Thermal isomerization of 1,1,2,2-tetrafluoroethane (FC-134) to 1,1,1,2-tetrafluoroethane (FC-134a) in the presence of hydrogen

Article abstract of DOI:10.1021/ja002359t

The mechanism of the high-temperature, gas-phase isomerization of 1,1,2,2-tetrafluoroethane (FC-134) to 1,1,1,2-tetrafluoroethane (FC-134a) in the presence of H₂ has been explored both experimentally and computationally. Studies of the impact of temperature, H₂/FC-134 ratio, and contact time on conversion and yield, as well as a study of deuterium incorporation when D₂ was used in place of H₂, led to the conclusion that a free radical chain mechanism involving rearrangement of CHF₂CF₂^· to CF₃CHF^· is involved.

Full text of DOI:10.1021/ja002359t

J. Am. Chem. Soc. 2001, 123, 6773-6777
6773
Thermal Isomerization of 1,1,2,2-Tetrafluoroethane (FC-134) to
1,1,1,2-Tetrafluoroethane (FC-134a) in the Presence of Hydrogen
Raphaele Romelaer, J. Marshall Baker, and William R. Dolbier, Jr.*
Contribution from the Department of Chemistry, UniVersity of Florida, GainesVille, Florida 32611-7200
ReceiVed June 29, 2000
Abstract: The mechanism of the high-temperature, gas-phase isomerization of 1,1,2,2-tetrafluoroethane (FC-
134) to 1,1,1,2-tetrafluoroethane (FC-134a) in the presence of H2 has been explored both experimentally and
computationally. Studies of the impact of temperature, H2/FC-134 ratio, and contact time on conversion and
yield, as well as a study of deuterium incorporation when D2 was used in place of H2, led to the conclusion
‚
that a free radical chain mechanism involving rearrangement of CHF2CF2 to CF3CHF‚ is involved.
In the accompanying paper, it is reported that the high-
temperature gas-phase pyrolysis of CHClF2 (FC-22) and CHF3
(FC-23) in the presence of hydrogen leads to selective forma-
tion, mainly via a free radical chain process, of CH2F2
(FC-32), CHF2CHF2 (FC-134), and CF3CH2F (FC-134a).1 In
the course of this study it became apparent that, particularly at
the higher temperatures studied, FC-134 was isomerizing to
FC-134a under the pyrolysis conditions. Because the non-Lewis-
acid-catalyzed isomerization of FC-134 to FC-134a is unprec-
edented, it was decided to take a closer look at this apparent
H2-induced process in order to define its mechanism.
The thermal reactivities of FC-134 and FC-134a have been
previously examined, with both undergoing virtually exclusive
elimination of HF to form CF2dCHF at temperatures >900
°C.2,3 Although small amounts of CH2F2 (FC-32) and
CF2dCF2 (TFE) were detected, particularly at higher temper-
atures, no mention was made of isomerization of FC-134 to
FC-134a in these early investigations.
FC-134 to FC-134a.6,7 This is understandable in view of the
unfavorable thermodynamics of this two-step reaction at the
temperatures necessary to establish equilibrium between
FC-134 and FC-134a, via the logical and precedented unimo-
lecular HF elimination-bimolecular HF addition mechanism
depicted by eqs 2 and 3 in Table 1, which provides all relevant
experimental8 and calculated (DFT) thermodynamic data.
Although the thermodynamics of the HF elimination/addition
sequence (eqs 2 and 3) are quite acceptable at 25 °C (where
the process is not kinetically feasible), and even at 350 °C (where
the chromia-catalyzed equilibration is observed to occur), at the
temperatures that are required to induce the thermal unimo-
lecular first step (eq 2) of the sequence (>700 °C), the second,
addition step (eq 3) has become a highly unfavorable reaction
in terms of free energy (at 700 °C, the T∆S component of ∆G°
is + 35.4 kcal/mol, more than enough to outweigh the
exothermic ∆H° of the reaction.)
The forbidding thermodynamics of the equilibria in Table 1,
combined with the high activation energy of the first step (eq
2), virtually exclude this elimination-addition two-step process
as a possible mechanism for the high-temperature conversion
of FC-134 to FC-134a.
The present study was therefore initiated, first to demonstrate
that the thermal conversion of FC-134 to FC-134a in the
presence of H2 actually occurs, and, if so, to determine the
mechanism of this process. Indeed, it was found that, under
conditions (700 °C) where FC-134 in the presence of inert gas
(He) undergoes but 3.4% conversion to form 1.4% FC-134a
(plus 1.2% CF2dCHF), a 29% conversion of FC-134 is observed
in the presence of H2 to form 25% FC-134a (plus only 0.6%
CF2dCHF). A free radical chain mechanism is proposed to
rationalize the results.
Within the past decade, work by Kemnitz demonstrated that
FC-134 will isomerize to FC-134a when it is passed over a
conditioned chromia catalyst at 320-445 °C.4,5 Such reactions
are considered to proceed via Lewis-acid-catalyzed elimination
of HF, followed by recombination of the absorbed CF2dCHF/
HF to give the more thermodynamically stable CF3CH2F isomer.
Until the present work, however, there had been no reports
of non-Lewis-acid-catalyzed, gas-phase thermal conversion of
Experimental Section
The CHF2CHF2 isomerization reaction was carried out at atmospheric
pressure and isothermally in a continuous-flow reactor, which is
depicted in the accompanying paper, and using a procedure analogous
to that described there.1
(1) Romelaer, R.; Kruger, V.; Baker, J. M.; Dolbier, W. R., Jr. J. Am.
Chem. Soc. 2001, 123, 6767-6772.
(2) Millward, G. E.; Hartig, R.; Tschuikow-Roux, E. J. Phys. Chem. 1971,
75, 3195-3201.
(6) While this work was in progress, patents from Elf Atochem appeared
which provided the first mention of the H2-induced isomerization of FC-
134 to FC134a.7
(3) Millward, G. E.; Tschuikow-Roux, E. J. Phys. Chem. 1972, 76, 292-
298.
(4) Kemnitz, E.; Niedersen, K.-U. J. Catal. 1995, 155, 283-289.
(5) Kemnitz, E.; Niedersen, K.-U. J. Fluorine Chem. 1996, 79, 111-
119.
(7) Hub, S.; Guillet, D., FR2758556; WO983165; EP975567
(8) Burgess, D. R., Jr.; Zachariah, M. R.; Tsang, W.; Westmoreland, P.
R. Prog. Energy Combust. Sci. 1996, 21, 453-529.
10.1021/ja002359t CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/20/2001

6774 J. Am. Chem. Soc., Vol. 123, No. 28, 2001
Romelaer et al.
Table 1. Calculated (and Experimental) Thermodynamic Data
(B3LYP/6-311+G(2df,2p)//B3LYP/6-31G(d))
Computational Methodology. Density functional theory calculations
were performed using the Gaussian 98 program package. Reactants,
products, intermediates, and transition structures were optimized using
Becke’s hybrid three-parameter functional (B3LYP) and the 6-31G(d)
basis set. Restricted and unrestricted wave functions were used for
closed- and open-shell species, respectively. Using the same level of
theory, vibrational frequency calculations were performed on all
stationary points to identity transition structures and determine thermal/
zero-point energies. Transition structures were characterized by a single
imaginary frequency. Thermochemical information at temperature T
and P ) 1.00 atm was obtained using frequencies scaled by 0.9804.9
An intrinsic reaction coordinate (IRC) calculation was performed for
each transition structure to examine the reaction pathway for each
elementary step. Single-point energies for each structure and transition
state were calculated using the B3LYP level of theory, using the
6-311+G(2df,2p) basis set, with restricted and unrestricted wave
functions being again used for closed- and open-shell species, respec-
tively.
T,
∆H°
∆S°
∆G°
eq
1
reaction
°C (exptl)a (exptl)a (exptl)a
CHF2CHF2 h CF3CH2F
25
-5.9
(-4.3) (-1.3)
-5.8
-5.7
23.5
(27.7)
24.7
-0.75
-5.7
(-3.9)
-5.5
-5.3
13.1
(17.4)
2.0
-11.2
-18.8
350
700
-0.53
-0.44
35
(34.6)
36
36
-35.7
2
3
CHF2CHF2 h CF2dCHF + HF 25
350
700
25 -29.4
23.8
CF2dCHF + HF h CF3CH2F
(-31.6) (-35.9) (-20.9)
350 -30.5
700 -29.5
-36.9
-36.4
-7.5
6.0
a Experimental data, in parentheses, are from ref 6.
The reagents are as follow: CHF2CHF2 (SynQuest Laboratories Inc.,
99% minimum), CHClF2 (Elf-Atochem), CH4 (Matheson Gas Products
Inc.), and H2 and He (Strate Welding Supply Co., 95-97% and
99.995%).
The gas mixtures are passed through either a quartz or an Inconel
600 reactor, which is heated by a furnace (Applied Test Systems, Series
3210) with a temperature range from 650 to 850 °C (maximum
temperature 700 °C in the case of the quartz reactor).
Results and Discussion
The experimental investigation of the thermal behavior of
CHF2CHF2 (FC-134) with and without the presence of H2 was
carried out at atmospheric pressure and isothermally in the
above-described continuous-flow reactor: from 650 to 700 °C
in a quartz reactor, and from 700 to 850 °C in an Inconel 600
reactor.
The intent of the investigation was to determine the effect of
the presence of hydrogen (H2) on the outcome and dynamic
behavior of the reaction as compared to the results when the
pyrolysis is carried out in the presence of inert gas (He), and to
determine the mechanism of the observed reactions.
Studies in the Quartz Reactor. (i) Pyrolysis of CHF2CHF2
(FC-134) in the Presence of He. The pyrolysis of FC-134 in
He was carried out at 700 °C, with ratios of He/CHF2CHF2
ranging from 9 to 20 and the times of reaction ranging from 5
to 9 s. The FC-134 conversion and the yields of products are
given in Table 2. These data indicate that, in the presence of
He, FC-134 undergoes but a low conversion (2-3%), with small
amounts of CF2dCHF and FC-134a being formed under these
conditions.
(ii) Pyrolysis of CHF2CHF2 (FC-134) in the Presence of
H2. In contrast to its inertness in He, FC-134 exhibited a
significantly enhanced reactivity when pyrolyzed in the pres-
ence of H2. As seen in Table 3, although giving only a modest
3-4% conversion at 650 °C, FC-134 experienced a dra-
matic increase in both the degree of its conversion (14-29%)
and the yield of product FC-134a (82-88%), along with an
excellent carbon balance (90-95%), when the temperature of
the reactor was increased to 700 °C. Contrasting these re-
sults with the mere traces of FC-134a produced in the absence
of H2, they demonstrate conclusively that the presence of
hydrogen during FC-134 pyrolysis promotes its isomerization
to FC-134a.
At the outlet, the gases are passed through a KOH solution (1 M) in
order to neutralize HF and are dried by anhydrous calcium sulfate
(Drierite). An internal standard, CH4 for the quartz reactor or CHClF2
for the Inconel reactor, is then introduced in order to determine the
carbon balance and the yields after reaction and GC analysis.
The GC analysis of the gas mixture was performed on an HP
chromatograph using the following operating conditions: column, 1%
1
SP 1000, 60/40 carbopack, 4 m × /8 in. stainless steel; carrier gas, N2
(8 mL/min); temperature, 40 °C (10 min) to 180 °C (at 4 °C/min);
detector, FID, 250 °C; injector temperature, 250 °C. The products were
identified by comparison of GC retention times and mass spectra of
pure samples.1 Quantitative analysis of the product/standard ratios was
obtained by comparison with mixtures prepared for calibration purposes.
The relative response coefficients (ki/kstd) of each compound were
determined by GC-FID and are reported in the accompanying paper.1
After the determination of the inlet flow of FC-134 and the outlet
flow rates of FC-134 and each product, the results given in the tables
and/or represented graphically in the figures were calculated as indicated
below:
n(CHF2CHF2)i - n(CHF2CHF2)o
conversion of FC-134 )
× 100
n(CHF2CHF2)i
∆n(CHF2CHF2)
)
× 100
n(CHF2CHF2)i
where n(CHF2CHF2)i is the inlet molar flow rate (mmol‚h-1) and
n(CHF2CHF2)o is the outlet molar flow rate (mmol‚h-1).
x(product flow rate) (mmol‚h-1
∆n(CHF2CHF2)
)
yield (%)
(from consumed FC-134)
)
×
(iii) Pyrolysis of FC-134a in He and H2. In contrast to the
dramatic reactivity exhibited by FC-134 in the presence of H2,
FC-134a remained virtually inert when subjected to identical
reaction conditions, undergoing <1% conversion in either H2
or He. Although traces of FC-134 were observed in the presence
of H2, none was detected when the pyrolyses were carried out
in He.10
Studies in the Inconel Reactor. Corroborative results were
obtained when the pyrolyses of FC-134 were carried out in the
Inconel 600 reactor. Use of the Inconel 600 allowed pyrolyses
100
where x ) 1 for C2 products and x ) 0.5 for C1 products.
carbon balance (from consumed FC-134) ) yields of products
∑
overall production of FC-134a ) conversion × yield of FC-134a
Reaction time:
volume of the reactor (cm3)
total flow rate at the reaction temperature (cm3‚s-1
t (s) )
)
(9) Foresman, J. B.; Frisch, A. Exploring Chemistry with Electronic
Structure Methods, 2nd ed.; Gaussian, Inc.: Pittsburgh, PA, 1996.
(10) See Supporting Information for detailed data.
For the quartz reactor, V ) 150 cm3; for the Inconel reactor, V ) 100
cm3.

Thermal Isomerization of FC-134 to FC-134a
J. Am. Chem. Soc., Vol. 123, No. 28, 2001 6775
Table 2. Pyrolysis of CHF2CHF2 in Helium at 700° (Quartz Reactor)
conditions
yield (%)
carbon
balance (%)
overall formation
of CF3CH2F (%)
He/CHF2CHF2
t (s)
conv (%)
CH2F2
CF2dCHF
CF3CH2F
9/1
17/1
12/1
20/1
5
5
8
9
1.9
1.8
3.5
3.4
5.7
> 0
6.2
30.1
36.2
29.7
34.1
43.1
36.4
46.3
41.0
78.9
72.6
82.2
81.8
0.8
0.7
1.6
1.4
6.7
Table 3. Pyrolysis of CHF2CHF2 in Hydrogen at 650 and 700 °C for Reaction Times of 5 and 9 s (Quartz Reactor)
conditions
yield (%)
carbon
balance (%)
overall formation
of FC-134a (%)
T (°C)
H2/CHF2CHF2
t (s)
conv (%)
CH2F2
CF2dCHF
CF3CH2F
650
650
650
650
700
700
700
700
11/1
21/1
11/1
21/1
11/1
19/1
12/1
22/1
5
5
9
9
5
5
9
9
2.1
2.7
3.9
0
0
0
0
3.3
3.2
3.2
3.2
0
0
0
0
2.0
2.9
2.2
2.2
97.3
96.1
95.8
97.8
88.4
88.0
81.5
84.8
97.3
96.1
95.8
97.8
93.7
94.1
86.9
90.2
2.0
2.6
3.7
4.4
4.3
14.3
14.7
26.0
29.3
12.7
12.9
21.2
24.8
Table 4. Pyrolysis of CHF2CHF2 in He at 700 °C (Inconel Reactor)
conditions
yield (%)
carbon
balance (%)
overall formation
of CF3CH2F (%)
He/CHF2CHF2
t (s)
conv (%)
CH4
CH2F2
CF2dCF2
CF2dCHF
CF3CH2F
23/1
14/1
4/1
4
4
12
12.7
25.7
34.8
0
0
0
1.2
1.8
2.6
36.4
20.9
7.1
10.8
6.8
5.6
5.8
4.5
6.6
54.2
34.0
21.9
0.7
1.1
2.3
Table 5. Pyrolysis of CHF2CHF2 in H2 at 700 °C (Inconel Reactor)
conditons
yield (%)
CF2dCF2
carbon
balance (%)
overall formation
of CF3CH2F (%)
H2/CHF2CHF2
t (s)
conv (%)
CH4
CH2F2
CF2dCHF
CF3CH2F
18/1
11/1
8/1
6/1
3/1
6
6
6
22
29
32
33.2
32.2
28.5
39.7
38.4
34.3
1.2
1.1
1.3
1.9
1.6
0.8
12.6
12.4
11.8
10.9
10.3
9.3
0.6
0.8
1.1
0
0
0
2.5
2.8
3.2
3.0
3.1
0.8
62.5
60.5
57.6
69.6
66.3
55.3
79.4
77.6
75.0
85.4
81.3
66.2
20.8
19.5
16.4
27.6
25.5
19.0
1/1
Table 6. Pyrolysis of CHF2CHF2 in H2 between 650 and 850 °C (Inconel Reactor)
conditions
yield (%)
CF2dCF2
carbon
balance (%)
overall formation
of CF3CH2F (%)
T (°C)
H2/CHF2CHF2
t (s)
conv (%)
CH4
CH2F2
CF2dCHF
CF3CH2F
650
700
750
800
850
17/1
18/1
15/1
15/1
18/1
6
6
6
5
5
14.7
33.2
65.3
92.0
100
1.2
1.2
2.2
6.5
28.9
3.5
12.6
11.3
12.2
11.9
2.0
0.6
0
0
0
1.8
2.5
1.4
1.0
0
25.1
62.5
74.9
59.6
20.3
33.5
79.4
89.8
79.3
61.1
3.7
20.8
48.9
54.8
20.3
to be carried out at considerably higher temperatures (up to 850
°C, where up to 100% conversion of FC-134 could be observed),
but disadvantageously, the metal reactor gave rise to competitive,
reactor-induced dehydrogenative/hydrogenative processes, which
lowered the yield of FC-134a formation.
The results given in Tables 2, 3, 4 and 5 allow a direct
comparison of results in quartz and Inconel at 700 °C. It can
be seen that, whereas conversion of FC-134 in the presence of
He (Table 4) increases in the Inconel reactor, the yields of
FC-134a do not, and carbon balances are low (mostly due to
formation of “heavy products”, such as CF2dCHCF3 and
CF3CH2CF3, which were detected by GC/MS, but not quanti-
fied). The increased conversions derive largely from the
emergence of a new product, C2F4 (TFE), the formation of which
was undoubtedly catalyzed by the reactor wall (since it was
not observed during the studies in the quartz reactor).
and the yields of FC-134a are not so high as those observed in
quartz (65% vs 88%). Nevertheless, H2 promotion of the
isomerization of FC-134 to FC-134a is clearly demonstrated
by the contrasting data in Tables 4 and 5.
Methane appears as a new product in the presence of H2,
formed by wall catalysis, with little TFE being observed. The
lack of TFE is not surprising, since it has already been
demonstrated that, at such temperatures, H2 promotes the
efficient conversion of TFE to the trio of products, CH2F2,
FC-134, and FC-134a.1
(i) Temperature Dependence. Pyrolysis of FC-134 in the
absence of H2, even at temperatures as high as 850 °C, leads to
little formation of FC-134a (no more than 5.8%), despite an
increase in conversion of FC-134 from 12% (at 650 °C) to 93%
(at 850 °C).10 In contrast, as seen in Table 6 and graphically
depicted in Figures 1 and 2, when H2 is present, conversions of
FC-134 increase sharply with temperature (65% at 750 °C, 92%
at 800 °C, and 100% at 850 °C), as do yields and oVerall
In the presence of H2, conversions increased significantly
(Table 5) compared to those in He, but mass balances suffer

6776 J. Am. Chem. Soc., Vol. 123, No. 28, 2001
Romelaer et al.
Table 7. Calculations on 1,2-F and 1,2-H Radical Rearrangements
calcd Eq a
reaction enthalpies a
equation
INDOb DFTc INDOb DFTc exptld
F-shift ‚CF2CHF2 f CF3CHF‚
H-shift ‚CF2CHF2 f CHF2CF2
25.9
56.1
29.2 -6.1 -6.6 -3.8
56.1
‚
0
0
0
a In kcal/mol. b Reference 13. c This work (UB3LYP/6-311+G-
(2df,2p)//UB3LYP/6-31G(d)). d Reference 8.
Although the homolysis step (4) has an Ea of ∼91-93 kcal/
mol (BDE of FC-134),8 since this is an initiation step, few such
radicals are required to get the free radical chain process started.
The essential role of hydrogen, as a promoter of the free radical
chain mechanism, derives from the relative facility of steps (5)
and (6), which apparently serve as an efficient pathway to radical
Figure 1. Conversion and carbon balance vs temperature during
pyrolysis of CHF2CHF2 in H2 (Inconel 600 reactor).
‚
CHF2CF2 . Values of 9.8 and 10.6 kcal/mol are reported as the
Ea’s for the H-atom-transfer steps (5)11,12 and (6),8 respectively.
Although bimolecular, with such low activation barriers these
steps should be kinetically viable.
Perhaps the most interesting if not controversial step in the
proposed mechanism is the fluorine radical shift in step 7.
Reports of fluorine atom shifts in carbon radical systems are
rare, but such rearrangements nevertheless appear to be relatively
facile, much more so than the analogous H atom shifts.13-18 In
1988, Kotaka examined the specific F-shift of eq 7 by semi-
empirical calculations,13 with his results being given in Table
7. Our own DFT calculations, also given in Table 7, were
consistent with Kotaka’s INDO calculations, and they indicated
an Eq of only 29.2 kcal/mol for this unimolecular 1,2-F atom
shift.
Using the DFT activation parameters for the F-shift in Table
7, a rate constant of 3 × 106 s-1 at 700 °C was calculated.
Although there are no reported kinetic parameters for the reverse
of eq 6, using the experimental rate parameters8 for the reaction
of CF3‚ with H2 (and assuming an [H2] of 0.012 M) to estimate
that process, it appears that the F-shift should be kinetically
competitive.
Figure 2. Product yields vs temperature during pyrolysis of
CHF2CHF2 in H2 (Inconel 600 reactor).
productions of FC-134a, with these reaching a maximum at
about 750 °C, where values of 75% and 49%, respectively, are
observed.
From these results, the optimal conditions for conversion of
FC-134 to FC-134a can be stated as follows:
(a) For Inconel 600 reactor: 750 °C, H2/FC-134 ) 10, contact
time ) 5 s; to obtain 60% conversion, 75% yield, and thus 49%
overall production.
(b) For quartz reactor: 700 °C, H2/FC-134 ) 12, contact time
) 9 s; to obtain 26% conversion, 81% yield, and 21% overall
production.
It is interesting that the calculated activation energy for the
degenerate 1,2-H atom shift is much higher (56.1 kcal/mol),
thus indicating that a 1,2-F radical shift is significantly more
facile than an analogous 1,2-H shift.
It is necessary to comment on why the radical chain
rearrangement process should be significantly less efficient in
the absence of H2. Although an analogous free radical chain
mechanism, depicted by eqs 10, 7, and 11, below, is potentially
Mechanistic Hypothesis. Although there can be no doubt
that the presence of H2 promotes the specific conversion of
FC-134 to FC-134a, the question of the mechanism of this
isomerization remained unsettled at this point in the study. One
can be certain, however, based on our knowledge of the
thermodynamics of the potential two-step elimination-addition
mechanism (Table 1) and confirmed by results of FC-134
pyrolyses in He, that this mechanism cannot be operative.
The formation of small but significant amounts of CH2F2
(FC-32) in the reaction, and the lack of viable mechanistic
alternatives, induced us to consider a free radical chain
mechanism for the process. As a hypothesis, it was proposed
that carbon-carbon bond homolysis of FC-134 could lead to
initiation of the free radical chain mechanism depicted below:
available when FC-134 is pyrolyzed either alone or in the
presence of He, such a process is observed to be much less
‚
(11) This is actually a mean value for the reaction of CF3 with H2.
(12) Gilliom, R. D. J. Am. Chem. Soc. 1977, 99, 8399-8402.
(13) Kotaka, M.; Sato, S.; Shimokoshi, K. J. Fluorine Chem. 1988, 41,
371-382.
(14) Kotaka, M.; Sato, S.; Shimokoshi, K. J. Fluorine Chem. 1987, 37,
387-396.
(15) Kotaka, M.; Kohida, T.; Sato, S. Z. Naturforsch. 1990, 45b, 721-
722.
(16) Sung, K.; Tidwell, T. T. J. Org. Chem. 1998, 63, 9690-9697.
(17) Kovtonyuk, V. N.; Kobrina, L. S.; Volodin, A. M. Russ. Chem.
Bull. 1995, 44, 95-98.
(18) Dolbier, W. R.; Palmer, K.; Koroniak, H.; Zhang, H. Q.; Goedkin,
V. L. J. Am. Chem. Soc. 1991, 113, 1059-1060.

Thermal Isomerization of FC-134 to FC-134a
J. Am. Chem. Soc., Vol. 123, No. 28, 2001 6777
Table 8. Conversion, Yield, and Carbon Balance Obtained during
the CHF2CHF2 Thermal Activation in H2 and in D2 at 700 °C
(Quartz Reactor)
Table 9. Deuterated Compounds Distribution Determined by
GC/MS and Yields Obtained during the CHF2CHF2 Pyrolysis in D2
(700 °C, D2/CHF2CHF2 ) 15, t ) 5 s, New Quartz Reactor)
conditions
yield (%)
compounds
GC/MS distribution (%)
overall production (%)
carbon
H2/D2/CHF2CHF2 t (s) conv (%) CH2F2 CF3CH2F balance (%)
CF3CH2F
CF3CHDF
CF3CD2F
CHF2CHF2
CHF2CDF2
CDF2CDF2
4
84
12
76
22
2
1
14
2
61
18
1
15/0/1
0/15/1
5
5
21
20
6
88
94
91
5a
86a
a Partially deuterated products are obtained (see Table 9).
efficient than that which occurs in the presence of H2. Its relative
inefficiency must derive from the smaller rate constants of the
propagation steps (10) and (11),8 when compared with those of
the alternative, better propagation steps (5), (6), and (8) that
are available in the presence of H2.
D2 Experiments. To determine the degree of involvement
of H2 in the rearrangement mechanism, as well as to probe the
nature of that involvement, the reaction was carried out using
D2 in place of H2. The quartz reactor was used in order to avoid
complications due to reactor-induced, catalytic H2 processes.
The results are given in Tables 8 and 9, which allow direct
comparison of H2 and D2 results.
From the data in Table 8, it can be seen that conversion of
FC-134 is not significantly affected by substitution of D2 for
H2, and that is there is no apparent kinetic isotope effect in the
rate-determining process of isomerization. There is also no effect
upon the yield of FC-134a.
Looking at the deuterium distribution in the product mixture
(Table 9), it is seen that deuterium incorporation is significant,
thus ruling out any purely intramolecular mechanism, or the
simple elimination-addition mechanism (eqs 2 and 3). The
results almost certainly confirm the free radical nature of
the rearrangement process. The major isotopomer of product
FC-134a formed is the monodeuterio product, with little
undeuterated product being observed.
content of the product mixture, an almost equal amount of
deuterium has been incorporated into recovered FC-134 (20%
deuterium “yield”) as into product FC-134a (18% deuterium
“yield”). The incorporation of deuterium into both starting
FC-134 and product FC-134a is readily explained within the
context of the proposed radical chain mechanism, but not by
other, nonradical mechanisms.
Conclusions
On the basis of the sum of the evidence, and in the absence
of a reasonable alternative, it is concluded that the mechanism
for the H2-induced isomerization of FC-134 to its more
thermodynamically stable isomer, FC-134a, proceeds via the
proposed free radical chain mechanism.
Acknowledgment. Preliminary work by S. Hub and D.
Guillet, and helpful discussions with S. Hub, E. Lacroix, and J.
P. Schirmann, are acknowledged with thanks. Support of this
research by Elf Aquitaine, Inc. through funding of a postdoctoral
appointment for R.R. and for funding construction of the flow
reactor is gratefully acknowledged.
Supporting Information Available: Computational data
tables and supplementary tables and figures that describe
thermolysis data (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org. <
Recovered, nonconverted FC-134 also exhibits significant
deuterium incorporation. Indeed, looking at the total deuterium
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