Intermediates of thermal transformations of perfluoro-organic compounds. New spectral data and reactions

Article abstract of DOI:10.1016/S0022-1139(98)00325-X

New completely fluorinated intermediates were identified from spectroscopic studies of thermal reactions of perfluoro-olefins, some carbocycles, and perfluoro-oxiranes in the gas phase. Detailed mechanisms of several complex reactions were established. For example, the mechanism of higher perfluoro-olefin synthesis from lower species under pyrolysis was established. Following this mechanism the first stage is the reversible reaction of biradicaloid formation which are isomeric to perfluoro-olefins. It was shown that biradicaloids are the primary intermediates in recombination of singlet completely fluorinated carbenes.

Full text of DOI:10.1016/S0022-1139(98)00325-X

Journal of Fluorine Chemistry 96 (1999) 35±42
Intermediates of thermal transformations of per¯uoro-organic
compounds. New spectral data and reactions
*
Nikolay N. Buravtsev, Yuly A. Kolbanovsky
Laboratory of Physical±Chemical Impulsive Processes, A.V. Topchiev Institute of Petrochemical Synthesis,
Russian Academy of Sciences, Leninsky Prospect, 29 Moscow, Russian Federation
Received 23 September 1997; accepted 25 August 1998
Abstract
New completely ¯uorinated intermediates were identi®ed from spectroscopic studies of thermal reactions of per¯uoro-ole®ns, some
carbocycles, and per¯uoro-oxiranes in the gas phase. Detailed mechanisms of several complex reactions were established. For example, the
mechanism of higher per¯uoro-ole®n synthesis from lower species under pyrolysis was established. Following this mechanism the ®rst
stage is the reversible reaction of biradicaloid formation which are isomeric to per¯uoro-ole®ns. It was shown that biradicaloids are the
primary intermediates in recombination of singlet completely ¯uorinated carbenes. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Fluorinated biradicals; Biradicaloids; Carbenes; Pyrolysis mechanism
1
. Introduction
2. Methods
Many different intermediate species appear in thermal
2.1. Experimental methods
transformations of ¯uoro-organic substances. Radicals, bir-
adicals, biradicaloids, and carbenes take part in these trans-
formations at moderate temperatures. Such a variety of
forms of neutral intermediate species in gas phase are not
met with in other sections of chemistry.
This paper is devoted to the results of time-resolved
spectroscopic investigations of these intermediates and their
reactions. The data are of de®nite interest for chemical
theory and the detailed mechanisms of their reactions are
of importance for optimization of their technology. Tetra-
Thermal transformations of ¯uoro-organic compounds
were studied under true vapor phase pyrolysis carried
out in the free-piston adiabatic compression set-up
(ACS) described in [5]. Pyrolysis under pulsed adiabatic
compression conditions is principally anisothermal.
Under initial pressure 1 atm the volume of the thermal
boundary layer is negligibly small as compared to the
total volume of the compressed gas, thus the reaction space
is uniform. The walls of the adiabatic unit remain cold
and the process always occurs in strictly homogeneous
conditions.
¯uoroethylene cyclodimerization and its pyrolysis are typi-
cal examples.
The formation of intermediates in thermal processes is
often interpreted as the result of the interaction of valence
saturated molecules although spectroscopic investigations
show that reaction mechanisms are much more complicated
containing many stages.
Recently signi®cant progress has been achieved in inves-
tigations of the intermediates in thermal reactions of ¯uoro-
organic compounds in the gas phase. Many of these inves-
tigations have been examined in detailed reviews (see [1±
Decomposition of ¯uoro-organic compounds under infra-
red multiphoton dissociation (IRMPD) has been investigated
using methods described in [6]. The beam of a pulsed CO2
laser crossed the molecular beam at right angles. The
features of the rotating source molecular beam machine
have been previously described [7]. The fragments created
by IRMPD were characterized by usual time-of-light tech-
niques. A multichannel scaler triggered by the laser, col-
lected ion counts as a function of the ¯ight time from the
interaction region to the detector. Ultimately the transla-
tional energy distribution of these fragments has been
obtained. This information is of importance for comparison
purposes for alternative mechanisms of reactions.
4]). The present review concentrates on studies which were
not included in [1±4].
*
Corresponding author. Fax: +70-95-2302224.
0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 9 8 ) 0 0 3 2 5 - X

3
6
N.N. Buravtsev, Y.A. Kolbanovsky / Journal of Fluorine Chemistry 96 (1999) 35±42
2
.2. Calculation methods
Among new calculated methods we will point to the lately
is now believed to include two stages:
(
(
2)
3)
developed approach for determination of the enthalpy of
intermediate formation reaction using kinetic spectroscopy
data [8]. This can be explained as follows. The intermediate
concentrations which are known from the Lambert±Beer
equation are equated to those from the ordinary thermo-
dynamic ratio, where these concentrations are expressed
through the concentration of the initial substance and the
equilibrium constant. This relation gives the ÁH of the
reaction. If the heat of formation of the initial compound is
known, it is possible to determine also the heat of formation
of the intermediate.
involving 1,4-biradical [14].
4
. Mechanisms of tetrafluoroethylene
‡2-cyclodimerization and
octafluorocyclobutane 2‡2-cycloreversion
2
(4)
The latter adds to a ꢀ-bond and forms a 1,4-biradical.
Further development of this method allowed extension of
its application to determine heats of reaction for two inter-
mediates with overlapping spectra using kinetic spectro-
scopy data [9].
(
5)
Kinetic estimates of the alternative to reaction (5) (recom-
bination of biradicaloids) were carried out in [15] and it was
found that it cannot compete with (5). It was also shown that
the direct square recombination of biradicaloids to octa-
3
. Formation of initial intermediates from
perfluoro-olefins ± biradicaloids
¯uorocyclobutane can be rejected (relative to (3)).
Unfortunately, we did not ®nd the 1,4-biradical in the
Tetra¯uoroethylene pyrolysis was studied in 1988 [10]
and for the ®rst time it was shown that at temperature near
00 K light absorption is observed in the UV-region of the
spectrum. It was ®rst assumed that tri¯uoromethyl¯uoro-
relatively low temperature process of cyclodimerization.
However, its formation was revealed in octa¯uorocyclobu-
tane pyrolysis and its spectrum was obtained. It was ascer-
tained that 1,4-biradical formation precedes the stage of the
tetra¯uoroethylene biradicaloid generation (Fig. 1). This
result con®rms that the cycloreversion and cyclodimeriza-
tion reactions go through the stages (3)±(5).
9
carbene :CF CF had been formed. It was shown [11] that
3
this carbene has an absorption band in the visible with a
maximum at 465 nm. The intermediate detected in [10] did
not have this band but had an absorption band in the visible
with a maximum at 500 nm. The reaction heat for the
intermediate formation was determined as 18Æ2 kcal/mol
and it was established that its formation and disappearance
show ®rst-order kinetics.
5. Mechanism of difluorocarbene formation and
recombination in tetrafluoroethylene pyrolysis
The formation of such intermediates is a property of all
lower per¯uoro-ole®ns. Their absorption bands in the visi-
ble region are shown in [12].
It is known that the enthalpy of tetra¯uoroethylene dis-
sociation to two di¯uorocarbenes is 70 kcal/mol (see, for
1
We have classi®ed them as 1,2-biradicaloids.
The simplest reaction including biradicaloids is the
reverse reaction (monomolecular ole®n formation). Also,
a biradicaloid as a structure having radical centers, must
add to multiple bonds. The activation energy of this
reaction for classical free radicals does not exceed a few
kcal/mol.
It is known for tetra¯uoroethylene that among all its
reactions (in the absence of any other substance) cyclodi-
merization has the least activation energy. It is also known
that direct 2‡2-cyclodimerization is forbidden by orbital
symmetry rules. Therefore the mechanism of the reaction
(
1)
1
Using IUPAC terminology [13] biradicaloids are biradicals having
substantial interaction between the unpaired electrons.
Fig. 1. Cyclo-C
4
F
8
pyrolysis (1 mol% in Ar,
T
maxˆ1214 K,
Pmaxˆ47 bar); curve 1: ꢁˆ230 nm and curve 2: ꢁˆ500 nm.

N.N. Buravtsev, Y.A. Kolbanovsky/ Journal of Fluorine Chemistry 96 (1999) 35±42
37
2
Substituting [CF ] from (b) into (d), one can obtain
example, [10]). If di¯uorocarbenes result from biradicaloid
decomposition, the enthalpy of this destruction will be
2
k7‰IŠ ꢁ k� 4‰C2F4Š:
Taking into account the reaction stoichiometry, one can
ꢀe†
7
0� 18ˆ52 kcal/mol.
How can it be proved that the tetra¯uoroethylene decom-
position is going through biradicaloid formation? Such an
evidence was recently obtained [16].
obtain
Ã
Á‰IŠ ˆ 1=2ꢀ‰C F Š ‡ ‰C F Š†;
2
4
2
4
Reaction (4), as was shown above, is the ®rst stage of the
tetra¯uoroethylene pyrolysis. It is suggested that di¯uoro-
carbene formation occurs
where Á[I]ˆ[I] � [I]. But as far as [C F *]ꢂ[C F ], then
0
2
4
2 4
Á‰IŠ ꢃ 1=2‰C2F4Š
and (e) can be represented as
(
6)
Thus CF2 recombination leads to tetra¯uoroethylene
through biradicaloid formation (see reverse reactions (6)
and (4)).
k7‰IŠ ꢁ 2k� 4Á‰IŠ:
ꢀf†
For the experimental conditions (Tˆ800 K, tˆ3 ms, and
To prove the above suggestions on the reaction mechan-
ism it is necessary to study the reaction at relatively low
temperatures. Under such conditions the equilibrium bir-
adicaloidconcentrations(reaction(4))isverylow.Ifthereisan
ole®n-independent low temperature source of carbenes, this
should show biradicaloid concentrations greater than those at
equilibrium. This fact will be used as the criterion of correct-
ness of the carbene recombination mechanism suggested.
Estimates given below can serve as a basis of experi-
mental research. We perform estimates for the well-studied
reaction of CF -cyclo-C F (I) pyrolysis. The kinetic path-
conversion of (I) yˆ0.9) using the value of rate constants
13:7
ꢀexp � 36 000=RT†c� 1
k7 ˆ 10
;
(
see e.g. [17]),
4:5
k� 4 ˆ 10 ꢀexp � 3000=RT†c� 1
;
(
see e.g. [18]) and relations (a)±(f), the following relation
Ã
Ã
can be obtained: ‰C2F Š=‰C2F Š ˆ1.7. Thus, correctness of
4
4 eq
Scheme 1 will be proved, if such a value of the ratio is
de®ned from the (I) pyrolysis investigation using the kinetic
spectroscopy method.
3
3 5
ways for the simple scheme (Scheme 1) are followed.
Ã
Ã
Two typical maxima of light absorption belonging to the
only intermediate can be seen in Fig. 2. These maxima are
Biradicaloids are now designated C F , C F , etc.
2
4
3
6
Ã
The concentrations of C2F4 and di¯uorocarbene are
assumed to be quasi-stationary for simpli®cation. Then
2 ms from each other. Our results show the evidence of the
Ã
existence of two mechanisms of C2F formation.
4
Ã
2
Ã
‰
‰
C2F Š ˆ k� 6‰CF2Š � k� 4‰C2F Š ‡ k4‰C2F4Š ˆ 0;
ꢀa†
It is known [18,19] that tetra¯uoroethylene is in equili-
brium with its biradicaloid. Obviously, in this case the band
absorption maximum must be placed at the temperature
maximum. However, there are two maxima in Fig. 2 and the
stronger (approximately twice, as can be expected from the
ratio calculated above) is observed at the lower temperature.
4
4
2
CF2Š ˆ k7‰IŠ � k6‰CF2Š ˆ 0;
ꢀb†
and correspondingly,
2
k� 6‰CF2Š
k� 4
k4
Ã
‰
C F Š ˆ
‡
‰C F Š:
2 4
ꢀc†
2
4
Ã
2
k4
As only C2F absorbs light at ꢁˆ500 nm ), therefore these
4
two maxima relate to it.
It is clear that the right-hand side of (c) corresponds to the
Ã
Thus it is necessary to explain: (1) the presence of two
absorption maxima; and (2) the relatively strong absorption
at lower temperatures.
equilibrium concentration of C F (reactions (4) and (� 4)).
2
4
To reliably detect the over-equilibrium concentration of
Ã
C2F , the inequality (d) should be satis®ed.
4
In our opinion, there is only one explanation of these data.
It is well known that di¯uorocarbenes form under (I)
pyrolysis (e.g. see [20]). Reaction of di¯uorocarbene recom-
bination leads to the tetra¯ouroethylene biradicaloid. There-
fore the C2F4 concentration is twice the possible
te eq mu ipl ei br ra it uu mr e.``biradicaloid $ ole®n'' concentration at this
2
k
‰CF Š
k
�
6
2
� 4
ꢁ
‰C2F4Š:
k4
ꢀd†
k
4
Ã
Therefore, when (I) completely decomposes, i.e. the
source of di¯uorocarbene disappears, the biradicaloid con-
centration falls practically to zero. When the temperature
rises (compression continues), the biradicaloid equilibrium
concentration increases (the second maximum).
2
3 4
The band absorption of perfluorobiradicaloids C ±C in the visible has
Scheme 1.
maxima at ꢁ<500 nm [12].

3
8
N.N. Buravtsev, Y.A. Kolbanovsky / Journal of Fluorine Chemistry 96 (1999) 35±42
at tˆ� 1.9 ms. The second kinetic curve (ꢁˆ250 nm) relates
*
to 2,3-C F absorption. At time tˆ� 1.9 ms there is a
4
8
shoulder on it which breaks the symmetry of the curve. It
was shown earlier [12,19] that a non-symmetric interme-
diate's absorption curve indicates the absence of chemical
equilibrium in the adiabatic compression method.
Thus, it is shown that recombination of three-atom and
more complicated singlet carbenes passes through two
stages. During the ®rst stage biradicaloids form before
the appearance of ole®ns. In our opinion, such experimental
results are of interest for theoretical organic chemistry.
6
. Dissociation of completely fluorinated biradicals
Fig. 2. CF
3
-cyclo-C
3
F
5
pyrolysis (1 mol% in Ar,
T
maxˆ872 K,
P
maxˆ19 bar); curve 1: ꢁˆ500 nm and curve 2: temperature.
Biradicals were considered to be the products of the
initial stage of strained carbocycles dissociation [21]. But
these biradicals were not observed, and views on the
mechanism of thermal per¯uoro-organic reactions were
advanced independently from this hypothesis.
But turning to be experimental results, it becomes clear
that some traditional aspects must be revised. In particular it
concerns the well-known reaction of singlet carbene addi-
tions to a ꢀ-bond, which is known as ``cyclopropanation
reaction''.
We have studied the kinetics of reaction, which was
reversed to cyclopropanation. Reversible, 1,3-biradical for-
mation occurs under pulse (I) pyrolysis under mild condi-
tions when other intermediates or stable products are not
observed.
Ã
Thus, biradicaloid C2F has been obtained by two inde-
4
pendent ways for the ®rst time. One of them is the biradi-
caloid synthesis from the low temperature source of
di¯uorocarbenes to C F , the other is its formation on the
2
4
way from C F decomposition to 2CF .
2
4
2
We believe that the interpretation given above has no
alternatives and our explanation may be regarded as the ®rst
direct con®rmation of the ground state (singlet) di¯uoro-
carbenes two-stage recombination.
Let us next show that biradicaloids are the initial products
not only of three-atom ¯uoro-organic carbenes recombina-
tion, but also of more complex ones.
It was shown [5] that a-elimination of carbene :CFCF3
occurs during C F SiF (II) pyrolysis. Carbenes recombine
to the stable 2-per¯uorobutene. If the concept formulated
2
5
3
above is ful®lled, then over equilibrium concentration of the
*
-per¯uorobutene 2,3-biradicaloid (2,3-C F ) should be
(8)
2
detected during (II) pyrolysis.
4
8
The :CFCF absorbance at ꢁˆ465 nm (this wavelength
As is clear from Fig. 4, this 1,3-biradical (ꢁˆ250 nm)
3
corresponds to the maximum of visible absorption [11])
versus time is shown in Fig. 3. The kinetic curve passes
through a maximum and practically completely disappears
really appears before all other intermediates of the process:
Fig. 4. CF
3
-cyclo-C
3
F
5
pyrolysis (1 mol% in Ar,
T
maxˆ818 K,
P
maxˆ18 bar); points are experimental, curves are calculated (Scheme
Fig. 3. C
2
F
5
SiF
3
pyrolysis (1.2 mol% in Ar, Tmaxˆ1172 K, Pmaxˆ33 bar);
1); curve 1: ꢁˆ250 nm, curve 2: ꢁˆ236.5 nm, curve 3: ꢁˆ271 nm, curve
Ã
curve 1: ꢁˆ465 nm, curve 2: ꢁˆ250 nm, and curve 3: pressure.
4: ꢁˆ500 nm, and curve 5: ꢁˆ236.5 nm (equilibrium C2F4 $ C2F ).
4

N.N. Buravtsev, Y.A. Kolbanovsky/ Journal of Fluorine Chemistry 96 (1999) 35±42
39
Ã
di¯uorocarbene (ꢁˆ271 nm), C F (which is the product of
2
4
di¯uorocarbene recombination, ꢁˆ236.5 and ꢁˆ500 nm).
Thus, a 1,3-biradical is really the ®rst intermediate of
pyrolysis (I). Di¯uorocarbene and hexa¯uoropropene are
the products of its decomposition. Therefore the same
biradical will be the initial intermediate in the reaction
of singlet carbene :CF addition to the hexa¯uoropropene
2
ꢀ
-bond.
This mechanism is believed to hold for all singlet
carbenes addition to ole®n ꢀ-bonds.
7
. Extrusion transformation of :CF2
The reaction of extrusion transformation is to be exam-
ined next. This problem is complex from the theoreti-
cal point of view and is important for practice, as will be
shown.
Fig. 5. C
points are experimental, curves are calculated (Scheme 2); curve 1:
3
F
6
pyrolysis (1 mol% in Ar, Tmaxˆ1455 K, Pmaxˆ71 bar);
ꢁ
ˆ250 nm, curve 2: ꢁˆ271 nm, and curve 3: ꢁˆ500 nm.
The term ``extrusion transformation'' has been recently
recommended by IUPAC [22], we should like to remind that
this de®nition was given for the back implantation reaction.
Extrusion transformation of di¯uorocarbene from some
It was found that appearance of di¯uorocarbene is not
accompanied by tri¯uoromethyl¯uorocarbene formation
during C F pyrolysis (the absorption at its typical wave-
3
6
¯
uoro-organic compounds is to be considered.
Let us ®rst examine hexa¯uoropropene pyrolysis. We
length ꢁˆ465 nm [11] was not observed). This is an evi-
dence of the absence of C F ꢀ-bond direct dissociation to
3
6
have already noted that homogeneous pyrolysis of C3F6
must go through the same stages as its synthesis. Until
recently clear evidence on a detailed synthesis mechanism
was not formulated. Now, in our opinion, it has become
possible [23].
two carbenes. In our opinion, di¯uorocarbene appears as a
result of extrusion from tri¯uoromethyl group in an a-
position with respect to the radical center of C3F6 (see
Scheme 2).
Activation energy for the overall reaction
*
Up to now, generation processes of di¯uorocarbene from
stable molecules and its interaction with such molecules
have been studied (see e.g. recent review [1±4]). This
section is devoted to di¯uorocarbene generation from bir-
adicals and biradicaloids, and di¯uorocarbene interaction
with these intermediates. Hexa¯uoropropene was selected
as the primary object of the investigation, as far as it is a
typical representative of the lower per¯uoro-ole®ns and a
most important monomer for the ¯uoroelastomer industry.
Studying the kinetics and detailed mechanisms of C F high
temperature conversion, we hoped to get not only informa-
tion about its possible secondary reactions under pyrolysis
but also information about its decomposition mechanisms.
These mechanisms must contain the same stages as C3F6
synthesis due to the detailed equilibrium principle.
CF2ˆCF� CF3 $ CF2ˆCF2‡ : CF2
(11)
was determined as E11ˆ83Æ1.5 kcal/mol in [24].
Reaction enthalpy (9) was estimated as ÁH
ˆ18Æ2 kcal/
9
mol in [12]. Using these data we get the activation energy for
reaction (10):
E10 ˆ E � ÁH ˆ ꢀ83 Æ 1:5† � ꢀ18 Æ 2†
11
9
ˆ
65 Æ 3:5 kcal=mol:
3
6
The Arrhenius parameters for reactions (6), (� 6) and (4),
(
� 4) were determined in [9,18]. The calculated light absorp-
tion versus time for the Scheme 2 is in good agreement with
the experimental data (Fig. 5). However, these data (appear-
ance of characteristic bands of CF (271 nm) in the absorp-
2
tion spectra) cannot be interpreted unambiguously as CF2
The typical data obtained from C F pulse pyrolysis
6
3
under adiabatic compression using time resolved spectro-
scopy are shown in Fig. 5.
From Fig. 5 the appearance of a 1,2-biradicaloid, iso-
meric with C F (ꢁˆ250 nm), precedes di¯uorocarbene
3
6
appearance (ꢁˆ271 nm). Together with di¯uorocarbene
the light absorption of the biradicaloid, isomeric with
C F (ꢁˆ500 nm), appears. The singlet di¯uorocarbene
2
4
recombination to tetra¯uoroethylene was shown earlier to
start with biradicaloid formation [16]. Tetra¯uoroethylene is
the ®rst stable product, which is in accord with literature
data [24].
Scheme 2.

4
0
N.N. Buravtsev, Y.A. Kolbanovsky / Journal of Fluorine Chemistry 96 (1999) 35±42
Clear evidence of CF extrusion from the CF -group a-
2
3
position to the radical center should be obtained if this
reaction is realized through an ordinary biradical (but not a
biradicaloid), subject to the condition that the initial mole-
cule (precursors the biradicaloid) does not have CF -groups.
2
Consider whether the extrusion transformation of CF2
Scheme 3.
from a CF -group is the result of CF interaction with a-
radical center, or with initial molecule's ꢀ-bond.
To answer this question it was necessary to choose a
3
3
extrusion transformation because there is another source of
CF -group in the CF3±CF=CF molecule.
2
2
To be sure that di¯uorocarbene extrusion from a tri¯uor-
omethyl-group is possible, we have examined 2-per¯uor-
obutene (it does not contain a CF -group) pyrolysis.
di¯uorocarbene source having neither CF -groups nor ꢀ-
2
bonds.
Per¯uoro-2,3-epoxybutane (III) was found to be such a
compound. Experimental data obtained from its thermal
transformations are shown in Fig. 7.
2
Data obtained from 2-per¯uorobutene pyrolysis are ana-
logous to those for C F pyrolysis (Scheme 3).
3
6
The ®rst stage of 2-per¯uorobutene pyrolysis is 2,3-
biradicaloid formation. Data on light absorption of inter-
mediates versus time of pyrolysis are shown in Fig. 6. As
shown for C F , the direct breakdown of the ꢀ-bond to two
Kinetic curve 1 displays the light absorption by the
primary intermediate of (III) pyrolysis. Its appearance pre-
cedes the appearance of all other species. Using methods
analogous to [26], it was shown that in equilibrium
``(III)$primary intermediate'' both direct and back reac-
tions have ®rst kinetic order. This corresponds to the
reversible reaction of epoxide dissociation. This reaction
is the initial stage of (III) thermal decomposition. The 1,3-
biradical formed is, as mentioned earlier in pyrolysis (I), the
primary intermediate of the process.
3
6
carbenes was absent. The same results were obtained for 2-
C F (the light absorption at ꢁˆ465 nm was not observed).
4
8
In spite of the absence of CF -groups in 2-C F , di¯uor-
4 8
2
ocarbene was observed by its absorption at ꢁˆ271 nm
Fig. 6). The stable products of the ®rst pyrolysis stages
are C F and C F , their molar ratio is 2:1 in accordance
(
3
6
2 4
with Scheme 3 and experimental data [25].
The CF -group forming the di¯uorocarbene, is always
3
Thus, experimental evidence of di¯uorocarbene as a
result of extrusion from a tri¯uoromethyl-group has been
obtained from 2-C F pyrolysis. The CF extrusion occurs
near the 1,3-biradical radical center and this does not depend
on the epoxide bond which has been broken (Scheme 4).
Secondary intermediates appear at higher temperatures.
4
8
2
Ã
from the CF -group in the a-position to the radical center of
Absorptions typical for CF (ꢁˆ271 nm) and for C2F
3
2
4
the biradicaloids both for pyrolysis of C F and 2-C F . The
activation energy for the overall reaction of 2-C F decom-
(ꢁˆ500 nm) are observed. The latter arises from CF
3
6
4
8
2
recombination. At the same time the stable pyrolysis pro-
ducts (C F and CF CFO in molar ratio 1:1) were found by
4
8
position to C F and CF was determined by the C F yield
6
3
6
2
3
2
4
3
as E ˆ92Æ1.5 kcal/mol. Taking into account the enthalpy
chromatography. This composition of products corresponds
to Scheme 4.
The activation energy for CF extrusion was estimated as
E13ˆ30Æ2.5 kcal/mol from absorption versus time data.
Æ
*
of equilibrium ``2±C F $2,3±C F '' as ÁHˆ18Æ2 kcal/
4
8
4 8
mol [12], the activation energy for the extrusion transfor-
2
*
mation of CF from 2,3-C F can be estimated:
2
4 8
Eextr ˆ EÆ � ÁH ˆ ꢀ92 Æ 1:5† � ꢀ18 Æ 2†
74 Æ 3:5 kcal=mol:
ˆ
Fig. 7. (III) pyrolysis (1 mol% in Ar, Tmaxˆ1120 K, Pmaxˆ38 bar); points
are experimental, curves are calculated (Scheme 4); curve 1: ꢁˆ250 nm,
curve 2: ꢁˆ271 nm, and curve 3: ꢁˆ500 nm.
Fig. 6. 2-C
4
F
8
pyrolysis (2 mol% in Ar, Tmaxˆ1245 K, Pmaxˆ45 bar);
curve 1: ꢁˆ250 nm, curve 2: ꢁˆ271 nm, and curve 3: pressure.

N.N. Buravtsev, Y.A. Kolbanovsky/ Journal of Fluorine Chemistry 96 (1999) 35±42
41
9
0 kcal/mol [27] and even 100±105 kcal/mol in accordance
with RRKM calculations [6]. This is much more than the
activation energy for the overall process obtained from all
known experimental data.
The pathway (b) was discussed in [28]. It was eliminated
in [6], as it is impossible to explain the translational energy
distribution of reaction products, determined experimen-
tally. Also we have estimated the rate constant for per¯uor-
otrimethylene decomposition. It is several orders of
magnitude greater than the rate constant for its hypothetical
isomerization to C F . C F synthesis and destruction can-
3
6
3 6
not pass through the same per¯uorotrimethylene stage, as
this would be in contradiction to the detailed equilibrium
principle.
Let us discuss in detail the pathway (c), combining it with
Scheme 2: Scheme 6.
A comparison of Schemes 2 and 6 shows that there is a
Scheme 4.
We emphasize that CF extrusion from the (III) 1,3-
2
transient structure from which the CF comes. In its turn,
2
Ã
biradical cannot be explained by F-atoms (belonging to a
CF -group) interaction with ꢀ-electrons, as there is no
C3F formation foregoes formation of this structure. Com-
parison of the C F synthesis and decomposition processes
6
3
3 6
double-bond in (III).
Thus, reactions of CF extrusion from biradicaloids and
biradicals tri¯uoromethyl group occur only if the tri¯uor-
omethyl group is ꢂ with respect to the radical center. The
shows that it is necessary to take into account processes (6)
Ã
and (4), as well as the circumstance that C2F forms before
C F itself during the C F pyrolysis.
2
4
2
4
3 6
From the C F synthesis, the activation energy for overall
3 6
activation energies for CF extrusion from biradicaloids
2
reaction
tri¯uoromethyl group are greater than the activation ener-
gies for CF extrusion from 1,3-biradicals.
CF ˆCF ‡ : CF ! CF ˆCF� CF
(-11)
was determined to be E� 11ˆ20Æ2 kcal/mol. It was shown
earlier that CF addition to a C ꢀ-bond leads to a
per¯uorotrimethylene biradical formation. However, the
latter is not an intermediate both for C synthesis and
pyrolysis, as shown earlier under consideration of mechan-
isms (a) and (b) of C pyrolysis. We suppose that reaction
(4) (ÁH
2
2
2
2
3
2
Consider C F pyrolysis as a speci®c example for the
3
6
detailed mechanism for CF extrusion.
2
2
F
2 4
Four possible pathways (a)±(d) for C F transformation
3
6
have been discussed in [6] (Scheme 5).
The pathways (a)±(c) lead to :CF and C F formation,
and the pathway (d) leads to :CF and tri¯uoromethyl¯uor-
F
3 6
2
2 4
F
3 6
2
ocarbene (:CFCF ) formation. Pathway (d) may be rejected
3
4
ˆ18Æ2 kcal/mol) foregoes reaction (� 11).
as :CFCF was not observed using spectroscopic methods
3
Thus the transient structure formation during pathway (c)
occurs under synthesis as a result of di¯uorocarbene inter-
under thermal initiation.
Pathway (a) may also be rejected. Indeed, the activation
energy for the process (a) is estimated to be greater than
Ã
action with the biradicaloid C2F (Scheme 6, pathway (3)).
The activation energy for this reaction is approximately 2±
4
Scheme 5.

4
2
N.N. Buravtsev, Y.A. Kolbanovsky / Journal of Fluorine Chemistry 96 (1999) 35±42
Scheme 6.
References
[
[
[
[
[
1] W.R. Dolbier Jr., Chem. Rev. 96 (1996) 1557.
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[
Fig. 8. Energy level diagram for C
3
F
6
system.
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(
1988) 885].
3
kcal/mol. The reaction can be interpreted as an implanta-
[
11] J.E. O'Gara, W.P. Dailey, J. Am. Chem. Soc. 114 (1992) 3581.
tion reaction of di¯uorocarbene to a C±F bond of a radical
center, that is the extrusion transformation back-reaction.
Thus, the speci®c properties of a tri¯uoromethyl group ꢂ
with respect to the radical center can be regarded as proved
from properties which were analyzed using kinetic and
spectral data. The conclusion is that the synthesis of higher
per¯uoro-ole®ns from lower species pyrolysis occurs by
interaction of singlet di¯uorocarbene with biradicaloid
radical centers.
[12] N.N. Buravtsev, Yu.A. Kolbanovskii, A.A. Ovsyannikov, Zh. Org.
Khim. 30 (1994) 1802.
[
[
13] IUPAC: Organic Chemistry Division: Commission on Physical
Organic Chemistry, in: P. Muller (Ed.), Glossary of Terms Used in
Physical Organic Chemistry, 2nd ed., 1994, p. 63.
14] J. March, Advanced Organic Chemistry, vol. 3, New York, 1985.
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[
16] N.N. Buravtsev, Yu.A. Kolbanovskii, Dokladi Akad. Nauk (1997), in
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[
17] N.N. Buravtsev, Yu.A. Kolbanovskii, T.S. Kuptsova, O.M. Nefedov,
Sixth All-union Conference on Fluoroorganic Compounds Chem-
istry, Thesis, Novosibirsk, 1990, p. 124.
The energy level diagram for systems having the general-
ized chemical formula C F was constructed using literature
3
6
[
[
[
[
[
18] N.N. Buravtsev, Yu.A. Kolbanovskii, A.A. Ovsyannikov, Mendeleev
Commun. (1994) 190.
and present data (Fig. 8). It is clear from the diagram which
pathway of C F thermal decomposition and synthesis is
preferable from the energy point of view. This is the path-
3
6
19] N.N. Buravtsev, Yu.A. Kolbanovskii, A.A. Ovsyannikov, Izv. Akad.
Nauk, Ser. Khim. (1995) 2048.
way through the reaction of di¯uorocarbene with the bir-
Ã
20] L.S. German, A.S. Grigor'ev, Yu.A. Kolbanovskii, S.D. Chepik,
Kinet. Katal. 30 (1989) 221.
adicaloid C F .
2
4
21] T.S. Chambers, G.B. Kistiakowsky, J. Am. Chem. Soc. 56 (1934)
399.
22] IUPAC: Organic Chemistry Division: Commission on Physical
Organic Chemystry, in: P. Muller (Ed.), Glossary of Terms Used in
Physical Organic Chemistry, 2nd ed., 1994, p. 248.
8
. Conclusion
New completely ¯uorinated intermediates were identi®ed
[23] N.N. Buravtsev, Yu.A. Kolbanovskii, Dokladi Akad. Nauk (1997), in
press.
from spectroscopic studies of thermal reactions of several
per¯uoro-organic compounds in the gas phase. The detailed
mechanisms of several complex reactions were established.
Among them are mechanisms of the higher per¯uoro-ole®n
synthesis from lower ones under pyrolysis, and of some
other reactions (e.g., reaction of C F oxidation for per-
[
24] N.N. Buravtsev, A.S. Grigor'ev, Yu.A. Kolbanovskii, Kinet. Katal.
0 (1989) 21.
25] N.N. Buravtsev, A.S. Grigor'ev, Yu.A. Kolbanovskii, Kinet. Katal.
0 (1989) 449.
3
[
3
[26] I.V. Bil'era, N.N. Buravtsev, Yu.A. Kolbanovskii, A.A. Ovsyannikov,
Dokladi Akad. Nauk 349 (1996) 764.
3
6
[
27] D.S. Bomse, D.W. Berman, J.L. Beauchamp, J. Am. Chem. Soc. 103
1981) 3967.
¯
uoroepoxypropane production). These results may be the
(
foundation for scienti®c-grounded recommendations for
technologists to create optimal schemes and conditions
for a number of processes.
[
28] N.D. Kagramanov, P.S. Zuev, A.A. Kutin, I.O. Bragil'evskii, A.K.
Mal'tsev, O.M. Nefedov, Izv. Akad. Nauk, Ser. Khim. (1989) 46.