THERMAL DECOMPOSITION OF DIFLUORODIAZIRINE
Difluorodiazirine. VIII.
1229
Kinetics of the Thermal Decomposition
of Difluorodiazirine
by Erwin W. Neuvar and Ronald A. Mitsch Contribution No. 977 from the Central Research Laboratories, Minnesota Mining and Manufacturing Co., St. Paul, Minnesota 66119 (Receined July 37,1966)
The gas-phase thermal decomposition of difluorodiazirine has been studied in a static system in the temperature range 123-170" and a pressure range 100-800 mm. The primary decomposition, yielding nitrogen and difluorocarbene, appears to be unimolecular and homogeneous. The rate constant is pressure dependent in the pressure range studied. At a total pressure of 200 mm, the first-order rate constants fit the Arrhenius equation, k200mm = 0.68k, = 1013JM.4exp(-32,200 700/RT) sec-I. The activation energy calculated from extrapolated values of k, at 123.2 and 170.0' agrees within limits of experimental error with that obtained from values of k at 200 mm.
*
Introduction Recent interest in the kinetics of thermal transformations of small-ring compounds, some of which are highly fluorinated, is exemplified by the work of several investigators. Herbert, Kerr, and Trotman-Dickenson,'* as well as Atkinson and McKeaganIb and the present authors12have studied the first-order thermal isomerization or decomposition of fluorinated cyclopropanes. In the area of small-ring heterocyclics, Lenzi and Mele3 obtained an activation energy of 31.6 kcal for the homogeneous unimolecular thermal decomposition of tetrafluoroethylene oxide yielding carbonyl fluoride and difluorocarbene. In the only reported kinetics work on diazirines, Frey and Stevens4 report an activation energy of 33.2 kcal for the thermal decomposition of dimethyldiazirine to dimethylcarbene and nitrogen. A study of the kinetics of the thermal decomposition of difluorodiazirine was undertaken to obtain data to further the understanding of the thermal behavior of small-ring compounds, particularly diazirines.
Results and Discussion The thermal decomposition of difluorodiazirine is known to yield primarily nitrogen, tetrafluoroethylene, hexafluorocyclopropane, and perfluoro-2,3-diaza-l,3butadiene.6 Traces of other fluorine-containing compounds are also detected. Hence, the following series
of competitive consecutive reactions accounts for the principal products
N
F
I F
N
iC ''
/ \
F
N
+ CF2: +CF-N-N=CF2 I1
(1) (a) F. P. Herbert, J. A. Kerr, and A. F. Trotman-Dickenaon, J. Chem. SOC.,5710 (1965); (b) B. Atkinson and D. McKeagan, Chem. C m m u n . , 189 (1966). (2) R. A. Mitsch and E. W. Neuvar, J. Phys. Chem., 70, 546 (1966). (3) M. Lenzi and A. Mele, J. Chem. Phye., 43, 1974 (1965). (4) H. M. Frey and I. D. R. Stevens, J. Chem. SOC.,3865 (1962). (6) R. A. Mitsch, J. Heterocyclic Chem., 1, 59 (1964); 3, 245 (1966).
Volume 71, Number 6 April 1067
ERWIN W. NEWARAND RONALD A. MITSCH
1230
The rate of consumption of CF2N2 via reaction 2 increases with the initial pressure. For example, in the temperature range 120-140" and at an initial pressure of 1 atm, approximately 10% of the CF2N2has been converted to I1 at completion of the reaction. Since our objective was a study of the kinetics of the decomposition of CF2N2 to difluorocarbene and nitrogen, we chose to minimize the competing side reaction by decomposing CF2N2 at a partial pressure of 50 mm in the presence of a 3 molar excess of C2F4. Tetrafluoroethylene is a suitable carbene trap since even at the upper temperature limit reaction 4 shows no detectable reversibility. Under these conditions the conversion of CF2N2to I1 is reduced to about 1%. Figure 1 shows plots of log (CF2N2peak height) (partial pressure) us. time in the temperature region 123-170". The linearity of the semilog plots, even when the decomposition was followed to more than 95% completion, indicates that the disappearance of CFzN2follows the first-order rate law. The same rate constants were obtained, within limits of experimental error, by monitoring either the disappearance of CF2N2or the buildup of N2 (compare runs 16 and 10, Table I). Table I contains values for the observed rate constants over the described temperature and pressure ranges. Figure 2 shows the Arrhenius plot which fits the expression kZwrnrn = 0.68k, = 1013.1M*4 exp(-32,200 i 700/RT) sec-' obtained by a least-squares treatment of the data. The homogeneity of the reaction was tested with one run at 158.5" in a reactor packed with Monel turnings (run 17, Table I) and one run at 156.3" with a reactor containing Teflon turnings (run 9, Table I). In both cases the surface-to-volume ratio was increased approximately sevenfold over the unpacked reactor. The rate constants thus obtained fit the Arrhenius plot within limits of experimental error indicating that the thermal decomposition of CF2N2 is predominantly homogeneous. The linearity of the Arrhenius plot reinforces this conclusion. For a unimolecular reaction the rate constant is pressure dependent in the pressure region in which the rate of transformation of activated molecules approaches the rate of collisional activation.g Indeed, the "falloff" in the CF2Nz thermal decomposition rate constants was observed in the total pressure range 100-800 mm (25-200 mm of CFzN2). The occurrence of "falloff" in this pressure region is consistent with semiquantitative predictions of the Rice-RamspergerKassel theory of unimolecular reactions.E Assuming a collision diameter of 4 A and using tabulated values of k / k , from Kassel's equation,' one obtains the effective number of oscillators contributing to dissociation in the RRK theory, s = 6, at 200 mm and 146". This is a The Journal of Phyeicd Chmietry
i'
im.o.c
' '
-
I
I 4
E
I 12
I 16
20
I
I
I
I
I
I
3 4 TIME(HRS)
5
6
I
I
2
I 24
Figure 1. Diiiuorodiazirine concentration (glpc peak height units) us. time. Total initial pressure 200 mm.
reasonable value for a molecule with only nine fundamental modes of vibration. The observed first-order disappearance of CF2N2, even when followed to 95'% completion, suggests that the decomposition products are approximately as effective as the parent molecule in maintaining the rate constant. The limiting high-pressure rate constants (k,) for 123.2 and 170.0" were determined by a linear leastsquares fit of k-' vs. p-' over the total pressure range 100-800 mm and extrapolating to p-' = 0 as illustrated in Figure 3.8 The resulting values of k,, (3.2 0.2) X sec-l at 123.2 and 170.0", and (2.5 A 0.3) X respectively, yield an activation energy of 32.5 1.0 kcal mole-', agreeing within limits of error with the value calculated from rate constants at 200 mm. In light of the uncertainty in the preexponential factor (a factor of approximately 3) measurement of the pressure dependence of activation energy is precluded. Although the rate data described herein do not afford any information concerning the actual mechanism of decomposition, we prefer a mechanism involving the transient existence of difluorodiazomethane or its equivalent prior to loss of nitrogen.
*
*
-~
~~
(6) L. S. Kassel, "The Kinetics of Homogeneous Gas Reactions," The Chemical Catalog Co., New York, N. Y., 1932. (7) E.W. Schlag, B. 8.Rabinovitch, and F. W. Schneider, J . Chem. Phya., 3 2 , 1699 (1960).
THERMAL DECOMPOSITION OF DIFLUORODIAZIRINE
F
F
\e
C ' ~'
-+-
/ \
F
1231
N
17QO.C
e-'
e C-N=N-
/
F
F
F
\
e e C=N=N--+
/
\ C:+Nz /
ko"
F
F
This rationale gains some support from the report of Amrich and Bell,* who showed that diazirine undergoes photoisomerisation to diazomethane. It is also supported by the observation that photolysis of difluorodiazirine in an argon matrix at 4°K forms CF,N-N=CF2,e presumably by a bimolecular matrix reaction of the electrophilic diffuorocarbene with e e CF2-N=N (or its equivalent) formed by photoisomerization.
Figure 3. Pressure dependence of rate constant for the thermal decomposition of difluorodiazkine.
Table I : Thermal Decomposition of Difluorodiazirine Total initial
kobdpb
Temp, OC
pressure,
seo-1 X
mma
104
123.2 123.2 123.2 123.2 126.3 131.4 141.0 150.7 156.3 160.0 166.2 170.0 170.0 170.0 170.0 160.0 158.5
100 200 400 800 200 200 200 200 200 200 200 100 200 400 800 200 200
0.19 0.22 0.26 0.31 0.29 0.50 1.27 3.31 4.98 7.58 11.10 11.97 16.99 19.07 22.57 7.70 6.07
Experimental Section All rate measurements were made using the same gaseous mixture containing 25% CF2N2, 70% CZF+ and 5% CF2Cl2 (mole per cent). Gas chromatographic analysis of each component of the mixture indicated a purity of not less than 98%. The small amount of CF2C12, which is inert in this system, serves TEMR ('Cf
Run
1 2 3 4 5 6 7
8 go 10 11 12 13 14 15 16d 17'
' Initial partial pressure of CF& is 25% of total initial preasure. Uncertainty limits (90% confidence level) for k o b d are estimated from least-squares analysis to be about 15%. Rate S/V = 10 cm-1; for all other runs S/V = 1.4 cm-1. constant calculated from the buildup of nitrogen.
'
NT X 10'
Figure 2. Arrhenius plot for the thermal decomposition of difiuorodiazirine a t a total initial p m u r e of 200 mm: 0,runs 2, 5, 6, 7, 8, 10, 11, and 13 ( S / V = 1.4), from which the Arrhenius parameters were calculated; A, run 9, S/V = 10, Teflon packing; 0, run 17, S/V = 10, Monel packing.
as an internal standard to correct glpc analytical data for instrument drift and sample depletion. The apparatus used is identical with that described previously, except that the temperature of the thermostated Monel reaction vessel was measured to an ab(8) M. J. Amrich and J. A. Bell, J . Am. Chem. Soc., 86, 292 (1964). (9) D. E. Milligam, D. E. Mann, M. E. Jacox, and R. A. Mitsch, J. C h m . Phus., 41, 1199 (1964).
Volume 71, Number 6 April 1967
1232
solute accuracy of about 0.1" with a calibrated copperconstantan thermocouple. Before collection of rate data, the reaction vessel was conditioned by a 6-hr exposure to difluorodia~irine a t an initial pressure of 1 atm and a temperature of 140". The evacuated reaction vessel at the desired temperature was charged to the required pressure by expansion of the gas mixture from a glass bulb. Periodic sampling was made by expanding an aliquot (0.7501,) of the gaseous mixture into the gas-sampling valve of a gas chromatograph. A 0.25411. X 24-ft column of Kel-F polymer oil No. 8126 (33% on Chromosorb P) a t room temperature adequately separated all of the components of the system. Inasmuch as the height of a gas chromatographic peak for small samples was found to be directly proportional to the number of moles of the corresponding compound and since each sample represents a constant fraction of the volume of the re-
The Journal of Physical Chemhtru
ERWINW. NEUVAR AND RONALD A. MITSCH
action vessel, it follows that the peak height is directly proportional to concentration (partial pressure) of the corresponding component in the reactor. All chromatographic operating parameters were held constant throughout the entire investigation. Most of the runs, consisting of six to ten points taken at convenient time intervals, were followed to a t least 90% completion. Rate constants and Arrhenius factors were calculated from the glpc analytical data by the least-squares method.
Acknowledgments. We gratefully acknowledge the assistance of Mr. Raleigh C. Ormerod, who prepared the computer program for the least-squares analysis of the data, and Mr. Wayne H. Swanson, who performed many of the decomposition runs. We extend special thanks to Dr. Alan S. Rodgers, Stanford Research Institute, Menlo Park, Calif., for his helpful counsel and critique of the manuscript.
