292
G . E. MILLWARD AND E. TSCHUIKOW-ROUX
A Kinetic Analysis of the Shock Wave Decomposition
of 1,1,1,2-Tetrafl~oroethane~~
by G. E. Millwardlb and E. Tschuikow-Row* Department of Chemistry, The University of Calgary, Calgary 44, Alberta, Canada
(Received August 18, 1971)
Publication costs borne completely by The Journal of Physical Chemistry
The kinetics of the thermal decomposition of 1,1,1,2-tetrafluoroethanehave been studied using a single pulse shock tube in the temperature range 1170-1410°K at total reflected shock pressures of about 2850-3800 Torr. Although the main reaction involves the molecular elimination of hydrogen fluoride, CzHzF4 kl_ C~HFI HF, a radical mechanism is proposed to explain the formation of additional products derived from the scission of the C-C bond. From the analysis the first-order rate constant was found to be log kl (sec-l) = 13.42 f 0.28 - (70,700f 1700)/2.303RT. With this value of kl it was possible to evaluate the C-C bond dissociation energy, D(CHtF-CF8) = 92.3 f 2.9 kcal mol-’.
+
Introduction Previous work on the decomposition of fluoroethanes has established primarily the Arrhenius parameters for the molecular elimination of hydrogen fluoride. This reaction governs the kinetics of these compounds a t temperatures below 1350”K, as has been demonstrated for the series CzHa+F, (where n = 1-3).2-7 However, studies of the pyrolysis of 1,1,2,2CzHzF48lg and CzHFs, lo using the single pulse shock tube (SPST) technique, have indicated the increasing importance of the parallel C-C bond scission reaction. I n these cases the major product was still the fluoroethylene derivative, although several other stable species arose as a result of the radical reactions. Mechanisms, which were consistent with the observed products, were proposed in each instance. From the analysis it was possible to evaluate not only the Arrhenius parameters for HF elimination but also the C-C bond dissociation energies D (CHF2-CHF2) and D(CHFrCF8). The general trend of the activation energy for H F elimination with additional fluorine atom substitution has been e ~ t a b l i s h e d . @However, ~~~ the effect of p-fluoro substitution upon this trend has not been resolved. Additional fluorine substitution seems also to provide a concurrent increase in the C-C bond dissociation energy. Thus, the present work has three important objectives: firstly, to provide a mechanistic interpretation of the observed reaction products, which is consistent with the previous investigations of 1,1,2,2CzH2Fd9 and CzHFs;’O secondly, to evaluate the Arrhenius parameters for H F elimination from 1,1,1,2CzHzF4 and to examine the effect of structure on the activation energy of this reaction; finally, to estimate the important thermochemical quantity, D(CHzF-CF3). The Journal of Physical Chemistry, Vol. 7%,No. 8,1978
Experimental Section The 1,1,1,2-tetrafluoroethane was donated by the Du Pont de Nemours Co., and a gc analysis showed it had a purity of 99.95%. The methyl fluoride, fluoroform, vinyl fluoride, and hexafluoroethane were Matheson Co. research grade gases, as was the argon diluent (99.99801, purity). Pentafluoroethane and trifluoroethylene were obtained from Peninsular Chemresearch Inc., and were found to be 99.9% pure. Tetrafluoroethylene was prepared by heating polytetrafluoroethylene (Teflon) shavings in vucuo, and the product was purified by trap-to-trap distillation. (1) (a) Work supported by the National Research Council of Canada; (b) Postdoctoral Fellow, 1970-1971. (2) D. Sianesi, G. Nelli, and R. Fontanelli, Chim. Ind. (Milan), 50, 619 (1968). (3) M. Day and A. F. Trotman-Dickenson, J . Chem. floc. A , 233 (1969). (4) P. Cadman, M. Day, A. W. Kirk, and A. F. Trotman-Dickenson, Chem. Commun., 203 (1970). (5) (a) P. Cadman, M . Day, and A. F. Trotman-Dickenson, J . Chem. Soc. A , 2498 (1970); (b) ibid., 1356 (1971). I n these references errors in the calculated reaction time may arise from boundary layer formation. Small bore (1-in. i.d.) shock tubes are particularly prone to this phenomena, and the problem is magnified in a reflected shock wave environment [see, for example, A. G. Gaydon and I. R. Hurle, “The Shock Tube in High-Temperature Chemical Physics,” Chapman and Hall Ltd., London, 1963; or R . L. Belford and R. A. Strehlow, Ann. Rev. Phys. Chem., 20, 247 (1969)l. However, using a highfrequency pressure transducer to monitor the wave history it is possible t o obtain an experimental reaction time. This can then be compared directly with the reaction time calculated from the knowledge of the decomposition of the internal standard. (6) E. Tschuikow-Roux, W. J. Quiring, and J. M. Simmie, J . Phys. Chem., 74, 2449 (1970). (7) E. Tschuikow-Roux and W. J. Quiring, ibid., 7 5 , 295 (1971). (8) G. E. Millward, R. Hartig, and E. Tschuikow-Roux, Chem. Commun., 465 (1971). (9) G. E. Millward, R. Hartig, and E. Tschuikow-Roux, J. Phy8. Chem., 75, 3195 (1971). (10) E. Tschuikow-Roux, G. E. Millward, and W. J. Quiring, ibid., 75, 3493 (1971).
293
SHOCK WAVE nECOMPOSITION OF 1,1,1,2-TETRAFLUOROETHANE Table I : Experimental Results -Shook Mach. No.w 1 1 WZI
2.173 2.181 2.231 2.245 2.245 2.255 2.245 2.282 2.272 2.283 2.293 2.303 2.288 2.314 2.327 2.314 2.336 2.323 2.330 2.345 2.367 2.350 2.365 2.413 2.413 2.418 a
1.300 1.282 1.303 1.289 1.306 1.294 1.302 1.290 1.303 1.332 1.323 1.314 1.305 1.308 1.306 1.329 1.33'7 1.320 1.350 1.339 1.346 1.348 1.356 1.351 1.357 1.365
Ps,
1'6,
t.
Torr
"K
psec
R1
2851 2829 3033 3043 3088 3087 3077 3161 3164 3269 3282 3288 3219 3309 3346 3364 3458 3367 3473 3493 3583 3533 3602 3751 3768 3810
1177 1174 1224 1228 1237 1238 1235 1253 1251 1272 1285 1278 1271 1293 1296 1304 1327 1306 1325 1335 1353 1349 1361 1394 1397 1411
580 680 681 674 688 765 824 893 764 681 865 873 779 835 864 897 916 948 888 840 857 840 894 836 914 924
0.0010 0,0013 0.0039 0.0054 0.0058 0.0080 0.0086 0.0125 0.0137 0.0136 0.0197 0.0226 0.0201 0.0287 0.0310 0.0390 0.0478 0.0503 0.0484 0.0669 0.0897 0.0931 0.116 0,216 0.290 0.478
,-----
Product ratiosa RZ Ra
...
...
... ... ...
...
...
... ... ...
... ...
... ... ...
... ... ... ...
...
R4
...
... ...
... ... ...
...
... ... ... ...
0.0138 0,0090
0.0018 0.0026
0.0010
0.0170 0.0129 0.0036 0.0386 0.0300 0.0455 0.0598 0.0944 0.0713 0.111 0.238 0.369 0.546
0.0024 0.0033 0.0027 0.0039 0.0030 0.0035 0,0044 0.0085 0.0067 0.0085 0.0141 0.0196 0.0210
0.0029 0.0038 0.0037 0.0049 0.0040 0.0052 0.0063 0.0099 0.0125 0.0168 0.0410 0.0650 0.0860
...
...
...
kuc?
kl,
kua'9b
k',
sec -1
sec-1
sec-1
seo-1
1.71 1.97 5.84 7.92 8.44 10.5 10.4 13.9 17.8 19.8 22.5 25.5 25.6 33.7 36.0 42.2 50.2 51.0 52.4 75.5 96.1 103 118 215 251 372
1.52 1.83 5.27 7.30 7.73 9.58 9.61 12.9 16.3 18.2 20.8 23.2 23.5 30.6 32.0 38.7 47.0 47.7 48.7 69.7 89.8 94.9 109 200 237 348
...
...
... ... ... ... ... ...
*..
...
... ... ...
.*.
...
... ... ... ...
32.4 34.7
30.2 31.8
.,.
*.. 49.9 50.5 67.3 73.6 79.9 86.5 122 164 168 201 394 486 67 1
e . .
45.7 46.5 62.1 69.2 75.2 80.9 113 152 158 188 370 460 630
R; = [Xi]/[C:H9CF8] where Xi
mentally.
= CHFCF2, CHF,, CHFCH2, CFzCFzfor i = 1, 2, 3,4, respectively, and Ri is determined experiRate constants uncorrected for the finite cooling rate of the rarefaction fan.
A dilute reaction mixture (1% 1,1,1,2-C8HzF4 in Ar) was prepared in a large stainless steel tank and allowed to mix thoroughly before use. The design and operation of the modified SPST has been fully described previously.11*12After evacuation the tube was filled with pure argon, while the ball valve was filled to the same pressure with the reaction mixture. Using amplified signals from two highfrequency pressure transducers, the incident and reflected shock. transit times were registered on microsecond counters. Also, the wave history near the end plate was obtained from a photographic record of the oscilloscopetrace. Following each shock a sample of the fully mixed gases was withdrawn and subjected to analysis using a gas chromatograph (Varian, Model 1740). The mixtures were separated on a 12-ft silica gel column at 125" with helium as carrier gas (30 cms min-*). Quantitative identification of the product/reactant ratios was obtained by comparison with standard mixtures prepared for calibration purposes. The conversions ranged from -O.l?& at 1170°K to 46% a t 1410°K.
Results The reflected shock wave parameters P g and T6 given in Table I were calculated from the incident and reflected shock wave Mach numbers WII and WZI.
The reaction dwell time, t, was determined by a previously described method.ll The initial cooling rate of the rarefaction fan, m = (dTs/dt){, was derived from the oscillographic pressure record. The values of -m varied between 4 X lo5 and 1 X lo6 OK sec-'. Between 1170 and 13OO0I E,(14). Finally, the self-consistent electrostatic (semi-ion pair) model of Benson and Haugenso predicts the same value of E, for reactions 14 and 15. This is because account is not taken of the differences in the transition state polarizabilities and bond lengths in the reverse addition reactions of (14) and (15). It is interesting to compare some relative rate constants for H F elimination from CH&HF2 and CHaCF3 obtained from several laboratories (data are taken from ref 2, 5b, 6, and 7, respectively). For CHaCHFz at 1300°K the ratio of rate constants is 0.25:0.13: 1
k1)
=
'/2kZ(l
+ k3/k-z)
- 1016.7*o.6 exp[- (92,300 f 2900)/RT] sec-' where the error limits are standard deviations. The ratio ka/k-2 has not been evaluated for CHzF and CFa radicals. However, we assume the ratio to be 0.2 and temperature independent by andogy to the disproportionation/combination ratio of CHF2 radicals which is well established.'* Therefore
kz (sec-l) = 101a.g+o.6 exp[-(92,300
f
2900)/RT]
This provides us with the first experimental determination of the C-C bond dissociation energy in CHzFCFa. The magnitude of the preexponential factor is consistent with those determined for similar Table 111: C-C Bond Dissociation Energies of Some Fluorocarbon Analogs of Ethane
Fluorocarbon
CHa-CH3 CH2F-CH2F CHs-CFg CHzF-CFa CHFr-CHFz CHFz-CF3 CFa-CFs
C-C bond dissociation energy, koa1 mol-1
88.0 f 2,O
Temp range, OK
298 89.6 298 298 99.7 .& 2 . 0 9 2 . 3 & 2 . 9 1280-1410 9 1 . 4 f.3.7 1320-1450 9 3 . 5 f 5 . 2 1300-1450 9 4 . 4 f.4.0 1300-1600 96.5i. 1.0 298
Method
Reference
a
C
b b
32
SPST SPST SPST SPST b
d
This work 9 10
e 32
a Static pyrolysis of CH3CH3. Calculated from known thermochemical data. M. C. Lin and M. H. Back, Can. J . Chem., 44, 505 (1966). See Table 11, footnote b. e E. Tschuikow-Roux, J. C h m . Phys., 43,2251 (1965).
(28) H. W.Chang and D. W. Setser, J . Amer. Chem. Soc., 91, 7648 (1969). (29) J. R.Lacher and H. A. Skinner, J . Chem. SOC. A , 1034 (1968). (30) 8. W.Benson and G. R. Haugen, J . Amer. Chem. Soc., 87, 4036 (1965). (31) E. Tsohuikow-Roux, J. M. Simmie, and W. J. Quiring, Astronaut. Acta, 15, 511 (1970).
The Journal of Physical Chemistry, Vol. 76, No. 5, 1979
298
J. B. TELLINGHUISEN, C. A. WINKLER,AND L. F. PHILLIPS
reactions. It should be noted that the use of the assumptions regarding k3/k-Z leads to k l / k z 'v 1 at 1420°K, and kl/kz < 1 above this temperature. Table 111gives recently determined values of the C-C bond dissociation energy in ethane and several of the fluoroethanes. The interesting feature is the relatively high value of D (CHa-CF3) calculated from known thermochemical data.32 It is difficult to fault this result, and CNDO calculationsa3of charge distributions, in CHaCFa, indicate that it may be correct. I n the past it had been generally that the highly electronegative character of the fluorine atom led to a diminishing positive character in a carbon atom chain, i e . , Fa- + C*t -+ C**+. However, the CNDO calculations suggest that the charge distribution should decay as follows: Fa- + C*+ + C*". This means that the negative character of the @-carbon atom
increases in going from CH~CHZFto CH3CFa and decreases in going from CHICFI to CHF2CF3. The maximum charge separation along the C-C bond exhibited by CHsCF3 is in accord with a strong C-C bond. The results tabulated are in reasonable agreement with the above hypothesis, but further experimental determinations of D(CHI-CH2F), D(CH3CHFZ),and D(CH3-CF3) are needed to substantiate it. Acknowledgments. We thank Dr. R. F. Hein of the Du Pont de Nemours Co. for a high purity sample of 1,1,1,2-CzHzF4. We also thank Mr. K. Maltman for stimulating discussions. (32) J. W. Coomber and E. Whittle, Trans. Faraday SOC., 63, 1394 (1967). (33) J. A. Pople and D. L. Beveridge, "Approximate Molecular Orbital Theory," McGraw-Hill, New York, N. Y., 1970, p 119.
Quantum Yields in the 58.4-nm Photolyses of Ammonia and Water by J. B. Tellinghuisen, C. A. Winkler, and L. F. Phillips* Chemistry Department, University of Canterbury, Christchurch, New Zealand
(Received September 19, 1971)
Publication costs borne completely by The Journal of Physical Chemistry
Quantum yields have been determined by mass spectrometric analysis and by measurements of the absorption of 58.4-nm radiation by the photolysis products. At pressures where ion-electron recombination is predominantly homogeneous we find the quantum yields of H2 to be 0.93 i 0.15 from NH3 and 2.1 f. 0.5 from HzO. Yields from HzO are sensitive to wall effects. The results are discussed in terms of known photofragmentation patterns and ion-molecule reactions, together with less well known dissociative recombination processes and neutral radical reactions. To account for the observed H2yield from H2O (though not for t h a t from "8) it is necessary to assume that most of the electron-ion recombination occurs by reactions of t h e type Haof e- + H2 0 H. The relative effects of 58.4-nm ionizing photolysis and radiolysis are and COZ. compared for H20, "3,
+
+ +
Introduction We have recently described studies of processes initiated by 58.4-nm radiation in gaseous COZ and other small molecules.1-3 I n a continuation of this work we now report product yields for the ionizing photolysis of NH3 and HzO. As before, the results are discussed in terms of known photofragmentation patterns and ion-molecule reactions, together with less well understood dissociative recombination processes and reactions of neutral radicals. Studies of the y radiolysis of water and ammonia4+ have shown that whereas NH3 is destroyed quite efficiently (GHa = 4.42 f 0.1, GNa = 1.45 A= 0.074),HzO is extremely resistant to decomposition (GHa ct! 0.0076). It is therefore of interest to compare the behavior of The Journal of Physical Chemistry, Vol. 76,No. 3,197.9
these two molecules in ionizing photolysis at 58.4 nm. We find that with 58.4-nm (21.2 eV) radiation both are decomposed, the quantum yield of Hz being greater Some possible reasons for from HzO than from "3. these differences are given in the Discussion. (1) 8. W. Bennett, J. B . Tellinghuisen, and L. F. Phillips, J . Phys. Chem., 75, 719 (1971). (2) J. B. Tellinghuisen, S. W. Bennett, C . A. Winkler, and L. F. Phillips, J. Phys. Chem., in press. (3) C. A. Winkler, J. B . Tellinghuisen, and L. F. Phillips, Trans. Faraday Soc., in press. (4) J. A. Eyre and D. Smithies, Trans. Faraday SOC.,66,2199 (1970). (5) A. R. Anderson, B. Knight, and J. A. Winter, Trans. Faraday Soc., 62, 359 (1966). (6) (a) F. T. Jones and T. J. Sworski, Trans. Faraday SOC.,63, 2411 (1967); (b) F. T . Jones, T. J. Sworski, and J. M.Williams, ibid., 63,2426 (1967).
