T H E
J O U R N A L
OF
PHYSICAL CHEMISTRY Registered i n U. S. Patent Office 0 Copyright, 1976, by the American Chemical Society
VOLUME 80, NUMBER21 OCTOBER 7, 1976
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Kinetics of the Reaction of Ozone with Tetrafluoroethene Frina S. Toby and Sidney Toby* School of Chemistry, Rutgers, The State Unlversity, New Brunswick, New Jersey 08903 (Received February 17, 1976)
The reaction between ozone and tetrafluoroethene was measured in the gas phase in the temperature range 0-110 "C. The rate law found could be represented by -d[03]/dt = kl[O3][CzF,] k2[03]2[CzF4]where log (k1lM-I s-l) = 8.2 f 0.5 - (9500 f 700)/2.3RT, and log (k2/M-2 s-l) = 14.6 f 0.4 (10 100 f 600)/2.3RT. O2 was found to be a minor product, but experiments done in the presence of added 0 2 showed an uptake of 0 2 . Loss of C2F4 was measured so that the stoichiometry AC2F4/A03 was obtained. Other products were not determined here, but have been previously reported. A mechanism was postulated which accounts for the major findings with respect to product formation, the observed rate law, and the chemiluminescence associated with this system.
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Introduction Both the kinetics and the accompanying chemiluminescence of the gas-phase reactions of ozone with alkenes have been much studied1 but there has been very little reported work on the corresponding reactions with fluoroalkenes. We have recently found that chemiluminescence accompanies the reactions of O3 with tetrafluoroethene, 1,l-difluoroethene, cis-1,2-difluoroethene, trans- 1,2-difluoroethene, and hexafluoropropene.2The emission from the O3 C2F4 system was particularly intense and was identified as due to CF2 in its first electronically excited singlet state. The reactions of O3 with some perfluoroalkenes were studied by H e i ~ k l e nProducts .~ were measured but very limited kinetic data were available for CzF4. Gozzo and Camaggi4 studied the reaction between O3 and C2F4 in gas and liquid phases and products were studied in some detail. In the present study the rate law, rate constants, and mechanism of the reaction were investigated and Arrhenius parameters are given for the first time.
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Experimental Section The apparatus and methods used have been described previ~usly.~ The quartz cylindrical reaction vessel of length 10.0cm and volume 450 cm3was mounted in a thermostatted (f0.5 "C) oven. The ozone concentration was monitored with a beam of 254-nm radiation and linear Beer plots were obtained over the ozone concentration range used. At the highest temperature employed the rate of disappearance of O3 alone was 0.1% of the corresponding rate in the presence of substrate. Under most conditions the transmitted light would return to its value for an evacuated reaction vessel showing that all 0 3 was destroyed and that products did not appreciably absorb 254-nm light.
Ozone was generated by passing oxygen (Matheson Ultrapure grade) through a discharge from a Tesla coil. The ozone was condensed at -196 "C and the oxygen ,pumped away. Tetrafluoroethene (Columbia Organic Chemicals) was distilled and degassed before use and check runs were done with aliquots which had been distilled twice. No impurities in the C2F4 were found by gas chromatographic analysis. Some runs were done in the presence of sulfur hexafluoride (Matheson) which had been distilled. The temperature range employed was 0-110 "C and the pressure range was 0.01-0.2 Torr for O3 and 0.3-15 Torr of CzF4. A t the end of some experiments, excess C2F4 was measured by gas chromatography using a 10-ft molecular sieve (13X) column at 100 "C.
Results Stoichiometry. Carbonyl fluoride, oxygen, tetrafluoroethylene oxide, and cyclohexafluoropropene have been identified as products in this system4 and only 0 2 was measured here. The stoichiometry of (C2F4 used/03 used) was measured at 30 "C and the results are shown in Figure 1. 0 2 has been reported as a minor product4 and we confirmed that when products were cooled to -196 "C there was some residual pressure. However, when runs were carried out in the presence of added 0 2 the pressure of noncondensables at the end of the run was actually reduced. With the assumption that the noncondensable pressure consisted of 0 2 (CO has not been reported as a product) the stoichiometry of 0 2 production and, consumption was measured for some runs and this is shown in Table I. Kinetics. The overall order of the reaction appeared to be between 1 and 2. Single order plots did not normally give 2313
2314
Frina S. Toby and Sidney Toby
TABLE I: Production and Consumption of 02 Temp, "C
Initial pressure, Torr
Final p , Torr
C2F4
0 2
02
(AOz)/ (Odn
3.0 3.3
0 0 0 0 0.17 0.35 0.41 0.11 0.22 0.55 1.62
0.007 0.002
+0.1 +0.03
0.004
+0.07
0.006 0.015
+0.1 -1.4
0 3
0.060 0.060 0.060 0.060
30
0.11 0.11 0.11
0.058 0.058 0.058 0.058
110
5.7 7.9 5.2 5.2
5.2 5.2 5.2 5.2 5.2
0.135
-1.9
0.275 0.006 0.015
-1.2
0.030 1.53
-9.0
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100
200
0
-1.6
1
,
2
3
M x1o6
Figure 2. Plot of k, and k b as a function of [03]0at 70 O C with [C2F4l0 = 1.4 X M. Units of k, are s-', and of kb are M-I s-l.
I
3 00
0
Figure 1. Stoichiometry plot of
(c2F4 used)/(03used) as a function of initial (C2F4)/(03) at 30 O C . Filled points referto runs with 0.4 Torr added
02.
straight lines although second-order plots were usually the least curved. We postulate that the rate law can be written
+ kz[0312[CzF41
(1)
For runs done with a large excess of C2F4 we define the pseudoconstants ha = k,[C2F4] and kb = h2[C2F4] n. Equation 1 may be integrated to give In ([03]/(1 + K[03I)I = kat
I
r0,1,
IC, F4/ O3lo
-d[03lldt = ki[031[CzF41
I
-1.8 -3.5
/
0
0
+ In {[03]0/(1+ K[03lo)!
(2)
where K = kblk,. A new method for obtaining the rate constants was used. Trial values of K were inserted in the lefthand side of eq 2 which was then plotted against time using an APL program.6 This typically resulted in a curved line but as the value of K was increased the line straightened and the correlation coefficient passed through a maximum which exceeded 0.99. The value of K for the maximum correlation coefficient together with the slope of that line gave k , and kb.6 In some cases no maximum correlation coefficient could be obtained but these were runs which obeyed simple order kinetics and only one rate constant could be measured. The resulting values of k , and k b were measured over a range of initial O3 concentrations at 0,30,70, and 110 OC with the CzF4 concentrations held constant at each temperature and typical results are shown in Figure 2. A series of experiments was then carried out with [O& held constant at each temperature over a range of C2F4 concentrations. The resulting values of k , and k b were plotted against [ C Z F ~and ] typical results are shown in Figure 3. These plots show reasonably good linearity and we therefore write the rate law as
The Journal of Physical Chemistry, Vol. SO, No. 21, 1976
I
2
[C,F41,
M x104
3
Flgure 3. Plot of k, and kb as a function of [C2F4]at 70 O C with [O3lO = 2.1 X M.
An Arrhenius plot of hl and k2 with estimated error limits is given in Figure 4 and yields log (kl/M-l s-l) = 8.2 f 0.5 (9500 & 700)/2.3RT, and log (k2/M-' s-') 14.6 f 0.4 - (10 100 f 600)/2.3RT. The reaction of 0 3 with CzF4 was slowed by the effect of added 0 2 and an induction period was noted after the reactants were mixed before appreciable reaction occurred. The plots of transmitted light vs. time were not smooth curves and we were unable to make a kinetic analysis of these runs. Some experiments were done in the presence of added SFe and no significant change in k, or kb was found.6 Discussion
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Heicklen3 studied the O3 C2F4 system a t room temperature with excess O3 present and found COF2 as a product together with some 0 2 which could have come from the background decomposition of 03.Gozzo and Camaggi4 measured products from the reaction in inert solvent at 0 "C. When C2F4 was in large excess the products were COFz and the epoxide C2F40 together with small quant,ities of 0 2 and c-CsF6. When 03/C2F4 was closer to unity, 0 2 became a major product and the ozonide CZF403was found. We found 0 2 to be a minor product (about 7% of the O3 used) as shown in Table I and also observed a rather surprising uptake of 0 2 when it had been added to the system. The reaction of CzF4 with 0 2 has been previously investigated by Gozzo and Camaggi7J' and was found to be very slow in the gas phase but strongly accelerated by ozone. Molecular oxygen can thus be a reactant and a product in the c2F4 + O3 system and we shall consider possible mechanisms for this effect shortly.
2315
Reaction of Ozone with Tetrafluoroethene
Taking steady states13 in C2F403, -CF202-, -CF&F20-, and CF2, and assuming that (k6 h7) >> hg[O3] yields
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i'09
+ k 7 ) (4) -d[03]/dt = h3[C2F4][03] + h&g[C~F4][03]~/(k~ which is of the same form as eq 3. We know of no published values for the Arrhenius parameters of O3 haloalkenes with which the present work can be compared, but comparisons with alkenes are possible. Our value for the Arrhenius factor A3 = 10s,zM-l s-l corresponds to ASc* = -25.0 f 2 cal mol-l deg-l a t 25 OC. The A factor is slightly larger than the values of 106.3and 106.9reported for O3 C2H4 by DeMore14 and Becker, Schurath and Seitz,15 respectively. DeMorel4 has estimated that a five-membered ozonide transition state would correspond to A = lo7.', whereas a linear complex would result in A = 1010.6,thus our results favor a cyclic complex for 0 3 C2F4. Our activation energy E3 = 9.5 kcal mol-l is much higher than the values of 4.9 and 4.7 reported for ethylene14J5 and comparable with the value E = 10.8 kcal mol-l measured for a~ety1ene.l~ Gozzo and Camaggi4measured products in various sdvents a t 0 "C using much higher 03/C2F4 ratios than used here, nevertheless some comparisons may be made. Our mechanism predicts that the order of abundance of products is c - C ~ < F~ CzF4O < 0 2 < COFz. This is the correct order with the exception of 02 which was always a minor product other than for two runs where 03/C2F4 approached unity. Our data (Table I) also show 0 2 is a minor product and that it is actually consumed in runs with added 0 2 . Our mechanism predicts that d[C~F40]/d[C3Fs]= k7/h6, and a plot of C Z F ~ O / C formed ~ F ~ vs. O3 flow rate (omitting one point where O3 was comparable to C Z F ~shows ~ ) no clear trend with an average value of h7/ks 40. The mechanism also leads to d[COF2]/d[C2F40] 2 2k9[03]/k7 and if we approximate this relationship by plotting COFz/CzF40 formed vs. O3 flow rate (again minus the highest 0 3 point4) there is a reasonably linear relationship with a correlation coefficient of 0.91 and an intercept of 1.95. Missing Steps in the Mechanism. Although the postulated mechanism explains most of the observed kinetics and formation of products, it is incomplete with respect to the consumption of 0 2 , and the overall uptake of O3 and of CzF4. We can only offer qualitative explanations for these aspects of the reaction. The consumption of added oxygen shown in Table I is in accord with the findings of Gozzo and Camaggi.7 They found that CzF4 reacted with 0 2 very slowly in the gas phase but that the reaction was very fast in the presence of 03.The uptake of 0 2 led to the formation of an unstable poly(tetrafluor0ethylene peroxide) ( C ~ F 4 0 2 )via ~ free radical intermediates:
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102:
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IO
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I I
2.6
I
I
I
I
I
2.0
3.0
3.2
3.4
3.6
1
I 03/T O K Flgure 4. Arrhenius plot of kr (in M-l s-l) and k2 (in M-* s-l). Vertical
bars are estimated error limits.
Because of the uncertainties involved in extracting two rate constants from the observed rate, individual points in Figures 2 and 3 are subject to appreciable error. Nevertheless the trends shown are clear: the values of ha and hb are reasonably independent of [O3lOover nearly all of the range studied. However, a t the lowest pressures employed (-0.03 Torr) hb showed an increase which has also been found for the rate constants corresponding to hb in the 0 3 allene and 0 3 butadiene system^^^^ and for which we have no satisfactory explanation. Both ha and hb are proportional to [C2F*] as shown by Figure 3, and in comparison plots of k , and k b against [C2F4I2showed distinct curvature. We can explain the basic features of the reaction in terms of the following simplified mechanism:
+
C2F4O3
4
C2F4 + -CF202-
COFZ
+ -CF202-
5 ---*
COF2
+ -CFzCF20-
6
-CF2CF20-
+
+
COFz
+ CF2
+
-
+
O3 -CF2CF20-
9
R + 0 2 ROz RO2 + CzF4 -,R-O-O-C2F4-
+
O2 2COF2
The chemiluminescence from this system2 is due to singlet excited CFz and is associated with energy of up to 117 kcal mol-l. The sequence suggested by Gozzo and Camaggi4 for the formation of CF2 is only 72 f 8 kcal mol-' exothermic: C2F4 + O3 COF2 CF202, CF202 -,CF2 + 0 2 . The sequence of reactions 3,4,5,6, and 7 is exothermic overall by 220 f 14 kcal mol-1 lo and is analogous to the multistep activation or energy pooling which has been proposed in peroxidell and singlet molecular oxygen12chemiluminescence.
-
+
-
etc.
A stable polymer (CZF~O),was also formed. Such reactions would consume far more C2F4 than 0 3 and account for the high AC2F4/A03 ratios which we found and which are plotted in Figure 1. In the absence of added 0 2 , the 0 2 normally obtained as a product in ozone reactions becomes a reactant. When 0 2 is deliberately added, the AC2F4/A03 ratio increases even more: Figure 1shows that the addition of 0.4 Torr of 0 2 approximately doubled the resulting value of AC2F4/A03. The quenching effect of added 0 2 on the CF2 chemiluminescence in this system2 can be similarly ascribed to the scavenging of precursor radicals by 0 2 . The Journal of Physical Chemistry. Vol. 80, No. 2 1, 1976
2318
M. Faraggi
Equation 4 may be written R = RI+ R2, where R1 and R2 are the rates of the primary and secondary processes consuming 03.Under the conditions of our experiments R2/R1 reached a maximum value of 2.5 and this is only possible if short chains of ozone decomposition occur. Such chains occur in the O3 alkene systems and have been shown to be thermochemically feasible for O3 C2H4.l Although the identity of the chain carriers for the O3 CzF4 system cannot be established from our work, such a chain leads to the catalytic destruction of 03.This would normally form 0 2 as a major product but, as previously discussed, the resulting 0 2 is consumed in this system to form oxygen-containing polymers.7 In summary, the reaction of O3 with CzF4 is an extraordinarily complex system involving chains of both reactants and exhibiting both 02 production and 02 uptake. We have attempted to account quantitatively for the observed rate law and have given a qualitative description of the complex aspects of the kinetics. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 12, 2015 | http://pubs.acs.org Publication Date: October 1, 1976 | doi: 10.1021/j100562a001
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Acknowledgment. We thank the Rutgers University Research Council for the support of this work. Supplementary Material Available: Tables A-F of kinetic
data at 0,30,70, and 110 "C and effects of added SFG at 30 OC along with a computer program for obtaining rate constants (6 pages). Ordering information is available on any current masthead page. References and Notes (1)F. S.Toby, S. Toby, and H. E. O'Neal, int. J. Chem. Kinet., 8 , 25 (1976). especially ref 1-11. (2)R. S.Sheinson, F. S. Toby, and S. Toby, J. Am. Chem. Soc., 97, 6593 (1975). (3) J. Heicklen, J. Phys. Chem., 70, 477 (1966). (4) F. Gozzo and G. Camaggi, Chim. ind. (Milan), 50, 197 (1968). (5) F. S.Toby and S. Toby, int. J. Chem. Kinet., 6, 417 (1974). (6) See paragraph at end of the text regarding supplementary material. (7) F. Gozzo and G. Camaggi, Tetrahedron, 22, 1765 (1966). (8) F. Gozzo and G. Camaggi, Tetrahedron, 22, 2161 (1966). (9)F. S.Toby and S. Toby, ht. J. Chem. Kinet., Symp. 1, 197 (1975). (10)S. W. Benson, "Thermochemical Kinetics", W h y , New York, N.Y., 1968. (11) S.R. Abbott, S. Ness, and D. M. Hercules, J. Am. Chem. Soc., 92, 1128 (1970). (12)E. A. Ogryzlo and A. E. Pearson, J. Phys. Chem., 72, 2913 (1968). (13)Gozzi and Camaggi4 found small quantities of ozonide in the products of the liquid phase reaction at 0 OC at high 03/C2F4ratios, but not in the gas phase reaction. (14) W. B. DeMore, int. J. Chem. Kinet., 1, 209 (1969). (15)K. H. Becker, U. Schurath, and H. Seitz, lnt. J. Chem. Kinet., 6, 725
(1974).
Steady State and Pulse Radiolysis Studies of Molybdenum Octacyanate in Aqueous Solutions M. Faraggl Nuclear Research Centre-Negev, P.O.B. 900 1, Beer Sheva, Israel
(Received January 8, 1976)
The oxidation of oxygen saturated solutions of Mo(CN)s4- at different pH values was studied by steady state and pulse radiolysis techniques. I t was shown that Mo(CN)& is the oxidation product formed. G values of the Mo(V) octacyanate complex varied with the pH of the solution. It was of the order of 13 at pH 0 (1M HC104) and approaching zero at pH >7. The high G values in acid solutions are explained by the oxidation of the Mo(1V) complex ion in the reactions with OH and H02 radicals and by H202. The H202 reaction with Mo(1V) ion was found to be a very slow reaction and 75% effective only. In neutral and alkaline solutions the low G values are interpreted by the reduction of the MOW)ions formed by the 0 2 - radicals. Confirmation of the above mechanism was established by using the pulse radiolysis technique. The formation of Mo(CN)s3-, the oxidation product of M O ( C N ) ~in~ the - reactions with OH and HO2 radicals, was followed at 385 nm. The rate constants (in M-1 s-l) are (5.8 f 0.5) X lo9 and (5.7 f 0.6) X lo4, respectively. These values were unaffected by the presence of H + or unreactive alkaline cations. 0- reacts with Mo(CN)s4much more slowly (estimated to be -1 X lo7 M-l s-l). 0 2 - reduces the Mo(CN)& to Mo(CN)x4-, the rate constant found was 3.0 f 0.5 X lo5 M-l s-l.
Introduction In recent years, two studies have been published on the steady state (y rays) and pulse radiolysis of aqueous molybdenum(1V) octacyanate solutions.l.2 Sharmal reported that M o ( C N ) ~ ions ~ - in 0.4 M H2S04 solutions (oxygenated) react with OH radicals via a single electron transfer reaction to give Mo(CN)s3-
+
M o ( C N ) ~ ~ -OH
+
Mo(CN)s3-
+ OH-
The Journal of Physical Chemistry, Voi. 80, No. 21, 1976
(1)
This study' also reports that G(Mo(CN)s3-) = 2.7 and concludes therefore that neither HO2 nor Hz02 oxidize the Mo(CN)& complex. However, a close examination of his results (ref 1, Figure 1) indicates that G(Mo(CN)&) = 1.3 rather than 2.1. Waltz et a1.2 measured the rate constant of reaction 1via the competition pulse radiolysis method3 and found a value of (5.8 f 0.6) X lo9 M-' s-l. Cyanide complexes have been extensively studied in the
