REACTION OF OZONE WITH PERFLUOROOLEFINS
Santelli and V. Lemke, members of the Analytical Group of th3 Chemical Engineering Division under the
477
direction of R. P. Larson, who performed the tellurium analysis.
The Reaction of Ozone with Perfluoroolefinsl
by Julian Heicklen Aeroepace Corporation, El Segundo, California (Received August 10,1966)
-
Ozone reacts with C2F4, CaF6, and C4Fs-2 at room temperature to give CF20, CFaCFO, and O2 as products. No other products were found, and, for the C2F4-03system, both CFaCFO and CF2CF20were definitely absent. The ratio of CF20to CF3CF0 is invariant to conditions and is 4.0 and 2.8, respectively, for the CaF6 and the C4Fssystems. A reaction mechanism is presented. For the reaction R2 O3 --+ R203where R is either CF2 or CF3CF, the rate constants are 81 X lo3, 13 X lo3, and 1.1 X lo3 M-’ sec-’, respectively, for the C2F4, C3F6, and C4Fs systems. At low pressures in the C4Fs system, the rate expression becomes second order in C4Fs; this is explained in terms of the competition between the reactions R2Oa -t R2 0 3 and R203 R2 + 3 R 0 R.
+
+
+
+
I. Introduction
11. Experimental Section
The reaction of ozone with hydrocarbon olefins has been studied extensively. The results have been reviewed and summarized by Leighton2 in his excellent book. The reactions are complex, and many products are formed. However, through several investig a t i o n ~ ~the - ~ relative rate constants of some of the simpler olefins at room temperature have been obtained. The most extensive set of rate constants was obtained by Vrbagki and Cvetanovi6,6 who found values of 1.8 X lo3, 5.1 X loa, and about 15 X loa M-l sec-l for C2H4, C3H6, and C4Hs-2, respectively, based on the Cadle and Schadt value of 1.8 X lo3 M-l sec-l for C2H4. The more recent work of Wei and Cvetanovi? slightly alters the values for CBH6 and C4H8-2to 8.1 X lo3 and about 18 X lo3 M-l sec-’, respectively. As part of a continuing program of fluorocarbon oxidations, we have investigated the reactions of ozone with C2F4, C3Fe, and C4Fs-2. Those results are reported here.
A mixture of cis- and trans-perfluorobutene-2 obtained from the Matheson Co. and perfluoropropene obtained from Peninsular ChemResearch, Inc., were used without further purification. Gas chromatograms showed no impurity peaks for C3F6, but C4Fe had one impurity peak of about 1.5% in the C4-Cs fluorocarbon region. The C2F4 was prepared by slowly adding 1,2-C2F4Br2to a mixture of zinc in methanol (1) This work was supported by the U. 8. Air Force under Contract No. AF 04(695)-469. (2) P. A. Leighton, “Photochemistry of Air Pollution,” Academic Press Inc., New York, N. Y., 1961. (3) R. D.Cadle and C. Schadt, J. Am. Chem. Sac., 74,’ 6002 (1952). (4)R. D. Cadle and C. Schadt, J. Chem. Phys., 2 1 , 163 (1953). (5) P. L. Hanst, E. R. Stephens, W. E. Scott, and R. C. Doers, “Atmospheric Ozone-Olefin Reactions,” The Franklin Institute, Philadelphia, Pa., 1958. (6) T. Vrbas’ki and R. J. Cvetanovi;, Can. J. Chem., 38, 1053, 1063 (1960). (7)Y. K.Wei and R. J. Cvetanovi;, ibid., 41, 913 (1963).
Volume 70, Number 1 February 1966
478
JULIANHEICKLEN
so as to keep the reaction temperature a t 60". The effluent gas passed through a reflux condenser to retain the methanol and through water to remove the last traces of methanol. Finally, the gas was dried with Drierite. A gas chromatogram showed the gas to be over 99% CzF4 with only two impurities. A distillation a t - 126" effectively eliminated these impurities. Ozone was prepared by the electrodeless discharge of research grade oxygen. The reaction cell was of Pyrex and had a Teflon stopcock with Viton 0 rings. Salt windows were a t either end of the cell to permit i n situ infrared analysis. The vacuum seal between the cell and the windows was made with Viton 0 rings lubricated with Kel F grease. The cell was situated in the sample beam of a Beckman IR-4 spectrophotometer. Pure ozone was allowed to stand in the cell. It decomposed slowly but measurably. Thus, all runs were done by adding the ozone last, so that its decomposition would have negligible effect. The olefin was degassed at -196" and placed in the cell at known pressure. The ozone was then degassed at -196" outside the reaction cell, and the stopcock to the cell was quickly opened and closed. The total pressure was then measured on a suitably calibrated Alphatron gauge. The rate of growth of a product peak (either the 5.129.4 band of CFzO or the 5.30-p band of CF3CFO) was monitored, and the initial rate of growth was obtained by extrapolation to zero time. I n some cases, after the run, the pressure of gas noncondensable a t - 196" (ie.,02)was measured.
111. Results The products of the reaction are CF20, CFsCFO, and 02.No other products were found. I n the C2F4 7
1
experiments, careful checks were made for CFzCFzO and CF3CF0, and they were definitely absent. Furthermore, some runs of the CzF4-03 system with excess O3 were taken to completion, and the CFzO pressure was twice the initial CzF4 pressure. The 0 2 formed could come from either the ozone-perfluoroolefin reaction or the background decomposition of 0 3 or both. From our results it is difficult to ascertain the situation. The initial rates of product formation R{ RO ), where RO is either CFzO or CSCFO, are shown in Tables I through V. At any perfluoroolefin (hereafter referred to as Rz) pressure, R{RO) increases linearly with (03)a t small 0 3 pressures but then levels off or even drops as ( 0 3 ) continues to rise. For the region where R( RO is linear with (03), the ratio R( RO )/(os) rises with the R2 pressure, as shown in Figures 1
1
The Journal of Physical Chemistry
Table I : CFZOProduction from C~F4-03Reaction (Cdd, mm
0.20 0.22 0.22 0.20 0.22 0.63 0.54 0.66 0.58 0.61 1.10 1.03 2.2 2.2 2.2 5.7
0.73 2.1 5.7 14.6 24 1.07 1.85 4.7 11.3 17 1.7 18 2.4 9.8 22 6.2
14 35 27 20 26 39 47 128 83 78 123 190 165 370 280 340
Table 11: CFZOProduction from C3F6-O3 Reaction R(C F ~ O ) ,
(CaFd, mm
(OS), mm
dsec
0.22 0.23 0.46 0.48 0.41 0.36 0.41 0.70 0.73 0.70 1.07 1.14 1.07 2.0 2.0 7.0
4.7 14.8 2.0 2.0 4.9 6.5 14.7 2.1 4.5 9.3 2.4 4.3 14.0 1.9 7.4 10.6
7.6 7.8 17 23 21 10.1 20 16 30 34 26 44 150 23 200 615
~~~
Table 111: CFaCFO Production from C3Fe-03 Reaction (Os),
0.186 0.43 0.75 1.02 1.96 1.89 5.5
R{CF:CFO) ,
mm
dsec
5.0 4.7 4.2 4.2 2.1 7.9 6.2
42.5 3.9 8.4 10.4 9.3 38 88
through 3. Under all conditions, the ratio of R (CF20) to R(CF3CFO[ remains constant at 4.0 in the C3Fe runs and 2.8 in the C4Fs runs.
REACTION OF OZONEWITH PERFLUOROOLEFINS
479
Table IV : C F 2 0 Production from C4F8-03 Reaction
1.8 3.5 6.0 4.8 19 7.2 18 25 30 27 33 31 37
39 85 46 9.9 33 4.0 11.3 19.4 24.5 37 65 11.9 6.0
0.19 0.19 0.39 0.84 0.86 1.31 1.29 1.33 1.33 1.28 1.31 2.6 5.3
10-3:/rl
:
RO'CF3CFO
~
I
~
10-4
Table V : CFsCFO Production from C4F8-Oa Reaction
-
I
I l l
I
I
I I
Figure 2. Plots of R{RO]/(Os) V8. (CaFe). 0.19 0.22 0.33 0.46 0.42 0.70 0.88 0.82 1.07 1.09 1.28 1.32 2.2 2.3 3.5 3.4 3.5 6.7
0.65 1.45 1.44 0.71 2.2 4.2
33 63 37 9.6 37 28 20 40 20 27 3.9 12.8 16 20 1.3 7.0 16 5.6
3.8 5.0 7.7 6.1 1.48 5.0 12.0 13.9 1.75 9.1 17.2
10-4
12.5
-
I
1
I
I I
1 10-5
lo-'
I
I l l
I
I (C4Fg1, mm.
10
Figure 3. Plots of R{RO}/(Os) us. (CdF8-2).
IV. Discussion At low reactant pressures, the rate of reaction should I
(C2F4),
mm.
Figure 1. Plot of R[CFzO)/(Os) us. (CZF4).
be slow and diffusional mixing of reagents should be fast. On the other hand, at high reactant pressures, the reverse is true. Thus, at low pressures, the chemical reaction should be rate controlling, but at high pressures diffusion should be rate controlling. Volume YO, Number B February 1966
JULIANHEICKLEN
480
Table VI: Rate Constant Ratios M -1 sec -1
mm-1 aec-1
CzF4
16 x 10-3 2.5 X 10-3 0.22 x 10-8
C&'B CaFS
-----
k1-
7
Olefin
mm - 2
81 X lo8 13 X loa 1.1 x 108
>O .OB
>o .012 1.9 x 10-4
Our results conform to this model. At low pressures, RIRO} increases linearly with (03) at any R2 pressure, thus indicating that the chemistry controls the rate. However, at high 0 3 pressures (which is then similar to the total pressure), R( RO ] becomes independent of (03)or actually falls as the (03)is enhanced. This effectis most marked in the C2F4 system where chemical reaction is the fastest and is least noticeable in the C4FS system where chemical reaction is the slowest. Clearly, diffusion must be the controlling step. The effect is so pronounced in the C2F4 system that there is only a very limited experimentally accessible region where the chemistry controls. We are only interested in the region in which chemistry controls, so that all of the ensuing discussion will be limited to the region where R( RO ] is proportional to (03). The mechanism that most easily explains the results is R2
+
0 3
-
R203 +R2
+ Rz R +
R203
0 3
--f
--f
(1)
R203
+
3R0
RO
(2)
0 3
+
+R
(3)
(4)
0 2
where R20a is the unstable ozonide intermediate. Reaction 3 cannot be a one-step process for its reverse would be quadrimolecular. Perhaps the intermediates are (RO)2 and RZO. The (RO)2 would fall apart to 2R0, and R 2 0 could either decompose to RO R 02. In the or react directly with O3 to yield 2 R 0 latter case reaction 4 would not be needed. Apparently, since CF20 is the principal product even in the C4F8 experiments, some fraction of the CFaCFO formed has sufficient energy to react further with ozone
+
+
CF&FO*
+
2CF7,O
+
(5) where CF3CFO* represents those CF3CF0 molecules with suffcient energy to react via ( 5 ) . 0 3 ---j
The Journal of P h y s h l Chemistry
0 2
hka/kp---880 -1
M -2 8eo-1
>2.0
x
10'2
>O. 13 X 10'2 4.8
x
109
R{CFIOI / R~CF~CFO) W
4.0 2.8
The mechanism predicts that
R { CF2O ] = constant R( CFaCFO 1
(7)
where LY is 2, 3, or 4, respectively, for C2Fa, CSFB, or C4F8. Thus, the quantities R{RO ]/(03) should be independent of the ozone pressure and rise linearly with (R2) at high R2 pressures or with (RJ2 a t low R2 pressures. The appropriate plots are shown in Figures 1 through 3. For C2F4, only very limited data are available, but the log-log plot (Figure 1) can be represented by a straight line of unit slope. From the intercept, a value for kl of 81 X 103 M-l sec-l is obtained. The data for C3F6 are shown in Figure 2, and the log-log plots can be well fitted by a line of slope 1. The ratio R( CFtO ] / R (CF3CF0] is 4.0, and kl is 13 X loaM-l sec-I. For neither C2F4 nor C3F6 is the pressure sufficiently low to enter the second-order region, but the situation is considerably different for C4F8, as shown by Figure 3. Both the first- and second-order regions are readily apparent. The ratio R { CF20] / R1CF3CF0] is 2.8, kl is 1.1 X loa M-' sec-l, and klk3/k2 is 4.8 X log M -2 sec-'. The rate constant data for the perfluoroolefins are summarized in Table VI, and they are comparable to those for the hydrocarbon analogs. However, contrary to the hydrocarbons, the reactivity increases in the perfluoroolefin series from C4F8 to C2F4 corresponding to the diminution of the double-bond strength for this series. Acknowledgment. The author wishes to thank Mr. Dennis Saunders for preparation of C2F4 and Mrs. Barbara Peer for assistance with the manuscript.
