicals.
experimental absorption bands reasonably well, there are large No evidence for formation of peroxy acetic acid was been
discrepancies in the absorption frequencies especially for the observed in the residual product spectrum, however, an OH
carbonyl bands. The computed spectrum, however, does lend absorption was observed at ꢄ3585 cmꢂ1 suggesting formation
additional support to the other spectral experimental evidence of a product containing either an OH or OOH group. Although
that the residual product spectrum is very probably mainly due CO2 cannot be quantitatively determined in the system because
to formation of CF2ClC(O)OC(O)H from reaction of CF2ClC(O) of high background levels, there where indications that its
OCH2Oc radicals with O2. Secondary reactions of CF2ClC(O) concentration increased during the course of the reaction. The
OC(O)H with Cl are the most probable source of the small CH3 radical will be oxidized to formaldehyde (HCHO) the
amounts of F2CO observed in the reaction system.
further reaction of which will give CO. Formaldehyde could not
b
Atmospheric fate of CF2ClC(O)OCHO$CH3 radicals. be detected in the reaction system, which is not surprising given
Mishra et al.27 have reported that H-atom abstraction from the the high reactivity of HCHO with Cl. Orlando and Tyndall42 in
CH3– group in CF2ClC(O)OCH2CH3 at 298 K to give the CF2- their study on the reaction of Cl with ethyl acetate could only
ClC(O)OCH2CH2c radical is very slow (k ¼ 3.88 ꢁ 10ꢂ14 cm3 per detect traces of HCHO. Interestingly, the acid to CO ratio in this
molecule per s) compared to H-atom abstraction from the study on Cl with CF2ClC(O)OCH2CH3 and in the studies on Cl
32
42
–CH2– group to give the CF2ClC(O)OCH$CH3 radical (k ¼ 1.82 ꢁ with CF3C(O)OCH2CH3 and Cl with CH3C(O)OCH2CH3 are
10ꢂ12 cm3 per molecule per s). The authors reported that the all ꢄ0.4 indicating other fates for the CH3C(O) radical rather
barrier heights for hydrogen abstraction from the –CH2– group than oxidation through HCHO to CO. It was not possible to
by Cl at both the MPWB1K and G2(MP2) levels of theory are
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Fig. 4 Atmospheric fate of the alkoxy radicals formed after H-atom abstraction by Cl from the –CH2– group in ECDFA. The dashed boxes
indicate the identified and quantified products formed.
elucidate the other reaction pathways from the present FTIR cmꢂ1. The two carbonyl absorptions at 1864 and 819 cmꢂ1 are
data.
separated by 45 cmꢂ1, whereby 1819 cmꢂ1 is the stronger of the
The CF2ClC(O)OCHO$CH3 alkoxy radical could potentially two. As discussed for the assignment of CClF2C(O)OC(O)H as
decompose via a C–O–C bond cleavage forming CF2ClC(O)Oc a product in the reaction of Cl with MCDFA, bands at ꢄ1302,
radicals and acetaldehyde. The CF2ClC(O)Oc radicals would be ꢄ1169 and ꢄ978 cmꢂ1 are consistent with C–F and C–Cl
expected to decompose to form CO2 and cCF2Cl radicals and the vibrations from the ClF2C– group and a strong absorption at
subsequent reactions of which would be expected to produce ꢄ1125 cmꢂ1 would t with a C–O–C type vibration. These
CF2O. Neither the formation of CF2O nor formation of the observed absorptions are consistent with the formation of the
decomposition co-product acetaldehyde was observed in the anhydride CF2ClC(O)OC(O)CH3 from the reaction of CF2ClC(O)
product spectra suggesting that the contribution from this OCHO$CH3 radical with O2 (Fig. 4, channel B). This anhydride
channel was negligible. The absence of CF2O also shows that no is not commercially available for quantication purposes,
other reaction product was oxidized through to CF2O in the however, the high yield of CF2ClC(O)OH and evidence for
reaction system.
the formation of CF2ClC(O)OC(O)H suggest that its yield is
The absorption bands in the residual product spectrum from probably <10%.
the reaction of Cl with ECDFA are not as distinct as in the
residual product spectrum for Cl with MCDFA with more over-
Atmospheric implications
lapping of bands which suggests more than one compound is
present. Indeed, a comparison of the residual product spectrum
from Cl with MCDFA, which has been identied as belonging
mainly to the anhydride ClCF2C(O)OCHO, with that of Cl with
ECDFA shows that ClCF2C(O)OCHO is formed in the reaction of
Cl with ECDFA. Aer subtraction of the absorptions due to
ClCF2C(O)OCHO, the following absorptions remain at ꢄ1864,
ꢄ1819, ꢄ1302, ꢄ1169, ꢄ1125, ꢄ1093 (shoulder) and ꢄ978
Methyl chlorodiuoracetate and ethyl chlorodiuoroacetate are
removed from the troposphere mainly by their rapid reactions
with OH radicals16 which results in atmospheric lifetimes of the
order of 10–50 days. However, in some marine and continental
regions close to industrial emissions, peak values of 1 ꢁ 105
atoms per cm3 for Cl atoms concentrations have been
observed.3–11 Under such circumstances the reaction of Cl with
the chlorouoroacetates can be a signicant if not dominant
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loss process for the compounds. Photolysis is not an important
The product investigations presented in this work have
atmospheric loss process for these chlorouorinated shown that the major product of the Cl-atom initiated oxidation
compounds since they do not absorb radiation at actinic of methyl chlorodiuoroacetate under NOx-free conditions will
wavelengths.16
be formic dichlorouoroacetic anhydride (CF2ClC(O)OC(O)H).
As reported for other haloesters44,45 we expect relatively low We expect that chlorouoro anhydride will be further oxidized
values of the Henry laws constants at 298 K for both fairly rapidly by reaction with OH radicals and may also
compounds, indicating that wet deposition of these chloro- photolyze since the compound contains the –C(O)H chromo-
uoro acetates will not be signicant compared to the phore. The other product observed in the reaction of Cl with
OH-radical or Cl-atom initiated degradation. The volatility of MCDFA was chlorodiuoracetic acid (ꢄ30%). Both, anhydrides
these compounds will render dry deposition an unlikely and acids are highly soluble compounds and may be rapidly
removal mechanism.
incorporated into cloud droplets (where hydration of the
The temperature dependences of the Cl-rate coefficients re- anhydrides leads to acids) and rain out, contributing to the
ported in this work were used to calculate the rate of loss of the precipitation acidity.
chlorouoroacetates studied by their tropospheric reaction with
For the reaction of Cl with ETFA the main product observed
Cl atoms. At a given temperature (T) corresponding to a given was chlorodiuoroacetic acid (ꢄ88%). Haloacetic acids are
altitude in the troposphere, the rate of loss of the chloro- ubiquitous environmental pollutants and their presence in the
uoracetates is dened as the product between the Cl rate environment is due both to natural processes and to human
coefficient (T) and the [Cl] at this altitude.
activities. They partition into water because of their large
Using a 24 h daytime average global tropospheric Cl atom Henry's law constants.48,49 They could be accumulated to rela-
concentration of [Cl] ¼ 1 ꢁ 104 atoms per cm3,46 the Arrhenius tively high concentrations in aqueous reservoirs and would
parameters reported in this work and considering a lapse rate in likely accumulate to toxic levels. Furthermore, haloacetic acids
the troposphere of ꢂ6.5 K kmꢂ1
,
we have calculated the have been detected in many places such as soil, conifer needles
47
temperature prole between 0 and 10 km. Table S1† shows the and lichens, snow and ice in Antarctica, fog and rainwater, sea
Cl removal rates for the chlorouoroacetates studied as a func- water, lake water, wastewater and drinking water.50,51 Haloacetic
tion of altitude in the troposphere considering temperature of acids, as chlorodiuoracetic acid are generally phytotoxic and
298.15 K at 0 km. The results showed in Table S1† are plotted in toxic to green algae,52 while some haloacetic acids, e.g.,
the Fig. 5.
dichloroacetic acid and trichloracetic acid, have hep-
The loss rates (in sꢂ1) of the haloacetates studied at sea level atocarcinogenic potential.53
(0 km) are 9.93 ꢁ 10ꢂ10 and 1.55 ꢁ 10ꢂ8 and near to the
In this sense, studies of the tropospheric degradation of
tropopause (ꢄ10 km) around 2.78 ꢁ 10ꢂ10 and 5.51 ꢁ 10ꢂ9 for chlorouoroacetates are important and necessary, since these
methyl chlorodiuoroacetate and ethylchlorodiuoroacetate, compounds break down relatively rapidly to several products
respectively.
including persistent substances such as chlorodiuoroacetic
The positive temperature dependence of the reactions of Cl acid. Concentrations of CDFA with a maximum of 0.0002 ng Lꢂ1
atoms with both MCDFA and ECDFA is consistent with the were reported in water samples taken in Canada.48 However,
dominance of a hydrogen atom abstraction mechanism.
a continuing monitoring of the environmental concentrations
of CDFA is recommended to conrm trends in surface and
rainwater.
In addition, further studies on the product distributions
from the OH-radical and Cl-atom initiated oxidation of hal-
oesters need to be performed over the range of temperatures
and NOx levels typically prevailing in the troposphere in order to
gain a better quantitative understanding of their contribution to
photooxidant formation and precipitation acidity.
Acknowledgements
The authors wish to acknowledge Deutsche For-
schungsgemeinscha (DFG), DAAD-MINCyT, FONCyT and
CONICET for nancial support of this research. M. B. Blanco
wishes to acknowledge to the Alexander von Humboldt Foun-
dation for support through a postdoctoral fellowship.
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