Formation of bromodifluoroacetyl fluoride, CF2BrC(O)F, in the thermal gas-phase oxidation of bromotrifluoroethene, CF2CFBr, initiated by trifluoromethylhypofluorite, CF3OF

Article abstract of DOI:10.1016/j.jfluchem.2006.02.003

The oxidation of CF₂CFBr by molecular O₂, initiated by CF₃OF, has been studied at 273, 253.5, 239 and 218 K. The initial pressure of CF₃OF was varied between 0.9 and 2.4 Torr, that of CF₂CFBr between 11.5 and 30.7 Torr and that of O₂ between 42.5 and 100.7 Torr. The main product was CF₂BrC(O)F (yields 81-95% based on the CF₂CFBr consumed). Minor amounts of C(O)F₂ and C(O)FBr and traces of bromotrifluoroethene epoxide were also formed. The reaction is a chain reaction with bromine atoms as chain carriers. Its basic steps are: the thermal generation of CF₃O{radical dot} radical by abstraction of fluorine atom from CF₃OF, chain initiation by addition of CF₃O{radical dot} to the double bond of alkene, leading, in presence of O₂, to the formation of bromine atom and chain propagation by the reaction of bromine atom with CF₂CFBr, originating CF₂BrCFBrO{radical dot} radical. The predominant fate of the latter is the bromine atom extrusion with C{single bond}C bond scission playing the minor role. The full mechanism is postulated.

Full text of DOI:10.1016/j.jfluchem.2006.02.003

Journal of Fluorine Chemistry 127 (2006) 736–739
www.elsevier.com/locate/fluor
Formation of bromodifluoroacetyl fluoride, CF2BrC(O)F, in the thermal
gas-phase oxidation of bromotrifluoroethene, CF2CFBr, initiated by
trifluoromethylhypofluorite, CF3OF
*
Joanna Czarnowski
´ ´
Instituto de Investigaciones Fisicoquımicas Teoricas y Aplicadas, INIFTA, Sucursal 4,
´
Casilla de Correo 16 (1900) La Plata, Republica Argentina
Received 8 December 2005; received in revised form 3 February 2006; accepted 5 February 2006
Available online 23 February 2006
Abstract
The oxidation of CF2CFBr by molecular O2, initiated by CF3OF, has been studied at 273, 253.5, 239 and 218 K. The initial pressure of CF3OF
was varied between 0.9 and 2.4 Torr, that of CF2CFBr between 11.5 and 30.7 Torr and that of O2 between 42.5 and 100.7 Torr. The main product
was CF2BrC(O)F (yields 81–95% based on the CF2CFBr consumed). Minor amounts of C(O)F2 and C(O)FBr and traces of bromotrifluoroethene
epoxide were also formed. The reaction is a chain reaction with bromine atoms as chain carriers. Its basic steps are: the thermal generation of
CF3Oꢀ radical by abstraction of fluorine atom from CF3OF, chain initiation by addition of CF3Oꢀ to the double bond of alkene, leading, in presence
of O2, to the formation of bromine atom and chain propagation by the reaction of bromine atom with CF2CFBr, originating CF2BrCFBrOꢀ radical.
The predominant fate of the latter is the bromine atom extrusion with C–C bond scission playing the minor role. The full mechanism is postulated.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Thermal gas-phase oxidation; Bromotrifluoroethene; Trifluoromethylhypofluorite; Bromodifluoroacetyl fluoride
1. Introduction
F2OCF2C(O)F and a compound containing CF(CF3)OC(O)CF3
group are also formed in the respective oxidations of
perfluoropropene and perfluorobutene-2.
The use of chemical initiators in absence of light permits
better control of reaction course in organic synthesis. The stable
and easily handled trifluoromethylhypofluorite, CF3OF, con-
taining a weak O–F bond (43.5 kcal/mol) [1,2,3] is an effective
initiator of oxidation of haloalkenes and perfluoroolefins. The
oxidations of CF2CCl2 [4], CHClCCl2 [5] and CCl2CCl2 [6] in
presence of CF3OF are chain reactions with chlorine atoms as
chain carriers, giving as the main products CX2ClC(O)Cl,
where X = H, Cl or F. Minor amounts of C(O)X2, are also
formed. In the case of C(O)HCl, it decomposes rapidly to CO
and HCl. In the oxidation of CF2CCl2 [4] initiated by CF3OF
the formation of small quantities of 1,1-dichlorodifluoroethene
epoxide was observed. The oxidations of perfluoropropene [7]
and perfluorobutene-2 [8] initiated by CF3OF are chain
reactions with CF3Oꢀ radicals as chain carriers, giving
C(O)F2, CF3C(O)F and CF3OC(O)F as products. CF3OC-
Recently we have studied at 313.4 K the gas-phase oxidation
of CF2CFBr initiated by NO2 [9], giving CF2BrC(O)F as the
main product, minor equivalent amounts of C(O)F2 and
C(O)FBr, and small quantities of bromotrifluoroethene epoxide
and peroxynitrate, CF2BrCFBrO2NO2. It is a chain reaction
propagated by bromine atoms. We have not found published
data on the addition of bromine atoms to the fluorinated olefins.
Only the Br-atom initiated oxidations of chlorinated ethenes
with chlorine atoms as chain carriers, were reported [10–12].
In this work the oxidation of CF2CFBr in presence of CF3OF
was studied.
2. Results and discussion
The experiments were carried out at 273, 253.5, 239 and
218 K. The initial pressure of CF3OF was varied between 0.9
and 2.4 Torr, that of CF2CFBr between 11.5 and 30.7 Torr and
that of O2 between 42.5 and 100.7 Torr.
* Tel.: +54 221 4220961; fax: +54 221 4254642.
E-mail address: bdq782@infovia.com.ar.
0022-1139/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2006.02.003

J. Czarnowski / Journal of Fluorine Chemistry 127 (2006) 736–739
737
The products were identified by their infrared spectra after
oxygen was eliminated from the reaction system. The main
product was CF2BrC(O)F (yields 81–95% based on the
CF2CFBr consumed). Minor quantities of C(O)F2 and C(O)FBr,
traces of CF3OCF2C(O)F and bromotrifluoroethene epoxide,
previously removing O2, an explosion occurred with emission
of light, giving CO2, C(O)F2, CF4 and Br2 as only products.
The reaction is a chain reaction. At the pressures of O2 used
in this work, the reaction does not depend on the concentration
of O2, showing the pseudo-zero order with respect to O2 as
reactant and indicating the third body character of O2.
In order to propose the reaction mechanism, in addition to
the analysis of the products, the following reactions were
considered: the oxidations of CF2CCl2 [4], CHClCCl2 [5] and
CCl2CCl2 [6] initiated by addition of CF3OF to the double bond
and the oxidation of CF2CFBr initiated by NO2 [9].
The following reactions are consistent with the experimental
results obtained in this work:
, were also formed. The compound CF2BrC(O)F
was characterized by its IR spectrum consistent with the gauche
structure, by comparing it with the calculated one for this
molecule using ab initio and Density Functional Theory methods
[13]. For this molecule the most intense absorption bands appear
at1887, 1195, 1102and937 cmꢁ1, andareassignedtothe n(CO),
nas(CF2), ns(CF2) and n(CF), respectively. The compounds
C(O)F2 [14], C(O)FBr [15,16] and CF3OCF2C(O)F [17] were
identified by their respective IR spectra. The bromotrifluor-
oethene epoxide, present in the reaction mixture, was
characterized by its infrared band at 1540 cmꢁ1, assigned to
the ring-breathing mode, that is characteristic of fluoroepoxides.
This band appears at 1500 cmꢁ1 for 1,1-dichloro-2.2-difluor-
oethene epoxide [18], at 1545 cmꢁ1 for chlorotrifluoroethene
epoxide [18] and at 1551 cmꢁ1 for perfluoropropene epoxide
[19].
CF3OF þ CF2CFBr ! CF3CFBrꢀ þ CF3Oꢀ
(1)
(2)
CF3CFBrꢀ þ O2 þ M ! CF3CFBrO2 þ M
ꢀ
2CF3CFBrO2 ! 2CF3CFBrOꢀ þ O2
(3)
(4)
(5)
ꢀ
CF3CFBrOꢀ ! CF3CðOÞF þ Brꢀ
ꢀ
CF3CFBrOꢀ ! CF3 þ CðOÞFBr
The very strong absorption band of CF3OOCF3 at 1166 cmꢁ1
[20] and that of CF3C(O)F [21] at 1122–1119 cmꢁ1 were not
observed.
ꢀ
ꢀ
CF3 þ O2 þ M ! CF3O2 þ M
(6)
(7)
ꢀ
2CF3O2 ! 2CF3Oꢀ þ O2
To determine the concentration of CF2BrC(O)F and that of
non-consumed CF2CFBr, the room temperature infrared
calibration curves were made, using the pure compounds, thus
allowing conversion of the absorption intensities at 1887 cmꢁ1 to
the pressures of CF2BrC(O)F and those at 1779 cmꢁ1 to the
pressures of CF2CFBr. The pressures corresponding to the
temperature of each run were calculated using the pressures of
CF2BrC(O)F and CF2CFBr at room temperature obtained from
the calibration curves.
CF3Oꢀ þ CF2CFBr ! CF3OCF2CFBrꢀ
(8)
CF3OCF2CFBrꢀ þ O2 þ M ! CF3OCF2CFBrO2 þ M (9)
ꢀ
2CF3OCF2CFBrO2 ! 2CF3OCF2CFBrOꢀ þ O2
CF3OCF2CFBrOꢀ ! CF3OCF2CðOÞF þ Brꢀ
Br þ CF2CFBr ! CF2BrCFBrꢀ
(10)
(11)
(12)
(13)
(14)
ꢀ
The analytical data of nine experiments are summarized
in Table 1, where indices i and f denote initial and final and
Dt is the time of permanence of the reaction system
CF3OF + CF2CFBr + O2 in the reaction vessel for each run.
These data show that CF2CFBr was completely consumed in
the temperature range 273–239 K.
CF2BrCFBrꢀ þ O2 þ M ! CF2BrCFBrO2 þ M
2CF2BrCFBrO2 ! 2CF2BrCFBrOꢀ þ O2
ꢀ
ꢀ
CF2BrCFBrO2 þ CF2CFBr ! CF2BrCFBrO2CF2CFBrꢀ
ꢀ
When the temperature of the reaction system, initially at
218 K, was raised rapidly to the room temperature without
(15)
Table 1
Analytical data of nine experiments, where indices i and f denote initial and final and Dp is the time of permanence of reaction system CF3OF + CF2CFBr + O2 in
reaction vessel
Run
T (K)
Dt (min)
CF3OFi (Torr)
CF2CFBri (Torr)
CF2CFBrf (Torr)
O2 (Torr)
CF2BrC(O)F (Torr)
2
8
273.0
253.5
239.0
239.0
239.0
239.0
218.0
218.0
218.0
10.0
42.0
1.6
2.1
2.4
1.0
0.9
1.1
0.9
0.9
1.0
12.4
18.9
16.5
30.7
12.3
12.7
11.5
20.7
19.0
–
–
42.5
63.6
95.2
100.7
76.0
69.2
48.2
65.9
66.3
10.0
16.5
15.6
27.5
11.6
11.9
10.1
–
4
121.0
64.5
–
5
–
6
65.2
–
7
120.0
180.0
180.0
66.5
–
10
11a
13
0.8
–
13.5
5.2
a
In this run the temperature of the reaction system CF3OF + CF2CFBr + O2, initially at 218 K, was raised rapidly to room temperature without previously removing
the oxygen. An explosion occurred with emission of light, giving CO2, C(O)F2, CF4 and Br2 as products.

738
J. Czarnowski / Journal of Fluorine Chemistry 127 (2006) 736–739
3. Experimental
(16)
The experiments were performed in a grease-free conven-
tional static system allowing pressure measurements at constant
volume and temperature. A spherical quartz bulb with a volume
of 270 cm3 was used as reaction vessel. The pressure was
measured with a quartz spiral gauge. Infrared spectra were
recorded on a Shimadzu IR-435 spectrometer and a Perkin-
Elmer 1600 Series FTIR spectrometer, using 10 cm cells
provided with NaCl and KBr windows, respectively.
CF2BrCFBrOꢀ ! CF2BrCðOÞF þ Brꢀ
CF2BrCFBrOꢀ ! CF2Brꢀ þ CðOÞFBr
(17)
(18)
CF2Brꢀ þ O2 þ M ! CF2BrO2 þ M
(19)
ꢀ
All reactants were purchased commercially. CF3OF (PCR,
97–98%) was washed with 0.1 mol dmꢁ3 NaOH solution and
filtered at 80 K [30]. The CF2CFBr (PCR, 97–98%) contained
CF4, and CF3CF3 as impurities. These impurities are more
volatile than CF2CFBr, but could not be separated by fractional
condensation, distilling together. The CF2CFBr was purified by
intermittent brief evacuation cycles at 153 K, opening and
closing the trap valve. This procedure was repeated several
times until the disappearance of the respective very strong
absorption bands of CF4 [31] and CF3CF3 [32] at 1279 and
1250 cmꢁ1 in the IR spectrum of CF2CFBr. Oxygen (La
Oxigena, 99.99%) was bubbled through 98% analytical-grade
H2SO4, and passed slowly through a Pyrex coil at 153 K.
Fourteen experiments were made in the temperature range
273–218 K. The initial pressure of CF3OF was varied between
0.9 and 2.4 Torr, that of CF2CFBr between 11.5 and 30.7 Torr
and that of O2 between 42.5 and 100.7 Torr.
2CF2BrO2 ! 2CF2BrOꢀ þ O2
(20)
(21)
ꢀ
CF2BrOꢀ ! CðOÞF2 þ Brꢀ
The primary path is the homolytic cleavage of a rather
weak O–F bond in a bimolecular reaction between CF3OF and
CF2CFBr, originating radicals CF3Oꢀ. The radicals CF3Oꢀ
add rapidly to the double bond forming CF3OCF2CFBrꢀ
radical. The lack of formation of CF3OOCF3 confirms that
the addition of CF3Oꢀ to the double bond is faster that any
other reaction of this radical. The values of order
1010 dm3 molꢁ1 sꢁ1 were obtained for the addition of CF3Oꢀ
to the alkenes [22–25].
The radicals CF3OCF2CFBrꢀ react with O2, producing
oxyradicals CF3OCF2CFBrOꢀ through the reacꢀtions (9) and
(10). The equilibrium study of Rꢀ + O2 $ RO2 [26], where
Rꢀ are alkyl radicals, suggests that at the pressure of O2 used
in this work the haloalkyl radicals generated in this reaction
system are almost completely eliminated by O2. It was also
reported [27], that the rate constants for the reactions of
methyl and halomethyl radicals with O2 exceed those for
fluorine-atom abstraction from CF3OF by three to four orders
of magnitude. It indicates that the reaction between radical
CF3OCF2CFBrꢀ and CF3OF to give the corresponding adduct
CF3OCF2CF2Br and CF3Oꢀ can be neglected. The oxyradicals
CF3OCF2CFBrOꢀ decompose eliminating bromine atoms,
which, analogously to the chlorine atoms [4–6,28] add rapidly
to the double bond, initiating the chain reaction and leading to
the formation of radicals CF2BrCFBrOꢀ in presence of O2.
The obtained products point out that the CF3Oꢀ and bromine
atom add to the less bulky CF2 group of the CF2CFBr, which
is consistent with the fact that the steric effects govern the
addition process.
To eliminate the oxygen in excess from the reaction mixture,
the reaction vessel was rapidly cooled down to liquid–air
temperature and O2 evacuated.
Acknowledgments
This work was supported by the Consejo Nacional de
´
´
Investigaciones Cientıficas y Tecnicas, CONICET. The author
thanks Mr. Z. Czarnowski for helpful comments and the Max
Planck Institute for Biophysical Chemistry, Goettingen,
through the Partner Group for Chlorofluorocarbons in the
Atmosphere, for financial support.
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