CHEMPHYSCHEM
ARTICLES
radicals, or with themselves, resulting in the corresponding
chloroalkoxy radical. This radical can decompose by CꢁC bond
scission or react with O2,[25] leading to different reaction prod-
ucts.
Table 2. Summary of rate constants of the reactions of OH and Cl radicals
with fluroxene and atmospheric lifetimes, t.
Oxidant
Concentration
[moleculescmꢁ3
k[a]
t[b]
]
[cm3 moleculeꢁ1 sꢁ1
]
[h]
As expected, 2,2,2-trifluoroethyl formate is consistently ob-
served as a product in both OH and Cl reactions, which both
proceed through the addition of the radical to the double
bond and the subsequent decomposition of the alkoxy radical.
OH
Cl
1.0ꢂ106[c]
(2.96ꢀ0.61)ꢂ10ꢁ11
9
343
5.0ꢂ103[d,e]
(1.62ꢀ0.19)ꢂ10ꢁ10
[a] This work. [b] t=1/(k[oxidant]). [c] Blos et al.[36] [d] Pszenny et al.[37]
[e] Wingenter et al.[38]
C
In the fluroxene–Cl reaction, chloroacetaldehyde is also detect-
ed as a product, probably associated with the decomposition
of the alkoxy radical; 2,2,2-trifluoroethyl 1-chloro-acetate arises
from the reaction of the alkoxy radical with O2. In the oxidation
initiated by Cl radicals, all of the products detected result from
the addition of the oxidant to the terminal carbon atom of the
double bond; this forms the most stable radical.[26] In the reac-
tion initiated by OH radicals, the addition to both double bond
carbon atoms generates 2,2,2-trifluoroethyl formate, so it is
not possible to distinguish the priority site for the addition.
However, the same yield of this compound (ca. 79%) was ob-
tained with both OH and Cl, which suggests that the reaction
pathway may be the same in both cases, and, therefore, that
the addition preferentially occurs at the terminal carbon atom.
It is also interesting that the major fate of the alkoxy radical
formed in the oxidation process is the scission of a CꢁC bond,
and not its reaction with O2; this could be attributed to the
presence of the ether functionality, which reduces the CꢁC
bond strength, thereby facilitating the decomposition chan-
nel.[27] The dominance of this channel has also been observed
for the addition of OH and Cl radicals to other fluorinated and
non-fluorinated ethers,[18,28,29] even though the increase of fluo-
rinated substituents, for example in the alkoxy radicals derived
from HFE7200 or HFE7500, could lead to an increase in the rel-
ative importance of the reaction with O2.[27,30]
From the lifetime estimates in Table 2, it can be seen that
the reaction with OH radicals plays a major role in the atmos-
pheric destruction of fluroxene. On a global scale, oxidation in-
itiated by Cl radicals does not compete with that initiated by
the more abundant OH radicals. However, reactions of Cl radi-
cals are, in general, faster than the corresponding reactions
with OH radicals, rendering these reactions potentially impor-
tant in areas in which atomic chlorine concentrations are high.
Recent studies conducted in different parts of the world sug-
gest that in the early morning the production rate of Cl radi-
cals exceeds that of OH radicals for 2 to 3 h after sunrise,
owing to high concentrations of the precursor, ClNO2, which is
converted to Cl radicals through photolysis.[39–42] In this respect,
Cl concentrations of around 106 moleculescmꢁ3 have been cal-
culated from the concentrations and photolysis rate of ClNO2
in early-morning air masses.[39] Under such conditions, the life-
time of fluroxene with respect to this oxidant would be ap-
proximately 2 h and, therefore, the reaction of Cl radicals with
fluroxene could constitute a competitive channel for its atmos-
pheric removal.
3.2. Implications of the Oxidation Products
The increase in the yield of 2,2,2,-trifluoroethyl formate in
the presence of NOx in both OH- and Cl-initiated reactions can
be explained by considering the effect of the chemical activa-
tion of the alkoxy radicals.[31–33] The reactions of peroxy radicals
with NOx are exothermic and yield alkoxy radicals with consid-
erable internal-excitation energy, which is invested in the CꢁC
bond rupture, increasing the yield of the decomposition chan-
nel. However, the alkoxy radicals formed in the self-reaction of
peroxy radicals have little or no excitation energy and the de-
composition is less effective.
According to the atmospheric lifetimes, fluroxene rapidly
C
C
reacts with OH or Cl in the troposphere. The proposed
double-bond-addition mechanism, described in Scheme 1, sug-
gests that atmospheric oxidation of fluroxene leads almost ex-
clusively to 2,2,2-trifluoroethyl formate. However, some studies
indicate that the photolysis of fluorinated esters, such as
CF3CH2OC(O)H, is not relevant in the troposphere and that
their volatility probably renders atmospheric removal through
dry-deposition mechanisms unlikely.[10,43,44]
It is, therefore, expected that CF3CH2OCH=CH2 will rapidly be
converted into CF3CH2OC(O)H, which is the only product able
to remain in the troposphere for a considerable period of time,
until it is removed by OH. Oyaro et al. estimated a value of
0.44 yrs for the OH-lifetime of CF3CH2OC(O)H.[44] This value is
extremely high compared to that obtained in the present work
for CF3CH2OCH=CH2 (ꢄ1ꢂ10ꢁ3 yr). The oxidation of
CF3CH2OC(O)H primarily leads to the formation of the corre-
sponding fluorinated acid and anhydrides, as well as the hy-
drolysis products CF2O, CO2, and HF. Acids and anhydrides are
very soluble compounds and are rapidly incorporated into
cloud droplets.[16,43,45]
3. Atmospheric Implications
C
C
3.1. Atmospheric Lifetimes with OH and Cl
The rate constants determined in this study for the reactions
of OH and Cl radicals with fluroxene can be used to estimate
the atmospheric lifetime of the HFE. The optimal temperature
for such a scaling analysis is 277 K,[34] but by analogy with
other HFEs[27,35] the use of 298 K is not expected to have any
impact on the estimated atmospheric lifetimes. The concentra-
tions of tropospheric oxidants and the related rate constants
are listed in Table 2.
It is worth noting that fluorinated esters (FESs), such as
CF3CH2OC(O)H, are potential greenhouse gases, because they
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ChemPhysChem 2013, 14, 3834 – 3842 3839