Atmospheric chemistry of CF3CF{double bond, long}CH2: Products and mechanisms of Cl atom and OH radical initiated oxidation

Article abstract of DOI:10.1016/j.cplett.2007.11.051

The products of Cl atom and OH radical initiated oxidation of CF₃CF{double bond, long}CH₂ were studied in 700 Torr of N₂/O₂ diluent at 296 ± 1 K. The reactions of Cl atoms and OH radicals with CF₃CF{double bond, long}CH₂ proceed via electrophilic addition to the double bond. The reaction with chlorine atoms proceeds 56 ± 5% via addition to the central carbon. The chlorine atom initiated oxidation of CF₃CF{double bond, long}CH₂ gives CF₃C(O)F in a molar yield which is indistinguishable from 100% and independent of [O₂], and HC(O)Cl in a molar yield which increased from 30% to 59% as [O₂] was increased from 3 to 700 Torr. The OH radical initiated oxidation of CF₃CF{double bond, long}CH₂ gives CF₃C(O)F as major product in a yield of 91 ± 6%. The results are discussed with respect to the atmospheric chemistry and environmental impact of CF₃CF{double bond, long}CH₂.

Full text of DOI:10.1016/j.cplett.2007.11.051

Available online at www.sciencedirect.com
Chemical Physics Letters 450 (2008) 263–267
www.elsevier.com/locate/cplett
Atmospheric chemistry of CF3CF@CH2: Products and mechanisms of
Cl atom and OH radical initiated oxidation
a
a,
b
b
*
M.D. Hurley , T.J. Wallington , M.S. Javadi , O.J. Nielsen
a Systems Analytics and Environmental Sciences Department, Ford Motor Company, Mail Drop RIC-2122, Dearborn, MI 48121-2053, USA
b Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark
Received 22 October 2007; in final form 19 November 2007
Available online 23 November 2007
Abstract
The products of Cl atom and OH radical initiated oxidation of CF3CF@CH2 were studied in 700 Torr of N2/O2 diluent at 296 1 K.
The reactions of Cl atoms and OH radicals with CF3CF@CH2 proceed via electrophilic addition to the double bond. The reaction with
chlorine atoms proceeds 56 5% via addition to the central carbon. The chlorine atom initiated oxidation of CF3CF@CH2 gives
CF3C(O)F in a molar yield which is indistinguishable from 100% and independent of [O2], and HC(O)Cl in a molar yield which increased
from 30% to 59% as [O2] was increased from 3 to 700 Torr. The OH radical initiated oxidation of CF3CF@CH2 gives CF3C(O)F as
major product in a yield of 91 6%. The results are discussed with respect to the atmospheric chemistry and environmental impact
of CF3CF@CH2.
Ó 2007 Published by Elsevier B.V.
1. Introduction
Unsaturated hydrofluorocarbons are a class of com-
pounds, which are potential replacements for CFCs and
saturated HFCs in air conditioning units. In general, unsat-
urated hydrofluorocarbons react more rapidly with OH
radicals, have shorter atmospheric lifetimes, and have
lower global warming potentials than saturated hydro-
fluorocarbons. Prior to their large-scale industrial use an
assessment of the atmospheric chemistry and environmen-
tal impact of these compounds is needed. To address
this need the atmospheric chemistry of CF3CF@CH2
was investigated. Rate constants of k(Cl + CF3CF@CH2)
= (7.03 0.59) ꢀ 10ꢁ11, k(OH + CF3CF@CH2) = (1.05
0.17) ꢀ 10ꢁ12, and k(O3 + CF3CF@CH2) = (2.77 0.21) ꢀ
10ꢁ21 cm3 moleculeꢁ1 sꢁ1 in 700 Torr of N2/O2 diluent at
296 K have been reported [5]. In the present work smog
chamber/FTIR techniques were used to determine prod-
ucts of the chlorine atom and OH radical initiated oxida-
tion of CF3CF@CH2.
Recognition of the adverse environmental impact of
chlorofluorocarbon (CFC) release into the atmosphere
[1,2] has led to an international effort to replace these com-
pounds with environmentally acceptable alternatives. Satu-
rated hydrofluorocarbons (HFCs) have become widely
used CFC replacements. HFCs do not contain chlorine
and hence do not contribute to the well established chlorine
based catalytic ozone destruction cycles [3]. The atmo-
spheric lifetime of HFCs is determined by their reactivity
towards OH radicals. CF3CFH2 (HFC-134a) is used as
the working fluid in all modern vehicle air conditioning sys-
tems. Saturated HFCs such as HFC-134a display modest
reactivity towards OH radicals and hence have modest glo-
bal warming potentials. HFC-134a has a global warming
potential of 1430 (100 year time horizon) which is approx-
imately a factor of 8 lower than that of CFC-12 (10890)
which it replaced [4].
2. Experimental
*
Experiments were performed in a 140-l Pyrex reactor
interfaced to a Mattson Sirus 100 FTIR spectrometer [6].
Corresponding author.
E-mail address: twalling@ford.com (T.J. Wallington).
0009-2614/$ - see front matter Ó 2007 Published by Elsevier B.V.
doi:10.1016/j.cplett.2007.11.051

264
M.D. Hurley et al. / Chemical Physics Letters 450 (2008) 263–267
The reactor was surrounded by 22 fluorescent blacklamps
(GE F15T8-BL), which were used to photochemically initi-
ate the experiments. The products of the atmospheric oxi-
dation of CF3CF@CH2 were investigated by irradiating
CF3CF@CH2/Cl2/O2/N2, CF3CF@CH2/CH3ONO/O2/N2,
and CF3CF@CH2/CH3ONO/NO/O2/N2 mixtures. All
samples of CF3CF@CH2 used in this work were supplied
by the Honeywell International Inc. at a purity >99.9%
and were used without further purification. Chlorine atoms
were produced by photolysis of molecular chlorine
1.0
0.5
0.0
A: Before UV
B: After UV
0.5
0.0
0.6
Cl2 þ hm ! 2Cl
OH radicals were produced by photolysis of CH3ONO in
the presence of NO in air
ð1Þ
C: B - 0.23*A
0.3
0.0
CH3ONO þ hm ! CH3O þ NO
CH3O þ O2 ! HO2 þ HCHO
HO2 þ NO ! OH þ NO2
ð2Þ
ð3Þ
ð4Þ
0.4
D: CF C(O)F
3
CH3ONO was synthesized by the drop wise addition of
concentrated sulfuric acid to a saturated solution of
NaNO2 in methanol. Other reagents were obtained from
commercial sources at purities >99%. All experiments were
conducted in 700 Torr of N2/O2 diluent at 296 1 K.
Concentrations of reactants and products were moni-
tored by FTIR spectroscopy. IR spectra were derived from
32 coadded interferograms with a spectral resolution of
0.25 cmꢁ1 and an analytical path length of 27.1 m. Unless
stated otherwise, quoted uncertainties are two standard
deviations from least squares regressions.
0.0
0.6
0.3
E: HC(O)Cl
F: CO
0.0
0.3
3. Results and discussion
0.0
800
1200
1600
2000
2400
3.1. Cl atom initiated oxidation of Cl + CF3CF@CH2
Wavenumber (cm-1)
Fig. 1. IR spectra of
a mixture of 9.6 mTorr CF3CF@CH2 and
To investigate the products and mechanism of the Cl
atom initiated oxidation of CF3CF@CH2, mixtures con-
sisting of 9.6–40.6 mTorr CF3CF@CH2, 92.0–124.2 mTorr
Cl2, and 3.1–700 Torr O2 in 700 Torr total pressure of N2/
O2 diluent were introduced into the chamber and subjected
to UV irradiation. Comparison of the IR features formed
in low and high O2 partial pressure experiments revealed
that three distinct products, or sets of products, were
formed in the chamber. Fig. 1 shows IR spectra obtained
before (A) and after (B) a 30 s irradiation of a mixture con-
taining 9.6 mTorr CF3CF@CH2 and 101.4 mTorr Cl2 in
700 Torr total pressure of air diluent. Subtraction of IR
features attributed to CF3CF@CH2 from panel B gives
the product spectrum shown in panel C. By comparing
panel C with the reference spectra in panels D, E, and F,
it is clear that CF3C(O)F, HC(O)Cl, and CO are formed
as products in the system.
101.4 mTorr Cl2 in 700 Torr of air before (A) and after (B) 30 s UV
irradiation. Panel C shows the residual IR features after subtraction of
features attributable to CF3CF@CH2 from panel B. Reference spectra of
CF3C(O)F, HC(O)Cl, and CO are given in panels D, E, and F.
CF3CF@CH2 þ Cl !CF3CFClCH2
!CF3CFCH2Cl
The reaction mechanism for the Cl atom initiated oxida-
tion of CF3CF@CH2 is shown in Fig. 2. The radicals pro-
duced in reaction (5) react with O2 to give peroxy radicals
ð5aÞ
ð5bÞ
CF3CFClCH2 þ O2 ! CF3CFClCH2OO
CF3CFCH2Cl þ O2 ! CF3CFðOOÞCH2Cl
In the atmosphere the peroxy radicals will react with NO to
produce the corresponding alkoxy radicals
ð6aÞ
ð6bÞ
Reaction of Cl atoms with CF3CF@CH2 occurs via
electrophilic addition to the double bond. Addition can
occur to the central or terminal carbon atom to give
CF3CFClCH2 or CF3CFCH2Cl radicals:
CF3CFClCH2OO þ NO ! CF3CFClCH2O þ NO2 ð7aÞ
CF3CFðOOÞCH2Cl þ NO ! CF3CFðOÞCH2Cl þ NO2
ð7bÞ

M.D. Hurley et al. / Chemical Physics Letters 450 (2008) 263–267
265
CF3CF(
•
)CH2Cl
O2
CF3CF=CH2 + Cl
CF3CFClCH2
•
O2
CF3CF(OO
•
)CH2Cl
CF3CFClCH2OO
NO
•
NO
CF3CF(O
•
)CH2Cl + NO2
CF3CFClCH2O• + NO2
O2
Δ
CF3C(O)F + CH2Cl(
•
)
CF3CFCl(
•
) + HCHO
F3CFClCHO + HO2
Cl
O2
O2
CH2ClOO
NO
•
CF3CFClOO
•
HCl+CF3CFClC(O)
O2
NO
Δ
Δ
HCl + HCO
O2
CH2ClO
• + NO2
CF3CFCl(O
•
)+NO2
CO + CF3CFCl(•)
CF3CFClC(O)(OO
NO
•)
O2
CO2 + CF3CFCl(
•)
CF3CFClC(O)(O•)
HO2 + CO
HC(O)Cl + HO2
CF3C(O)F + Cl
Fig. 2. Mechanism of the Cl atom initiated oxidation of CF3CF@CH2.
In the chamber the conversion of the peroxy into alkoxy
40
30
20
10
0
radicals is accomplished by peroxy radical self- and cross-
reactions. As shown in Fig. 2, the alkoxy radical
CF3CF(O)CH2Cl is expected to decompose via C–C bond
scission leading to the formation of CF3C(O)F, HC(O)Cl,
and CO products. The alkoxy radical CF3CFClCH2O is
expected to undergo both decomposition via C–C bond
scission leading to the formation of CF3C(O)F and
HCHO, and reaction with O2 giving the aldehyde
CF3CFClCHO. As indicated in Fig. 2, the aldehyde is
expected to be reactive towards Cl atoms and be oxidized
to give CF3C(O)F, CO, and CO2.
Fig. 3 shows a plot of the formation of CF3C(O)F,
HC(O)Cl, CO, and HCHO versus the loss of CF3CF@CH2
following UV irradiation of a mixture of 33.8 mTorr
CF3CF@CH2 and 111.7 mTorr Cl2 in 700 Torr of air dilu-
ent. CF3CHO yield data for experiments conducted with
3.1–700 Torr partial pressure of O2 (at total pressure of
700 Torr, made up with N2) are also included in Fig. 3.
As seen from Fig. 3, the yield of CF3C(O)F is independent
of [O2]. The line through the data is a linear least squares fit
which gives a molar CF3C(O)F yield of 92 5 %. The line
through the HC(O)Cl data gives a yield of 54 6%. The
curves through the CO and HCHO are second order poly-
nomial fits to aid visual inspection of the data trends. The
curvature in the CO data indicates it is formed as a second-
ary product, the curvature in the HCHO data indicate that
it is lost via secondary reactions (probably with Cl atoms).
In contrast to the behaviour observed for CF3C(O)F,
the yield of HC(O)Cl varied with [O2]. The insert in
Fig. 3 shows the yield of HC(O)Cl observed in experiments
using [O2] = 3.1–700 Torr. The variation of HC(O)Cl with
[O2] reflects a competition between reaction with O2 and
decomposition via HCl elimination as fates for CH2ClOÅ
radicals in the system. This competition has been studied
previously and a rate constant ratio of k8/k9 = 5.0 ꢀ
10ꢁ18 cm3 moleculeꢁ1 has been reported [7].
0.6
0.4
0.2
0.0
CF3C(O)F
HC(O)Cl
0
200
[O
400
600
800
2] (Torr)
CO
HCHO
0
10
20
30
40
Δ [CF3CF=CH2] (mTorr)
Fig. 3. Formation of CF3C(O)F (triangles), HC(O)Cl (circles), CO
(squares), and HCHO (crosses) versus loss of CF3CF@CH2 observed
following UV irradiation of a mixture of 33.8 mTorr CF3CF@CH2 and
111.7 mTorr Cl2 in 700 Torr of air diluent. Additional CF3COF data are
shown for experiments conducted with different [O2]: 3.1 Torr, (gray
diamonds); 7.5 Torr, (gray inverted triangles); 10 Torr, (filled inverted
triangles); 50 Torr (gray squares); 300 Torr, (gray stars); 400 Torr (filled
stars); 550 Torr (gray circles), and 700 Torr, (gray triangles). The insert
shows the HC(O)Cl yield versus [O2], see text for details.
CH2ClO þ O2 ! HCðOÞCl þ HO2
CH2ClO þ M ! HCðOÞ þ HCl þ M
M represents a third body (mainly N2 in this case). The
HC(O)Cl yield is described by the expression,
ð8Þ
ð9Þ
"
#
k8
k9
½O2ꢂ
½O2ꢂ þ 1
YðHCðOÞClÞ@YðCH2ClOÞ
þ C
ð10Þ
k8
k9

266
M.D. Hurley et al. / Chemical Physics Letters 450 (2008) 263–267
where Y(CH2ClO) is the yield of CH2ClO radicals in the
system, k8/k9 is the ratio of the rate constants for reactions
(8) and (9) and C is the [O2] independent HC(O)Cl yield in
the system (e.g., via the CH2ClO2 + HO2 reaction [8]). The
curve through the data in the insert in Fig. 3 is a fit of
expression (10) to the data with k8/k9 fixed at
5.0 ꢀ 10ꢁ18 cm3 moleculeꢁ1 [7]. It can be seen from Fig. 3
that the variation of the HC(O)Cl yield with [O2] is consis-
tent with that expected from the literature value of k8/k9.
At [O2] > 100 Torr the HC(O)Cl yield reaches a limiting
value. The experiments conducted using [O2] > 100 Torr
give a HC(O)Cl yield of 56 5% which we equate to the
fraction of Cl atom attack that proceeds via addition to
the terminal carbon atom; k5b/(k5a + k5b) = 0.56 0.05.
The observation of a small, but significant, yield of
HCHO (see Fig. 3) is interesting as it implies that decompo-
sition via C–C bond scission is a significant loss mechanism
for CF3CFClCH2O radicals. This observation is unex-
pected as it is well established that the atmospheric loss
mechanism of structurally similar molecules such as
CCl3CH2O, CF3CH2O, CFCl2CH2O, and CF2ClCH2O is
reaction with O2 [9]. Further work, for example a computa-
tional study of the rate of decomposition of CF3CFx-
Cl2ꢁxCH2O radicals would be of interest, but is beyond
the scope of the present work.
6
5
4
3
2
1
0
0
1
2
3
4
5
6
Δ [CF3CF=CH2] (mTorr)
Fig. 4. Formation of CF3C(O)F versus loss of CF3CF@CH2 observed
following the UV irradiation of mixtures of 8.7–14 mTorr CF3CF@CH2
and 106–203 mTorr CH3ONO (open symbols) or 10.6–16 mTorr
CF3CF@CH2, 105–108 mTorr CH3ONO and 17–25 mTorr NO (filled
symbols) in 700 Torr air diluent.
3.2. Study of the mechanism of the atmospheric oxidation of
OH + CF3CF@CH2
CF3CF=CH2 + OH
CF3CF(OH)CH2
•
CF3CF(
•
)CH2(OH)
O2
O2
To investigate the products and mechanism of the
OH radical initiated oxidation of CF3CF@CH2, mix-
tures consisting of 8.7–16.0 mTorr CF3CF@CH2, 105–
203 mTorr CH3ONO, and 0–25 mTorr NO in air diluent
were introduced into the chamber and subjected to UV
irradiation. The consumption of CF3CF@CH2 was in the
range 10–41%. CF3C(O)F was observed as the major
product in all experiments. As seen from Fig. 4, there
was no discernable difference in the yield of CF3C(O)F in
experiments conducted with, and without, added NO.
The line through the data is a linear least squares fit to
the combined data set which gives a CF3C(O)F yield of
0.91 0.06. The observed CF3C(O)F yield is less than
100%. This may reflect the formation of a small yield of
the nitrates CF3(ONO2)CH2OH and CF3CF(OH)-
CH2ONO2 in the reactions of the corresponding peroxy
radicals with NO shown in Fig. 5, or it may reflect uncer-
tainties in our calibration of the CF3C(O)F reference spec-
trum, or both.
CF3CF(OO
•
)CH2(OH)
CF3CF(OH)CH2OO
•
NO
NO
CF3CF(O
•
)CH2(OH) + NO2
CF3CF(OH)CH2O• + NO2
Δ
Δ
CF3C(O)F + CH2(OH)(
•)
CF3CF(OH)(
O2
•) + HCHO
O2
CF3C(O)F + HO2
HCHO + HO2
Fig. 5. Mechanism of the OH radical initiated oxidation of CF3CF@CH2.
available data suggest that while CF3C(O)OH is not a nat-
ural component of the freshwater environment [10], it is a
natural component of the background oceanic environ-
ment [11], and any additional burden associated with
CF3CF@CH2 oxidation will be of negligible environmental
significance. We conclude that the products of the atmo-
spheric oxidation of CF3CF@CH2 have negligible environ-
mental impact.
4. Atmospheric chemistry and environmental impact
The atmospheric lifetime of CF3CF@CH2 is dictated by
its reaction with OH radicals and is approximately 11 days
[5]. We show here that CF3C(O)F is the major atmospheric
oxidation product of CF3CF@CH2. The atmospheric fate
of CF3C(O)F is hydrolysis which occurs on a time scale
of approximately 10 days to give CF3C(O)OH [3]. The
Acknowledgement
M.S.J. and O.J.N. acknowledge financial support from
the Danish Natural Science Research Council for the
Copenhagen Center for Atmospheric Research (CCAR).

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