ALANS. RODGERS
254
Kinetics of Fluorination.
11. The Addition of Fluorine to
cis- and trans-Perfluorobutene-2l
by Alan S. Rodgers Contract Research Laboratory, Minnesota Mining and Manufacturing Company, St. Paul, Minnesota 66119 (Received July 24,1964)
~
+
The kinetics of the reaction CF3CF-CFCF3 FZ + CF3CF2CF2CF3have been investigated in the gas phase between 220 and 2.50" IC=C
C - C f F
+F
(1)
This suggestion received both favorable3 and unfavorable4 comment. However, recent independent work from two laboratorie~5~~ has indicated that the kinetics of the addition of fluorine to the carbon-carbon double bond conforms to the niechanisni 1 to 4, thus providing quantitative evidence in support of the proposed initiation reaction.
>C=C< F3C-e
C - C f F
(2)
+ F2+F + C - C f F + F
(3)
2 ~ c-C< +
+termination
+
Consepuently, this reaction should provide a simple test of the proposed mechanism.
Experimental The apparatus and experimental procedure have been described in detail previously.6 Briefly, the reactants were brought to the experimental temperature separately, and the reaction was started by mixing through pressure expansion. Pressure equilibration was attained in approximately 5 sec. with no measurable effect upon the gas temperature. The progress of the reaction was monitored by continuously recording the pressure in the reaction vessel. There were three reaction vessels used in this work. (a) a 140-cc. glass flask, coated with Teflon and packed with 10 g. of Teflon turnings, resulting in a surface-to-volume ratio ( S / V ) of 2.9 cin-'; (b) a 148-cc. glass flask packed , with 10 g. of Teflon turnings, S l V = 2.9 c ~ n . - ~33%
(4)
(1) This research was supported by t h e Advanced Research Projects Agency and was monitored by t h e Bureau of Naval Weapons.
Neither reaction, however, was an example of siniple fluorine addition. In the study of 2,3-dichloroperfluorobutene-2, side reactions resulted in the formation of 2-~hloroperfluorobutane,and, as a result, only the initial rates could be studied.6 Fluorine-substitution reactions were also present in the reaction of ethylene with fluorit1e.j Such side reactions should not, however, occur in the fluorination of perfluorobutene-2.
( 2 ) W. T . Miller and 4 . L. D i t t m a n , J . Am. Chem. SOC.,78, 2793 (1956).
The Journal of Physical Chemistry
(3) N. N. Semenov, "Chemical Kinetics and Reactivity," Vol. 1, Pergamon Press, New Tork, N. Y., 1958, pp. 260-271. (4) J. M. Tedder, "Advances in Fluorine Chemistry," Vol. 2. M. Stacey, J. C. Tatlow, and A. G. Sharpe, Ed., Butterworth and Co., Ltd., London, 1961, pp. 107-109. (5) G. A. Kapralove, L. T u . Rusin, A. M.Chaikin, and A. E. Shilov, Dokl. Akad. .\'auk S S S R , 150, 1281 (1963). (6) A. S. Rodgers, J . Phys. Chem., 67, 2799 (1963).
KINETICS OF FLUORINATION
255
of the total surface area being glass; (c) a 316-cc. glass flask packed with Teflon turnings, S / V = 1.6 cni.-’, 45% of the total surface area being glass. The surface of each reaction vessel was conditioned by exposure to 100 to 200 mm. of fluorine for 24 hr. prior to its use initially and was reconditioned only if exposed to the atmosphere or other sources of contamination. Perfluorobutene-2 was obtained from Matheson Co., Inc., and was found to be a mixture of the cis and trans isomers in a 1 :4iiiole ratio. The isomers were separated by preparative g.1.c. using a 1.27 cni. 0.d. X 4.87 in. column packed with 33% (by weight) KEL-F tetramer oil (Minnesota Mining and Nanufacturing Co.) on Celite. Analyses of reaction products were made on a 0.636 cm. 0.d. X 7.31 in. colunin with the same stationary and support phases. The separated isomers were found to be 99% trans and 97% cis (determined by peak height iiieasurenien ts), respectively, and were identified by infrared spectroscopy. The fluorine gas was obtained from Matheson Co., Inc., and was passed through a sodium fluoride scrubber and stored in a monel flask. It was analyzed by reaction with mercury, 99.5% of the gas reacting. Hexafluoroethane (E. I. du Pont de Semours and Co.) was used as the diluent gas in the reactions; g.1.c. analysis indicated less than 0.1% C2F4 by volume. Prior to each experiment, all condensable inaterials were thoroughly degassed at the temperature of liquid nitrogen and a t a pressure of min. less than 1 X
Results Products. Gas-liquid chromatographic analysis of the products of the reaction of Fz with cis- and trans-
Table I : Effect of Fluorination upon the Isomer Ratio in Perfluorobutene-2 YCreaction Run
To trona
(theor.)
%
trans-CF&F=CFCFs 0 66 69 69
1 6 9
1
0.8 1.1
1.2
cis-CFsCF=CFCFs 0 54 50 67
12 13 15
3.1 2.7 2 7 2 3
which was found for 2,3-dichloroperfluorobutene-2.6 The integrated form of eq. 6 is n
The subscripts 0 and t indicate the values of the given quantities a t tinies to and t. Table I1 shows the agreenient obtained by applying both equations to the data from one run. ( C 4 Fg1
“*
perfluorobutene-2 could detect only perfluorobutane, identified by infrared spectroscopy. The stoichiometry of the reaction is CF3CFzCFCF3
+ Fz
----f
CF3CFZCFzCF3
C M
60% Run 9
(5)
When given sufficient time, reactions characteristically attained more than 90% of the expected pressure decrease according to eq. 5, corresponding to discrepancies of about 0.5 nini. or less. Analyses for the cis-trans ratio of the residual olefin were made for those reactions with excess olefin. The results presented in Table I show that no change in the ratio of the isomers occurred as a result of fluorination, for either cis- or trans-perfluorobutene-2. Kinetics. One would anticipate that the rate of the reaction between fluorine and perfluorobutene-2 would follow
I
”?
0.5 O I M
I
I
I
200
400
600
TIME ( 6 e c )
Figure 1. Plots based on integrated rate equation for reaction of trans-perfluorobutene-2 with fluorine. Per cent reaction of last data point is indicated. Experimental conditions are given in Table 111.
Volume 69, Sumber 1
January 1966
ALAXS.RODGERS
256
Table I1 : Specific Rate Constant for the Reaction of Fz with truns-CF&F=CFCFaa X 106 mm. set.--.
---K
Time, sec.
A P , mm.
Rate X 10% mm. - l 8ec.
30 90 150 240 480 600
0 5 1.4 2.2 3 2 4.9 5 5
1.73 1.40 1.15 0.944 0.515 0,486
Eq. 7
Av. a
Eq. 6
(to = 0 )
15.0 12 3 14.4 15.0 13.4 l5,5 14.3
13.2 14.2 14.9 12.9 14.5 14.6 14.1
Temp. = 222°K.; POC,F~= 14.9 mm.; POF,
=
10.2 mm.;
POC~F~ = 76 mm. 4.0
The specific rate constants for the fluorination reactions mere routinely determined by eq. i' through plots of ( I C4F8]/[F2])'"us. time. The slopes were calculated by least-squares procedure. Such graphs were generally linear to 80% reaction, and typical exaniples are shown in Figure 1. The results for both cis- and trans-perfluorobutene-2 are summarized in Table 111.
Table 111: Summary of the Kinetic Data for Reaction of Fluorine with Perfluorobutene-2 Run no.
1. O
K
Initial preaeure, mm.--Totala FI
GFs
K , 1. mole-' 8ec.-L
4.2
4.6
4.4
1000/ T'K
Figure 2. Temperature dependence and comparison of the rate constants for the reaction of cis- and trans-perfluorobutene-2 with fluorine: ~-, least squares, trans isomer only; 0,average values for trans isomer; 0, data points for cis isomer.
These results are coinpared in Figure 2. The curve represents a least-squares treatment of log K as a function of 1/T for the trails isomer, and the circular points represent the data obtained for the cis isomer. It is evident that the rates of fluorination of the two isomers are experimentally indistinguishable. A least-
trans
1 2
348 248
11 8 7 7
7.8 5.0
3b 4b 5b
247 247 247
10 15 6 2 6 5
9.6 7.4 10.9
6
7
233 233 233
15 2 10 0 10 1
10.4 14.7 14.1
9 10
222 222
14 9 7 2
10.2 10.4
116
220
19 6
8.0
ab
197,5 15 9 99.0 14 6 Av. 1 5 2 f O 6 95.1 14 6 95.6 12 8 96.6 12 8 Av. 1 3 4 * 0 6 98.1 4 6 5 11 93.7 95.6 5 1 Av. 4 9 * 0 3 101,o 2 02 294.7 2 08 Av. 2 05 i 0 03 1 85 87.0
cis 12 13 14 15 16
5.6 7.0 11 0
101 100 5 99 1
218
10 2
16 0
200 5
218
15 2
10.2
101 0 Av.
* C2Fs was added as diluent. Teflon.
The Journal of Physical Chemistry
-3t
M
10 4 15 1 7 1
247 236 228
t
* Reaction
14 4 6 03 3 62 1 43 1 51 1 47 f 0 04 flask coated with
I 0
I
1
50
100 TIME ( s e c )
I50
Figure 3. Pressure us. time for reaction of trans-perfluorobutene-2 with fluorine at 232°K. Surface-tovolume ratio is 1.6 cm.-l: 0, PO-c = 12.1 mm., PF~ = 7.8 mm., PT = 200 mm.; -calculated by eq. 10and 11; 0,pc-c = 12.0mm.) PF?= 7.1 mm., PT = 107 mm.; - - - -, calculated by eq. 10 and 11. ~
KINETICS OF FLUORINATION
257
squares treatment of all the data of Table I11 resulted in log K (1. mole-] see.-')
=
8.413
f.
0.1 -
* 30
17"
T
(8) or
K
=
(2.59
f
0.6) X lo8 exp
(-81tT*
140)
(9)
The reactor used in the above experiments has a surface-to-volume ( S / V ) ratio of 2.9 cm.-'. To investigate the effect of surface, the 316-cc. reaction vessel was used and packed with 10 g. of Teflon turnings, reducing the SI V to 1.6 cm. -I. The packing was added with the hope that an isothernial reaction would still be obtained. This hope was not realized; thus, it was necessary to take into account self-heating of the reaction gases using a convective heat-transfer model. The necessary equations7r8are dX dt
- ==
K(A
-
order with respect to fluorine; it is independent of the total gas concentration and unaffected by a change in the surface-to-volume ratio (aside from effects due to self-heating). All these factors satisfactorily conform to the niechanisni represented by reactions 1 to 4. By making the usual steady-state assumptions, one obtains
,X)"'(B -
In eq. 10 and 11,the symbols X, A, B, and N represent the concentrations of product, olefin, fluorine, and total gas, respectively; Q denotes the heat of reaction; a,the heat transfer coefficient, and U and Uo,the temperature of gas and walls of the reaction vessel, respectively. The heat capacity of CzFs9was used for C,, and Q was cistimated a t 120 kcal./mole. aS/VC, was adjusted so that the niaxiniuni observed and calculated pressures corresponded; a value of 12 X mole 1.-' set.-' was thus obtained. The results of the calculations for two experiments are shown in Figure 3. The good agreement between the observed and calculated pressures indicates a negligible effect of surface upon this reaction. An inhibition period of 8 to 30 see. was observed for this reaction. Since this was only a few times greater than the mixing time of the reactants (-5 sec.), it was not considered quantitatively. It was presumably due to oxygen present in the fluorine, as was the case for 2,3-dichloroperfl~orobutene-2.~
Discussion The experimental results presented above have shown that the rate of addition of fluorine to both cisand t~ans-perfluorobutene-2 is one-half order with respect to the olefin concentration, and three-halves
Thus, from eq. 6, 9, and 12
+
E3 - l,12E4
lI2E1=
8.2 kcal./mole
Both E, and E4 should be small so that the activation energy for initiation is approximately 16.5 kcal./mole. The heat of isomerization of butene-2 is 1 kcal./mole,'O the trans isomer being more stable than the cis. If the same difference in heat of formation is assumed for cis- and trans-perfluorobutene-2, then the identity of their rates of fluorination (and therefore the rate of reaction 1) indicates that the transition states for each isomer in reaction 1 are different; each more closely resembling their respective reactants, rather than products. The absence of any significant change in the isomeric ratio of the residual olefin (Table I) indicates that reaction 2 is essentially irreversible and that concurrent geometric isomerization, such as observed by Ayscough, et al.," in the photochlorination of czs- and transdichloroethylene, does not occur to an appreciable extent in fluorination under these experimental conditions. In conclusion, we have shown that the reaction of fluorine with perfluorobutene-2 represents a simple, homogeneous, gas-phase addition of fluorine to the carbon-carbon double bond, that the kinetics of this addition are given by eq. 6 or 7 for a t least 80% of the reaction, and, finally, that this rate equation can be interpreted in ternzs of the proposed niechanisni, reactions 1 to 4. Acknowledgment. The author wishes to express his appreciation to A i r . ~ ( ~ Garton ~ ~ for~ assistance , ~ ~I n h the experinlental (7) N. N. Semenov, "Some Problems in Chemlcal Kinetics and Reactivity," Vol. 2, Pergamon Press, New Tork, N. T , 1959. pp. 1-10. (8) A more detailed explanation of the procedure maJ be found in ref. 6. (9) J. S. Wicklund, H. F. Flieger, and J. F. Masi, J. Res. .VatZ. Bur. Std., 51, 91 (1953). (10) A.P.I. Research Project 43, Carnegie Press, Pittsburgh, Pa., 1953, p. 715. ( 1 1 ) 1'. B. iiyscough, A. J. Cocker, and F. S. Dainton, Trans. Farad a y sot., 58, 284 (1962).
Volume 65>,Vtkmber 1
.January I965
