Kinetics and mechanism of thermal gas-phase oxidation of hexafluoropropene in the presence of trifluoromethylhypofluorite, CF3OF

Article abstract of DOI:10.1039/a908989f

The oxidation of hexafluoropropene with molecular oxygen in the presence of CF3OF has been studied at 303.0, 313.0 and 323.4 K, using a conventional static system. The initial pressure of CF₃OF was varied between 1.7 and 16.6 Torr, that of C₃F₆ between 10.0 and 120.2 Torr and that of O₂ between 82.0 and 599.8 Torr. Several runs were made in the presence of 325.3-499.2 Torr of N₂. Major products were COF₂, CF₃C(O)F, CF₃OC(O)F and a new compound, CF₃OCF₂OCF₂C(O)F. The latter was characterised by the determination of its relative molecular mass and its IR spectrum, which is consistent with the calculated one using theoretical approximations. Small amounts of CF₃OCF₂C(O)F were also formed. The oxidation is an homogeneous chain reaction, attaining, at the pressure of O₂ used, the pseudo-zero-order condition with respect to O₂ as a reactant. It is independent of the total pressure. The basic steps are: the thermal generation of CF₃O radicals by abstraction of the fluorine atom from CF3OF by C₃F₆, chain initiation by the addition of CF₃O to olefin leading, in the presence of O₂, to the formation of haloalkoxy radicals, which decompose via the C-C scission to give products containing C(O)F group and CF₃ radicals, reforming CF₃O. The full mechanism is postulated.

Full text of DOI:10.1039/a908989f

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Kinetics and mechanism of thermal gas-phase oxidation of
hexaÑuoropropene in the presence of triÑuoromethylhypoÑuorite,
CF OF
3
M. dos Santos Afonso,a R. M. Romano,b C. O. Della Vedova*bc and J. Czarnowski*d
a INQUIMAE, Departamento de Qu•mica Inorganica, Anal•tica y Qu•mica, Facultad de Ciencias
Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellon II, 3er Piso
(1428) Buenos Aires, Republica Argentina
b CEQUINOR (CONICET , UNL P), Departamento de Qu•mica, Facultad de Ciencias Exactas,
Universidad Nacional de L a Plata, 47 esq. 115, (1900) L a Plata, Republica Argentina
c L aboratorio de Servicios a la Industria y al Sistema Cient•Ðco (UNL P-CIC-CONICET ),
Departamento de Qu•mica, Facultad de Ciencias Exactas Universidad Nacional de L a Plata,
47 esq. 115, (1900) L a Plata, Republica Argentina
d Instituto de Investigaciones Fisicoqu•micas T eoricas y Aplicadas, Sucursal 4, Casilla de Correo
16, (1900) L a Plata, Republica Argentina
Received 12th November 1999, Accepted 4th February 2000
Published on the Web 10th March 2000
The oxidation of hexaÑuoropropene with molecular oxygen in the presence of CF OF has been studied at
3
303.0, 313.0 and 323.4 K, using a conventional static system. The initial pressure of CF OF was varied
3
between 1.7 and 16.6 Torr, that of C F between 10.0 and 120.2 Torr and that of O between 82.0 and 599.8
3 6
2
Torr. Several runs were made in the presence of 325.3È499.2 Torr of N . Major products were COF ,
2
2
CF C(O)F, CF OC(O)F and a new compound, CF OCF OCF C(O)F. The latter was characterised by the
determination of its relative molecular mass and its IR spectrum, which is consistent with the calculated one
using theoretical approximations. Small amounts of CF OCF C(O)F were also formed. The oxidation is an
homogeneous chain reaction, attaining, at the pressure of O used, the pseudo-zero-order condition with
respect to O as a reactant. It is independent of the total pressure. The basic steps are: the thermal generation
of CF O~ radicals by abstraction of the Ñuorine atom from CF OF by C F , chain initiation by the addition
of CF O~ to oleÐn leading, in the presence of O , to the formation of haloalkoxy radicals, which decompose
via the CÈC scission to give products containing C(O)F group and CF~ radicals, reforming CF O~. The full
3
3
3
2
2
3
2
2
2
3
3
3
3 6
2
3
3
mechanism is postulated.
main product. The observed reactivity and low stereo- and
regioselectivity indicated that its formation occurred via a free
radical mechanism.
Introduction
During the last two decades Ñuorine chemistry has been con-
tinuously growing in terms of scientiÐc contributions, as well
as in the variety of innovative technologies and applications.
HypoÑuorite chemistry has led to the synthesis of a variety of
new compounds with applications in many industrially impor-
tant areas such as alternative refrigerants, pharmaceutical
drugs, electronics, automotives, optics, aeronautical, mem-
branes and coatings. The stable and easily handled tri-
The investigation of kinetic and mechanistic aspects of the
additions of CF OF to haloalkenes was initiated by dos
3
Santos Afonso and Schumacher, studying the thermal gas-
phase reactions of CF OF with C F 6 and C F .7 It was
3
3 6
4 8
followed by the studies of thermal reactions of CF OF with
3
CF CCl ,8 CHClCCl 9 and CCl CCl 10 in the absence of
2
2
2
2
2
O
and those of the thermal oxidations of CF CCl ,11
ÑuoromethylhypoÑuorite, CF OF, containing a weak OÈF
2
2
2
3
CHClCCl 12 and CCl CCl 13 initiated by addition of
bond, is a versatile and selective Ñuorinating agent.1 The value
2
2
2
CF OF to the double bond.
of 43.5 kcal mol~1 for the dissociation energy of its OÈF bond
was determined by Czarnowski et al.2 and, independently, by
3
In the above reactions of haloalkenes (HA) with CF OF, in
3
the absence as well as in the presence of O , two initial steps,
2
Kennedy and Levy.3 The thermal addition of CF OF across
3
common to both conditions, were postulated: the generation
the double bond of a variety of oleÐns belongs to a very inter-
of CF O~ and F(HA)~ in the bimolecular reaction between
esting class of reactions, where a saturated compound with a
relatively weak bond initiates a series of reactions, important
from a synthetic and mechanistic point of view. It is expected
that the Ðeld of Ñuoroxy compounds will continue to grow
with beneÐts to organoÑuorine and general chemistry.
The synthetic aspects of thermal addition of CF OF to
several haloalkenes (HA) have been reported by Johri and
DesMarteau4 and Sekiya and Ueda.5 In their works the tri-
Ñuoromethylhaloalkyl ether, CF O(HA)F, was formed as the
3
CF OF and oleÐn and the addition of CF O~ to the double
3
3
bond giving CF O(HA)~. The elemental steps, postulated in
these previous works, are referred to the analogue reactions in
3
the reactions scheme for the oxidation of C F , initiated by
3 6
CF OF, summarised in Table 1.
3
3
CF OF ] HA ] F(HA)~ ] CF O~
3
3
(see reactions (1) and (2), Table 1)
3
DOI: 10.1039/a908989f
Phys. Chem. Chem. Phys., 2000, 2, 1393È1399
This journal is ( The Owner Societies 2000
1393

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Table 1 Reaction mechanism for the oxidation of C F initiated by CF OFa
3 6
3
Initiation
(1) CF OF ] CF CF2CF ] CF C~FCF ] CF O~
3
3
2
3
3
3
(2) CF OF ] CF CF2CF CF CF C~F ] CF O~
3
3
2
3
2
2
3
(3) CF C~FCF ] O ] CF CF(O ~)CF
3
3
2
3
2
3
(4) CF CF(O ~)CF ] RO ] CF CF(O~)CF ] RO ] O
3
2
3
2
3
3
2
(5) CF CF(O~)CF ] CF C(O)F ] CF ~
3
3
3
3
(6) CF CF C~F ] O ] CF CF CF O ~
3
2
2
2
3
2
2 2
(7) CF CF CF O ~ ] RO ] CF CF CF O~ ] RO ] O
3
2
2
2
2
3
2
2
2
(8) CF CF CF O~ ] COF ] CF CF ~
3
2
2
2
3
2
(9) CF C~F ] O ] CF CF O ~
3
2
2
3
2 2
(10) CF CF O ~ ] RO ] CF CF O~ ] RO ] O
3
2
2
2
3
2
2
(11) CF CF O~ ] COF ] CF ~
3
2
2
3
Propagation
(12) CF ~ ] O ] CF O ~
3
2
3 2
(13) CF O ~ ] RO ] CF O~ ] RO ] O
3
2
2
3
2
(14) CF O~ ] CF CF2CF ] CF C~FCF OCF
3
3
2
3
2
3
(15) CF C~FCF OCF ] O ] CF CF(O ~)CF OCF
3
2
3
2
3
2
2
3
(16) CF CF(O ~)CF OCF ] RO ] CF CF(O~)CF OCF ] RO ] O
3
2
2
3
2
3
2
3
2
(17) CF CF(O~)CF OCF ] CF C(O)F ] CF OC~F
3
2
3
3
3
2
(18) CF CF(O~)CF OCF ] CF OCF C(O)F ] CF ~
3
2
3
3
2
3
(19) CF OC~F ] O ] CF OCF O ~
3
2
2
3
2 2
(20) CF OCF O ~ ] RO ] CF OCF O~ ] RO ] O
3
2
2
2
3
2
2
(21) CF OCF O~ ] CF CF2CF ] CF OCF OCF C~FCF
3
2
3
2
3
2
2
3
(22) CF OCF OCF C~FCF ] O ] CF OCF OCF CF(O ~)CF
3
2
2
3
2
3
2
2
2
3
(23) CF OCF OCF CF(O ~)CF ] RO ] CF OCF OCF CF(O~)CF ] RO ] O
3
2
2
2
3
2
3
2
2
3
2
(24) CF OCF OCF CF(O~)CF ] CF OCF OCF C(O)F ] CF ~
3
2
2
3
3
2
2
3
(25) CF O~ ] CF CF2CF ] CF CF(OCF )C~F
3
3
2
3
3
2
(26) CF CF(OCF )C~F ] O ] CF CF(OCF )CF O ~
3
3
2
2
3
3
2 2
(27) CF CF(OCF )CF O ~ ] RO ] CF CF(OCF )CF O~ ] RO ] O
3
3
2
2
2
3
3
2
2
(28) CF CF(OCF )CF O~ ] CF C~F(OCF ) ] COF
3
3
2
3
3
2
(29) CF C~FOCF ] O ] CF CF(O ~)OCF
3
3
2
3
2
3
(30) CF CF(O ~)OCF ] RO ] CF CF(O~)OCF ] RO ] O
3
2
3
2
3
3
2
(31) CF CF(O~)OCF ] CF OC(O)F ] CF ~
3
3
3
3
Termination
(32) CF O~ ] CF O~ ] CF OOCF
3
3
3
3
(33) CF OCF O~ ] CF OCF O~ ] CF OCF OOCF OCF
3
2
3
2
3
2
2
3
a RO \ CF CF(O )~CF , CF CF CF O ~, CF CF O ~, CF O ~, CF CF(O )~CF OCF , CF OCF O ~, CF OCF OCF CF(O )~CF ,
2
3
2
3
3
2
2
2
3
2
2
3
2
3
2
2
3
3
2
2
3
2
2
2
3
CF CF(OCF )CF O ~ or CF CF(O )~OCF and RO \ CF CF(O)~CF , CF CF CF O~, CF CF O~, CF O~, CF CF(O)~CF OCF ,
3
3
2
2
3
2
3
3
3
3
2
2
3
2
3
3
2
3
CF OCF O~, CF OCF OCF CF(O)~CF , CF CF(OCF )CF O~ or CF CF(OCF )O~.
3
2
3
2
2
3
3
3
2
3
3
CF O~ ] HA ] CF O(HA)~
dynamics rules may be carried over to the solution reac-
tions.16
3
3
(see reactions (14) and (25), Table 1)
The photooxidation of C F by molecular O was studied
3 6
2
in the condensed phase at 233 K,17 using a high-pressure
mercury lamp. It was found that the reaction is a complex
free-radical process, leading to the formation of perÑuoro and
polyperÑuoro ethers and products containing the C(O)F
group, but the full reaction scheme has not been clariÐed.
The perÑuoro ethers are proposed as alternative refrigerants
and polyperÑuoro ethers are used as lubricants in the mag-
netic recording industry.
In the absence of O the CF O(HA)~ radical abstracts the
2
3
Ñuorine atom from CF OF, accompanied by the simultaneous
3
formation of another CF O~ radical, initiating a chain reac-
3
tion with triÑuoromethylhaloalkyl ether as the main product.
CF O(HA)~ ] CF OF ] CF O(HA)F ] CF O~
3
3
3
3
In the presence of O , the CF O(HA)~ radicals react rapidly
2
3
with O , leading to the formation of haloalkoxy radicals:
2
In this work the study of the thermal gas-phase oxidation of
C F , initiated by the addition of CF OF to the double bond,
CF O(HA)~ ] O ] CF O(HA)O ~
3
2
3
2
3 6
3
has been undertaken in order to elucidate the elementary
steps involved in this system and to characterise the Ðnal reac-
tion products.
(see reactions (15) and (26), Table 1)
2CF O(HA)O ~ ] 2CF O(HA)O~ ] O
3
2
3
2
(see reactions (16) and (27), Table 1)
Experimental
The principal way of decomposition of CF O(HA)O~
3
haloalkoxy radicals, where HA \ CF CCl , CHClCCl and
The reaction proceeded with pressure decrease. The experi-
ments were performed in a grease-free static system allowing
pressure measurements at constant volume and temperature.
Two reaction vessels were used: a spherical quartz bulb with
a volume of 270 cm3 (S/V \ 0.75 cm~1) and another one of
similar dimension Ðlled with small pieces of quartz tubing (S/
V \ 4.7 cm~1). The pressure was measured with a quartz
spiral gauge and the temperature maintained within ^0.1 K
using a Lauda thermostat. Infrared spectra were recorded on
Perkin-Elmer 325 and Perkin-Elmer 1600 Series FTIR
spectrometers, using 10 cm cells provided with NaCl and KBr
2
2
2
CCl CCl , is the detachment of Cl~ for the CCl O group. The
2
2
2
chlorine atoms add to CX CCl , where X \ H, Cl or F, initi-
ating, in the presence of O , a chain reaction with Cl~ as chain
carriers and CX ClC(O)Cl as the main products.
In recent studies of the reactions of CF OF with C F and
2
2
2
2
3
3 6
other haloalkenes in a non-polar solvent, the formation of the
F(HA)~ and CF O(HA)~ intermediate radicals was observed
3
using EPR and ENDOR techniques and CF O(HA)F was
3
detected as the product.14,15 It indicates that, when the
reactantÈsolvent interaction is weak, the gas-phase reaction
1394
Phys. Chem. Chem. Phys., 2000, 2, 1393È1399

View Article Online
windows, respectively. The resolution of the reported wave-
tent with the relative molecular mass of 244 ^ 6 determined
for this compound by gasÈdensity measurements with a cali-
brated Pyrex bulb (theoretical relative molecular
mass of CF OCF OCF C(O)F is 248). No data on
numbers was 1 cm~1.
All reactants were purchased commercially. CF OF (PCR,
3
97È98%) was washed with 0.1 mol dm~3 NaOH solution and
3 2 2
CF OCF OCF C(O)F were found in the literature. The
Ðltered at 80 K.18 C F (PCR, 97È98%) was puriÐed by
3 6
3
2
2
repeated low-pressure trap-to-trap distillations on a vacuum
absorption band at 1550 cm~1 characteristic of the epoxy
group of hexaÑuoropropene oxide was not observed in the
infrared spectra.
line, the middle fraction being retained each time. The puriÐed
reactants were spectroscopically pure.
O
(La Oxigena,
2
99.99%) and N (La Oxigena, 99.9%) were bubbled through
All experiments were carried out until the complete con-
2
98% analytical-grade H SO , and passed slowly through a
sumption of C F . For analysing the reaction mixture of each
2
4
3 6
Pyrex coil at 123 K and at liquid air temperature, respectively.
The reaction was followed measuring the pressure decrease
as a function of time. The experiments were made at 303.0,
experiment, the reaction vessel was rapidly cooled to liquid air
temperature and the mixture separated by fraction conden-
sation. Any N , if present, and non-consumed O were
2
2
313.0 and 323.4 K. The initial pressure of CF OF was varied
separated as volatile at liquid air temperature as the Ðrst frac-
tion, Fr . The second fraction, Fr , volatile at 123 K, consist-
3
between 1.7 and 16.6 Torr, that of C F between 10.0 and
3 6
1 2
ed of CF OF and COF . The third fraction, Fr , volatile at
120.2 Torr and that of O between 82.0 and 599.8 Torr.
Several runs were carried out in the presence of N , varying
its pressure from 325.3 to 499.2 Torr. Two experiments were
2
3 2 3
163 K, was composed of CF C(O)F and CF OC(O)F. To
2
3 3
determine the concentration of COF in Fr and that of
2 2
CF OC(O)F in Fr , infrared calibration curves were con-
made in the reactor with S/V \ 4.7 cm~1.
3
3
structed using pure compounds, thereby allowing conversion
of the absorption intensities at 1942 and 1028 cm~1 to the
pressure of COF and CF OC(O)F, respectively. The amounts
Results
2
3
The reaction is an homogeneous chain reaction, independent
of CF OF and CF C(O)F in the corresponding fractions Fr
3
3
2
of the total pressure. At the pressure of O used in this work,
and Fr were obtained by di†erence. The compound
2
3
the reaction attained the pseudo-zero-order condition with
CF OCF OCF C(O)F remained as a residue at 163 K.
3
2
2
respect to O as a reactant.
The analytical data of 6 experiments are summarised in
Table 2. In this table, subscripts i and f signiÐy initial and
Ðnal. From these data the following can be deduced: The con-
sumption of CF OF was \1%. The concentrations of COF
2
Within the temperature range of this work, and in the
absence of CF OF, no reaction was observed between per-
3
Ñuoropropene and O after several hours.
2
3
2
In the presence of CF OF, three products were detected by
and CF OC(O)F formed were equal to those of CF C(O)F
3
3
3
3
means of infrared spectroscopy, COF ,19 CF C(O)F20 and
and CF OCF OCF C(O)F, respectively. The Ðnal pressure
2
3
2
2
CF OC(O)F.21,22 The formation of an unknown compound
decrease of each run, *p, was equal to the sum of the follow-
ing products: [CF C(O)F] ] [CF OCF OCF C(O)F]
]0.5[COF ]. The amount of O consumed in each run was
3
was also observed. According to the experimental evidence,
3
3
2
2
supported by theoretical Ðndings, the structure of
CF OCF OCF C(O)F has been assigned to this compound.
2
2
equal to the sum of [CF C(O)F] ] [CF OCF OCF C(O)F]
3
2
2
3
3
2
2
Its infrared spectrum, shown in Fig. 1, compares well with the
calculated one for this molecule, using the HF/3-21G*
approximation. It is discussed in more details in the Theoreti-
cal calculations section. The proposed composition is consis-
]0.5([COF ] ] [CF OC(O)F]). The sum of the products:
2
3
3
[CF C(O)F] ] [CF OCF OCF C(O)F] ] 0.5([COF ] ]
3
3
2
2
2
[CF OC(O)F]), was approximately equal to 98.5% [C F ] .
3 6 i
Discussion
Johri and DesMarteau showed in their study of reactivity of
CF OF with some haloalkenes in the condensed phase, that
3
these reactions occur via a free radical mechanism consistent
with the observed reactivity and low stereo- and regioselecti-
vity.4
The basic steps postulated to explain the experimental
results of our kinetic and mechanistic studies of the thermal
gas-phase reactions of CF OF with C F ,6 C F ,7 CF CCl ,8
3
3 6
4 8
2
2
CHClCCl 9 and CCl CCl 10 in the absence of O and
2
2
2
2
those of the oxidations of CF CCl ,11 CHClCCl 12 and
2
2
2
CCl CCl 13 initiated by addition of CF OF to the double
2
2
3
bond, were: 1. The generation of CF O~ and F(HA)~ radicals
3
in the bimolecular reaction between CF OF and the oleÐn. 2.
The addition of CF O~ to the double bond generating
CF O(HA)~ radicals. 3. The abstraction of Ñuorine atom from
3
Fig. 1 FTIR spectrum of CF OCF OCF C(O)F at room tem-
3
3
2
2
perature: pressure 2.5 Torr, optical path 10 cm, resolution 1 cm~1.
3
Table 2 Analytical data of 6 experiments carried out to the total consumption of C F a
3 6
*p
CF OF
3
C F
/Torr
O
/Torr
Fr
/Torr
COF
/Torr
Fr
/Torr
CF OC(O)F
/Torr
CF OCF OCF C(O)F
/Torr
O
/Torr
i
3 6i
2i
2
2f
3
3
3
2
2
2f
Run
T /K
/Torr
/Torr
8
12
15
16
18
19
303.0
313.0
313.0
313.0
323.4
323.4
25.1
24.1
50.2
13.4
8.2
3.5
3.2
5.0
4.2
2.6
2.6
30.6
29.4
61.2
16.4
10.0
29.9
294.9
102.0
121.6
106.1
96.6
13.7
12.8
24.2
10.1
6.3
10.3
9.8
19.6
6.0
3.8
11.1
20.4
19.5
40.0
11.1
6.9
10.4
9.8
19.7
6.1
3.8
10.9
9.8
9.5
20.5
5.1
2.9
9.3
259.7
86.4
51.7
87.1
85.1
64.9
24.5
99.3
13.5
20.1
a Fr \ [CF OF] ] [COF ] and Fr \ [CF OC(O)F] ] [CF C(O)F].
2
3
2
3
3
3
Phys. Chem. Chem. Phys., 2000, 2, 1393È1399
1395

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CF OF by CF O(HA)~, giving CF O~ as chain carriers and
CF OC~F and CF OCF O~ radicals is not expected, as the
3
3
3
3
2
3
2
CF O(HA)F as the main products in the absence of O ; (4)
activation energy for the cleavage of the CÈO group in per-
Ñuorinated ethers is rather high, near 100 kcal mol~1.35 The
extrusion of Ñuorine atom adjacent to oxygen atom in per-
Ñuoroalkoxy radicals is unlikely for these processes are highly
endothermic.36
3
2
the rapid reaction of CF O(HA)~ radicals, where HA \
3
CX CCl and X \ H, Cl or F, with O , leading to the forma-
2
2
2
tion of haloalkoxy radicals and CX ClC(O)Cl as the main
2
products in the presence of O .
2
It is well known that the principal way of disappearance of
As during the reaction course the consumption of CF OF
3
haloalkoxy radicals, containing an oxygen atom attached to
was \1%, the production of CF C(O)F and COF through
3
2
the CCl group, is the detachment of chlorine atoms. The
reactions (5), (8) and (11) is very small in comparison with the
chain formation of these products.
2
addition of the chlorine atoms to CX CCl in the presence of
2
2
oxygen leads to the chain formation of CX ClCCl O~ radicals
that decompose reforming Cl~ and giving CX ClC(O)Cl.
Recently the radical mechanism was strongly reinforced by
Reaction (18) was proposed as an alternative route of
decomposition of the CF C(O~)FCF OCF radical, giving
2
2
2
3 2 3
CF OCF C(O)F and the CF~ radical. Comparison of the
3
2
3
the work of Navarrini et al. who studied the kinetics of the
infrared spectrum of CF OCF C(O)F37 with that obtained in
3
2
addition of CF OF to a series of electron poor haloalkenes in
this work, indicates that, relative to other products, only small
amounts of this product have been formed. This is conÐrmed
3
a non-polar solvent in the absence of O .15 They determined
2
the relative rate constants of the initiation, chain-propagation
by the yields of the major products: [CF C(O)F] ]
3
and termination steps. The Ðnal products were CF O(HA)F.
[CF OCF OCF C(O)F] ] 0.5([COF ] ] [CF OC(O)F]) B
3
Their work clearly concluded that CF OF reacts through the
3
3
2
2
2
3
98.5% [C F ]. It indicates that reaction (17) is more rapid
3 6
homolytic cleavage of its OÈF bond. The ultimate proof for
than reaction (18), suggesting that oxygen atoms attached to
the existence of a free radical mechanism was the observation
each of the adjacent carbon atoms in the CÈC bond, as in the
of F(HA)~ and CF O(HA)~ intermediate radicals by EPR and
OÈCÈCÈ(O)ÈCF group of the CF CF(O~)CF OCF radical,
enhance the weakening of this CÈC bond. Reaction (17) was
3
3
3
2
3
ENDOR techniques in the reaction system CF OF
3
] perÑuoroalkenes.14 It indicates that, when the reactantÈ
postulated to explain the reported formation of CF C(O)F in
3
condensed-phase photooxidation of C F .17 Rates of b scis-
3 6
solvent interaction is weak, the gas-phase reaction dynamics
rules may be carried over to the solution reactions,16 suggest-
ing that in this case the mechanism for the liquid-phase reac-
tion does not di†er substantially from that for the gas-phase
reaction.
sion are partially determined by the stability of radicals pro-
duced, depending on the nature and degree of substitution at
the radical centre.17,38 It appears that the introduction of the
CF OCF O group replacing the CF O group on the c-
3 2 3
carbon in the CF OCF OCF CF(O~)CF radical favours the
In order to propose a reaction mechanism for the gas-phase
oxidation of C F in the presence of CF OF, the above con-
3
2
2
3
a scission by inductive e†ect.
The addition of such a bulky radical as CF OCF O~ to the
3 6
3
siderations were taken into account. The mechanism present-
ed in Table 1 explains well the experimental results obtained
in our work.
3
2
b-carbon of C F was considered, but it was not included in
3 6
the mechanism, assuming that the rate of this addition would
At temperatures of this work CF OF and C F are stable
be much lower that the rate of reaction (21).
3
3 6
molecules and the reaction is homogeneous, thus it can be
concluded that the primary process of this reaction is the
bimolecular reaction between CF OF with C F , generating
Reactions (32) and (33) were proposed as chain termination
reactions. No products of these reactions were detected. It
indicates that, in comparison to the long-chain products, only
very small amounts of these compounds were formed. In the
reaction of CF OF with C F , in absence of O ,6 where
3
3 6
F(C F )~ and CF O~ radicals. The CF O~ radicals react
3 6
rapidly with perÑuoropropene, forming CF O(C F )~ radicals.
3
3
3
3 6
3
3 6
2
It was reported that the addition of CF O~ to the double
bond of alkenes is considerably faster than any other reaction
CF O~ was generated as a chain carrier by the abstraction of
3
3
Ñuorine atom from CF OF by CF O(HA)~, small amounts of
CF OOCF were observed by its characteristic absorption
3
3
of CF O~,23h28 the values of the corresponding rate constants
3
3
3
being of the order of 1010 dm3 mol~1 s~1 for the addition of
band at 1166 cm~1 in the infrared spectra of Ðnal reaction
CF O~
to
several
alkenes.25h28
The
compounds
mixtures of experiments where [CF OF] [ [C F ].
3
3 3 6
The CF O~ radicals generated from CF OF through reac-
CF OCF CF CF and CF OCF(CF ) were reported as pro-
3
2
2
3
3
3 2
3 3
tions (1) and (2) initiate chains with CF O~ and CF OCF O~
ducts of the reaction of CF OF with CF CF2CF in the con-
3
3
2
3
3
2
densed phase and in the absence of O ,4 indicating that the
radicals, originated by the CÈC bond scission of per-
2
addition of CF O~ radicals to the double bond of hexa-
3
Ñuoroalkoxy radicals, as the chain carriers.
Ñuoropropene proceeds via two channels: addition to a and b
Applying the steady-state approximation to this mecha-
mism the following expression was obtained for the consump-
tion of perÑuoropropene:
carbon, respectively.
The equilibrium study of R~ ] O % RO~ 29 indicates that
2
2
at the pressure of O used in this work, the perÑuoro alkyl
2
[d[C F ]/dt
radicals react rapidly with O . The rate constants for
haloalkyl radicals with O exceed those of Ñuorine atom
abstraction from CF OF by three or four orders of magni-
tude.30,31 The lack of formation of hexaÑuoropropene oxide
3 6
2
\ (k ] k )[CF OF][C F ]
2
1
2
3
3 6
3
](k ] k )[(k ] k )/2k ]0.5[CF OF]0.5[C F ]1.5
14 25 32 3 6
1
2
3
indicates that perÑuorinated peroxy radicals do not add to the
double bond, disappearing by their self-reaction to give per-
Ñuoroalkoxy radicals. The available data for the additions of
methyl peroxy radicals to alkenes,32 giving epoxy compounds
as products, indicate that the rate constants for these addi-
tions were several order of magnitude smaller than those for
the self-reactions of peroxy radicals.
By analogy to other perÑuoroalkoxy radicals,33,34 the per-
Ñuoroalkoxy radicals, containing the ether group, decompose
via the CÈC bond scission, generating, through a sequence of
]k (k /2k )0.5[(k ] k )/2k ]0.25
21 14 33 32
1
2
][CF OF]0.25[C F ]1.75
(I)
3
3 6
The kinetic chain length, the ratio between the consumption
rate of C F and the rate of initiation, (k ] k )[CF OF]
3 6
1
2
3
i
[C F ] may be written as follows:
3 6 i
CL \ 1 ] j[([C F ]/[CF OF])/([C F ] /[CF OF] )]0.25
3 6 3 6 i
3
3
i
(II)
reactions, compounds containing the C(O)F group and CF~
3
radicals, which reform the CF O~ chain carriers. The scission
3
where the subscript i denotes initial concentrations of reac-
tants.
of CÈO bond in the CÈOÈC ether group in CF O(C F )O~,
3
3 6
1396
Phys. Chem. Chem. Phys., 2000, 2, 1393È1399

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The initial chain length for each experiment is:
CL \ 1 ] j
(III)
i
where
j \ M[(k ] k )[CF O~]
14
25
3
] k [CF OCF O~]][C F ] N/(2k [CF O~]2) (IV)
21 3 6 i 32
3
2
3
j is the chain length of a special type of the chain reaction,
which propagates by chain carriers generated and successively
reformed by the scission of the CÈC bond of perÑuoroalkoxy
radicals. The values of j obtained, as described in the follow-
ing section, show that j is independent of the temperature, the
ratio of propagation rate to that of termination being practi-
cally constant. As the termination rate does not depend on
temperature, it follows that the propagation rate is also tem-
perature independent. Assuming that k is equal to the col-
32
lisional factor, 1010 dm3 mol~1 s~1, it indicates that the
Fig. 3 The plot of (CL [ 1)
vs. M([C F ]/[CF OF])/
exp
3 6
3
activation energies E for the addition of perÑuoroalkoxy rad-
icals to the double bond of perÑuorinated alkenes must be
B 0. The reported rate con-
stants for the addition of CF O~ to alkenes are of the order of
1010 dm3 mol~1 s~1.25h28 The behaviour of j also suggests,
that the activation energy for the rupture of the CÈC bond in
perÑuoroalkoxy radicals to give CF O~ and CF OCF O~ in
the presence of O must be very small, the rate constant for
([C F ] /[CF OF] )N0.25 at 313.0 K, where CL \ experimental
A
3 6 i
3
i
exp
chain length \ V /(k ] k )[CF OF][C F ]. The value of the slope,
exp 3 6
equal to j, is given in the Table 3.
1
2
3
very small: E
B E
B E
A14
A21
A25
3
The values of j and those of k ] k at di†erent tem-
peratures are given in Table 3. In the temperature range
studied, j is equal to 184.2 and does not depend on the initial
pressure of each experiment.
The results of 16 experiments¤ illustrate the good agree-
ment between the experimental rate of consumption of C F ,
1
2
3
3
2
2
the formation of these chain carriers being practically
independent of the temperature. It was determined from the
pulse radiolysis experiments at 296 K that CF CF CF O~
3
2
2
radicals decompose with
a rate greater than 1.5 ] 105
3 6
3 6
V
\ R(*p/*t), and V , the rate of consumption of C F ,
s~1.33 The atmospheric fate of the CF CF CF O~ 33 and
exp
cal
3
2
2
CF CF(O~)CF 34 radicals is the CÈC bond scission.
3
3
Data treatment
j was determined as a slope, plotting CL [ 1 as a function
exp
of M([C F ]/[CF OF])/([C F ] /[CF OF] )N0.25. The CL is
3 6
3
3 6 i
3
i
exp
/(k
exp
the experimental chain length equal to
V
1
] k )[CF OF][C F ], where V is:
2
3
3 6
exp
[d[C F ]/dt \
3 6
M1 ] j[([C F ]/[CF OF])/([C F ] /[CF OF] )]0.25N
3 6
3
3 6 i
3
i
] (k ] k )[CF OF][C F ] (V)
1
2
3
3 6
Plots at 303.0, 313.0 and 323.4 K are illustrated in Figs. 2È4,
respectively.
The sum of the constants k ] k was calculated by the fol-
1
2
lowing expression obtained previously:6
k ] k \ 7.6 ^ 0.68 ] 108
1
2
] exp([15.9 ^ 0.45 kcal/RT )dm3 mol~1 s~1
Fig. 4 The plot of (CL [ 1)
vs. M([C F ]/[CF OF])/
exp
3 6
3
([C F ] /[CF OF] )N0.25 at 323.4 K, where CL \ experimental
3 6 i
3
i
exp
chain length \ V /(k ] k )[CF OF][C F ]. The value of the slope,
exp 3 6
equal to j, is given in the Table 3.
1
2
3
Table 3 Constants (k ] k ) for the addition of CF OF to the
1
2
3
double bond of C F and j \ CL [ 1, where CL is the initial chain
3 6
length of each experiment, at di†erent temperatures
i
i
T /K
k /10~5 Torr~1 min~1
j
1
303.0
313.0
323.4
0.835
1.880
4.135
184.1
184.2
184.3
Fig. 2 The plot of (CL [ 1)
vs. M([C F ]/[CF OF])/
3 6
exp
3
([C F ] /[CF OF] )N0.25 at 303.0 K, where CL \ experimental
3 6 i
3
i
exp
chain length \ V /(k ] k )[CF OF][C F ]. The value of the slope,
exp 3 6
equal to j, is given in the Table 3.
¤ Available as electronic supplementary information. See http://
www.rsc.org/suppdata/cp/a9/a908989f
1
2
3
Phys. Chem. Chem. Phys., 2000, 2, 1393È1399
1397

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Table 4 Calculated energies for the gauche and cis conformers of CF OCF OCF C(O)F
3
2
2
Approximation
E
/Hartrees
E
/Hartrees
*E (E [ E
)/kcal mol~1
gauche
cis
cis
gauche
HF/3-21G*
B3LYP/6-31 ] G*
[1165.2261949
[863.852261572
[1165.2246511
[863.851952905
0.97
0.19
calculated by eqn. (V) and show the experimental chain
ing to the spectrum depicted in Fig. 1. Calculated values are
scaled by the 0.9 factor, due to the known overestimation of
the wavenumbers by ab initio methods in the HartreeÈFock
approximation. Theoretical results are in good agreement
with experimental values.
lengths CL \ V /(k ] k )[CF OF][C F ]. *p is the pres-
exp
exp
1
2
3
3 6
sure decrease, *t the reaction time and R is equal to
[C F ] /*p . The subscripts i and f signify initial and Ðnal.
3 6 i
f
The amounts of C F consumed, corresponding to *p, were
3 6
calculated, point by point, multiplying *p by R. The amounts
CF OCF (O)F was also investigated because of the follow-
3
2
of CF OF consumed at *t \ t [ t
were computed, point
ing reasons: the formation of this compound during the
thermal-reaction was postulated and was found consistent
with the experimental results. Its known IR spectrum allows
comparison with the theoretical data obtained for this com-
pound using the same procedure that was employed for
CF OCF OCF (O)F thus conÐrming the reliability of the
3
n
n~1
by point, by [CF OF] M1 [ exp[ [ (k ] k )*t[C F ] ]N,
3
n
1
2
3 6 m
where n \ 1, 2, 3, . . . n and [C F ] is the mean pressure of
3 6 m
C F in the interval *t.
3 6
3
2
2
Theoretical calculations
In order to compare the experimental vibrational data of the
obtained unknown compound with the proposed one,
CF OCF OCF C(O)F was theoretically studied with ab initio
Table 5 Calculated wavenumbers of the gauche and cis conformers
of CF OCF OCF C(O)F using the HF/3-21G* approximation and
3
2
2
3
2
2
experimental values corresponding to the spectrum depicted in Fig. 1
and density functional theory methods.
All calculations were performed using the GAUSSIAN 94
program system39 under a Linux environment on a personal
computer. First, the electronic energy surface was investigated,
using the HF/3-21G* and B3LYP/6-31 ] G* levels of theory.
Two stable conformers were found, as depicted in Fig. 5. The
Ðrst conformer presents the C(1)ÈO(2) bond gauche with
respect to the C(4)ÈO(5) bond and the C(1)ÈF(3) bond stag-
gered between the C(4)ÈF(6) and the C(4)ÈO(5) bonds. The
second conformer with the C(1)ÈO(2) bond cis with respect to
the C(4)ÈO(5) and the C(1)ÈF(3) bond staggered between the
C(4)ÈF(6) and C(4)ÈF(7) bonds. These structures are in agree-
ment with the expected ones for compounds of the type
RCC(O)X.40 Energy minima with respect to nuclear coordi-
nates for which no imaginary wavenumbers occur were
obtained by simultaneous relaxation of all geometric param-
eters for the two conformers depicted in Fig. 5. Energy di†er-
ences calculated for the two forms with the HF/3-21G* and
B3LYP/6-31 ] G* levels of the theory are listed in Table 4.
The gauche conformer is predicted by both approximations as
the most stable form.
Gauche conformer
l/cm~1a
Cis conformer
l/cm~1a
Experimental
l/cm~1
1959
1898
1887
1393
1347
1330
1295
1290
1280
1238
1229
1181
1116
1892
1393
1342
1323
1289
1283
1275
1237
1217
1199
1119
1317
1284
1250
1174
1147
1125
1104
1028
952
948
948
906
819
793
770
719
672
641
613
570
545
542
497
478
441
364
349
334
306
253
212
189
148
96
814
793
749
732
687
645
618
576
555
537
497
479
408
384
337
319
303
242
226
197
149
96
769
716
690
Table 5 lists vibrational wavenumbers of two conformers of
CF OCF OCF C(O)F calculated using the HF/3-21G*
3
2
2
618
approximation, together with the experimental IR wavenum-
ber values obtained for the unknown compound, correspond-
62
58
62
47
43
35
37
32
26
25
Fig. 5 Molecular
models
of
the
two
conformers
of
CF OCF OCF C(O)F calculated with the B3LYP/6-31]G* approx-
a Scaled by a factor of 0.9.
3
2
2
imation.
1398
Phys. Chem. Chem. Phys., 2000, 2, 1393È1399

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Table 6 Calculated wavenumbers of the gauche and cis conformers
Universidad Nacional de La Plata, Republica Argentina for
Ðnancial support and to Fundacion Antorchas for the Nation-
al Award to the Argentinean cooperation.
of CF OCF C(O)F using the HF/3-21G* approximation and experi-
3
2
mental values (ref. 37)
Gauche conformer
l/cm~1a
Cis conformer
l/cm~1a
Experimental37
l/cm~1
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2
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6
26 Ch. Kelly, J. Treacy, H. W. Sidebottom and O. J. Nielsen, Chem.
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3
2
2
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3
2
lated spectrum of the proposed new and unknown compound
is in good agreement with experimental results. According to
this evidence and taking into account its molecular mass
determined by gas-density measurements the formation of
CF OCF OCF C(O)F was thus proposed. Experiments to
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3
2
2
obtain a larger quantity of this compound and to complete its
analysis are currently being carried out in our laboratory.
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The authors thank Mr Z. Czarnowski for very helpful com-
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Nacional de Investigaciones Cient•Ðcas y Tecnicas. C.O.D.V.
and R.M.R. thank the Fundacion Antorchas (Republica
Argentina), Alexander von Humboldt Stiftung, the British
Council and DAAD (Deutsche Akademische Austauschdienst,
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39 GAUSSIAN 94, Revision D.3, M. J. Frisch, G. W. Trucks, H. B.
Schlegel, P. M. W. Gill, G. Johnson, M. A. Robb, J. R. Cheese-
man, T. Keith, G. A. Petersson, J. A. Montgomery, K. Raghava-
chari, M. A. Al-Laham, V. G. Zakrzewski, J. V. Ortiz, J. B.
Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayakkara, M.
Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong,
J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J.
Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-
Gordon, C. Gonzalez and J. A. Pople, Gaussian, Inc., Pittsburgh
PA, 1995.
Germany)
for
Ðnancial
support
and
for
the
DAADÈFundacion Antorchas and Alexander von Humboldt
StiftungÈFundacion Antorchas Awards to the GermanÈ
Argentine cooperation and the British CouncilÈFundacion
Antorchas Award to the BritishÈArgentinean cooperation.
They also thank Consejo Nacional de Investigaciones
Cient•Ðcas
y Tecnicas (CONICET) (PIP 4695), Agencia
40 J. R. Durig, J. F. Davis and G. A. Guirgis, J. Mol. Struct., 1994,
Nacional de Promocion Cient•Ðca y Technologica (PICT 122),
and Comision de Investigaciones Cient•Ðcas de la Provincia
de Buenos Aires (CIC), Republica Argentina, for Ðnancial
support. They are indebted to Facultad de Ciencias Exactas,
328, 19.
Paper a908989f
Phys. Chem. Chem. Phys., 2000, 2, 1393È1399
1399