Anomalous elimination of HCl from 2-chloro-1,1-difluoroethane. Likely involvement of a 1,2-FCl interchange mechanism

Article abstract of DOI:10.1016/S0040-4039(02)01952-4

A novel 1,2-FCl interchange mechanism is proposed to be involved in the unexpected thermal conversion of CH₂ClCHF₂ to 1,2-difluoroethylene.

Full text of DOI:10.1016/S0040-4039(02)01952-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 8075–8077
Anomalous elimination of HCl from 2-chloro-1,1-difluoroethane.
Likely involvement of a 1,2-FCl interchange mechanism
William R. Dolbier, Jr.,* Raphaele Romelaer and J. Marshall Baker
Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA
Received 22 August 2002; revised 2 September 2002; accepted 10 September 2002
Abstract—A novel 1,2-FCl interchange mechanism is proposed to be involved in the unexpected thermal conversion of
CH2ClCHF2 to 1,2-difluoroethylene. © 2002 Elsevier Science Ltd. All rights reserved.
The quantitative thermal behavior of haloethanes has
been a subject of consistent interest over the last half
century.1,2 More recently, there has been an increased
interest
in
chlorine-
and
fluorine-substituted
hydrochlorofluorocarbons (HCFC’s) and chlorofluoro-
ethanes in particular, because of their potential as
replacements for ozone-depleting Freon refrigerants.
When chlorofluoroethanes are heated in the gas phase,
although free radical chain processes can occasionally
intervene,2 they generally preferentially undergo a uni-
molecular, four-center, b-elimination of HX,3 the barri-
ers to which are for the most part determined by
whether HCl or HF is eliminated as well as by the total
number and relative positions of the halogens in the
molecules. The relative reactivities of these C2-HCFC’s
and their products of elimination have generally been
quite predictable. For example, when both HCl and HF
eliminations are possible, and polar factors similar, the
HCl eliminations generally dominate, as in the pyrolysis
of 1-chloro-1-fluoroethane, where HCl elimination was
found to be virtually exclusive.4 b-Eliminations usually
dominate a-eliminations, and rearrangements have
never previously been known to play a significant role
in the 1,2-elimination reactions of haloethanes.
The reaction was studied over a temperature range
500–650°C, with no reaction being observed at 500°C,
less than 1% at 550°C, and a 3% conversion at 600°C.
Product ratios exhibited little variation with respect to
temperature or He:F142 ratio.
What can be the source of the unexpected, rearranged
1,2-difluoroethylene product? In this preliminary letter
we propose that the above reaction comprises what is
perhaps the clearest example of a novel unimolecular
1,2-FCl interchange mechanism that was recently pro-
posed by Holmes and co-workers in their computa-
tional and experimental study of the 1,3-HCl
elimination reaction of CF2ClCF2CH3.5,6
Therefore, it was with some amazement that in the
course of a recent study of the thermal behavior of C1
and C2 HCFC’s we observed 1,2-difluoroethylene to be
the major product from the flow thermolysis of 1-
chloro-2,2-difluoroethane (F142) (along with the
expected
difluoroethylene).
b-HCl
elimination
product,
1,1-
Readily available 1-chloro-1,1-difluoroethane (F142b) is
a good, clean thermal source of 1,1-difluoroethylene.4,7–9
However, the pyrolysis of its less accessible isomer,
2-chloro-1,1-difluoroethane (F142) has not previously
been reported.10
* Corresponding author.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01952-4

8076
W. R. Dolbier, Jr. et al. / Tetrahedron Letters 43 (2002) 8075–8077
Table 1. Activation parameters for HCl elimination from fluorinated monochloroethanes
Chloroethane
Log A
Ea (kcal/mol)
Ref.
Calcd barriera (kcal/mol)
CH3CH2Cl
CH3CHFCl
CH3CF2Cl (142b)
CH2FCH2Cl
CHF2CH2Cl (142)
CHFCHFCl
13.7
13.9
13.0
13.5
–
57.8
57.0
55.3
63.8
–
12
4
13
14
51.9
49.5
–
56.5
67.2
56.3
–
–
a Density functional theory calculations were performed using the Gaussian 98 suite of programs.11,15,16
As the available kinetic data in Table 1 indicates, the
F142b isomer, with a-fluorines is relatively reactive,
actually even more reactive than ethyl chloride towards
thermal b-elimination of HCl. This is probably because
the a-fluorines of F142b can stabilize the build up of
positive charge at the a-position that occurs within the
highly asymmetric and polarized elimination transition
state. In contrast, the presence of the b-fluorines in
F142 strongly inhibits such elimination, because of their
detrimental inductive effect on the transition state
(depicted in Fig. 1).
kcal/mol)11 would not be competitive, and even if it did
occur, the required subsequent 1,2-F shift would not
compete with the almost barrierless 1,2-H-shift.
Additional corroborating computational and experi-
mental evidence for the intervention of the 1,2-FCl
interchange mechanism in the thermal decomposition
of F142 will soon be forthcoming from the Holmes
group, and parallel work that will be reported in due
course continues in our group.
Lacking experimental kinetic data for F142, calcula-
tions were carried out on the related series of HCFC’s,
and it was found, as expected, that the b-elimination of
HCl from F142 should have the greatest barrier of the
series (67.2 kcal/mol). This apparently opens the door
for the 1,2-FCl interchange mechanism to become
energetically accessible and competitive with the normal
b-elimination mechanism. Once this interchange occurs,
loss of HCl from the resulting CHFCHFCl to form the
cis- and trans-1,2-difluoroethylenes should be very fast,
as indicated by its relatively low calculated barrier (56.3
kcal/mol) for b-elimination.
Acknowledgements
Support of this research in part by the National Science
Foundation and by Elf Atochem is gratefully acknowl-
edged. Ongoing discussions and communication of
unpublished results from Professor Bert Holmes of
UNC, Asheville have proved invaluable during the
course of this work.
References
We propose the intervention of the 1,2-FCl interchange
mechanism primarily because there is no logical alter-
native that can explain the formation of significant
amounts of 1,2-difluoroethylene in this reaction. Calcu-
lations indicate that the barrier to the interchange
process (Ea=61.9 kcal/mol, with DH°=6.4 kcal/mol)17
should be competitive with HCl elimination (Ea=67.2
kcal/mol).11 Once formed CH2FCHFCl should undergo
rapid b-HCl elimination (Ea=56.3 kcal/mol)11 to form
CHFꢀCHF. There is no evidence of significant inter-
vention of a free radical chain process in this reaction.
a-Elimination of HCl from F142 (calcd barrier of 88
1. Maccoll, A. Chem. Rev. 1969, 69, 33–60.
2. Barton, D. H. R.; Onyon, P. F. Trans. Faraday Soc.
1949, 45, 725–735.
3. Hiberty, P. C. J. Am. Chem. Soc. 1975, 97, 5975–5979.
4. Martens, G. J.; Godfroid, M.; Decelle, R.; Verbeyst, J.
Internat. J. Chem. Kinet. 1972, 4, 645–655.
5. Heard, G. L.; Holmes, B. E. J. Phys. Chem. A 2001, 105,
1622–1625.
6. Burgin, M. O.; Heard, G. L.; Martell, J. M.; Holmes, B.
E. J. Phys. Chem. 2001, 105, 1615–1621.
7. Huybrechts, G.; Van Assche, G. Internat. J. Chem. Kinet.
1999, 31, 283–289.
8. Jones, Y.; Holmes, B. E.; Duke, D. W.; Tipton, D. L. J.
Phys. Chem. 1990, 94, 4957–4963.
9. McDaniel, J. B.; Holmes, B. E. J. Phys. Chem. 1996, 100,
3044–3050.
10. Our studies of F142 were prompted by discussions with
Dr. J. P. Schirmann of Elf Atochem in which a number
of unusual observations made by Elf Atochem in their
studies of HCFC’s were mentioned.
11. Reactants, products, intermediates, and transition struc-
tures were optimized using Becke’s hybrid three parame-
ter functional (B3LYP)16 and the 6-31G(d) basis set.
Single-point energies were calculated using B3LYP level
of theory using the 6-311+G(2df,2p) basis set.
Figure 1. Calculated transition state for HCl elimination from
F142.11

W. R. Dolbier, Jr. et al. / Tetrahedron Letters 43 (2002) 8075–8077
8077
12. Evans, P. J.; Ichimura, T.; Tschuikow-Roux, E. Internat.
J. Chem. Kinet. 1978, 10, 855.
13. Huybrechts, G.; Van Assche, G.; Van Der Auwera, S.
Internat. J. Chem. Kinet. 1998, 30, 359–366.
14. Cadman, P.; Day, M.; Trotman-Dickenson, A. F. J.
Chem. Soc. A 1971, 1356–1360.
15. Frisch, M. J. T. G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.;
Montgomery, J. J. A.; Stratmann, R. E.; Burant, J. C.;
Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K.
N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.;
Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P.
Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challa-
combe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong,
M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.;
Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.7,
Gaussian, Inc., Pittsburgh, PA, 1998.
16. Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
17. Holmes, B. E.; Heard, G. L.; Beaton, P. T., unpublished
B3PW91/6-311+G(2d,p) calculations.