T H E J O U R N A L OF
PHYSICAL CHEMISTRY Registered i n U.S. Patent Office
0 Copyright, 1980, by t h e American Chemical Society
VOLUME 84, NUMBER 9
MAY 1,1980
Resonance Absorption Measurements of Atom Concentrations in Reacting Gas Mixtures. 3. Pyrolysis of CD4 behind Shock Waves Chi-Chang Chiang, James A. Baker, and Gordon B. Skinner" Department of Chemistry, Wright State University, Dayton, Ohio 45435 (Received August 22, 1979; Revised Manuscript Received January 2 I, 1980) Publication costs assisted by U.S.Department of Energy
Dilute mixtures (10-100 ppm) of CD4in argon were pyrolyzed behind reflected shock waves at temperatures of 1780-2440 K and total pressures of 2-3 atm. Progress of the reaction was followed by analysis for D atoms by using resonance absorption spectroscopy. Rate constants were determined as follows: CD4 CD3 + D, kl = 1.4 X 10'l exp(-81000/RT) s-'; CD4 + Ar CD3 + D + Ar, K1, = 2.1 X 1OI6 exp(-84900/RT) mol-I cm3 s-'; D + CD4 D2+ CD3,hz = 2.1 X 1015exp(-22300/RT) mol-' em3s-l, where activation energies are in calories. These data have been correlated with similar data by Roth and Just on pyrolysis of CH4,with laser schlieren data by Tabayashi and Bauer on pyrolysis of CHI and CD4,and with other data on CHI pyrolysis, via RRKM and activated complex theory calculations.
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Introduction Pyrolysis of methane has been studied by several inv e s t i g a t o r ~ ,most ~ - ~ of the recent studies involving shock tubes. The first group of studies involved single-pulse shock tubes in the 1300-1900-K range'-3 whereas a more recent group has used spectro~copic"~ and laser schlieren8 techniques at higher temperatures (1800-2800 K). Only the study of Tabayashi and Bauer (TB)8included a group of measurements on pyrolysis of CD4. The measurements of Roth and Just (RJ)6 used the resonance absorption technique of this work. Chen, Back, and Back (CBB)ghave carried out the one recent study using a static reactor in the 1000-1100-K range. Experimental Section Our apparatus and techniques have been described in an earlier paper.1° Briefly, we used a stainless steel shock tube with a test section 7.5 cm in diameter and 4.5 m long. Concentrations of D atoms were measured by resonance absorption behind the reflected shock wave. The microwave discharge lamp used to produce the Lyman-a radiation had been characterized in terms of emitted line shape and also empirically calibrated by the method of Appel and 0022-3654/80/2084-0939$0 1.OO/O
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Appleton.ll Temperatures were calculated from the incident shock speed while the reflected shock pressure was also monitored. Since the observed pressures were within 2-370 of those calculated and did not vary more than 2-370 during the measurements, we have estimated that the uncertainty in temperature is no more than 170 , or 20 K. We have estimated the effect of wall cooling on our experiments by using methods described p r e v i ~ u s l y . ~ ~ J ~ In typical experiments, the observed D concentrations are calculated to be 2-3% lower than would be the case without wall cooling. The effect is smaller than in our single-pulse experiments for several reasons: the diameter of the tube is greater; the measurements were made closer to the end plate; the experimental times were shorter; and the measurements were made linearly across the tube rather than on the entire volume. We did not make corrections for either our CalibrationslO or these experiments, so the cancelation of error should reduce the overall effect to 1-270. Gas samples were made from CD4obtained from Merck, Sharp and Dohme of Canada and Airco Research Grade argon (2 ppm. total impurity, with less than 0.5 ppm hy0 1980 American Chemical Society
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The Journal of Physical Chemistry, Vol. 84, No. 9, 1980
Chlang et al.
drocarbons reported as methane).
TABLE I: Experimental Values of Rate Constants from Absorption Profiles
Results Because methane absorbs deuterium (and hydrogen) Lyman-a radiation rather strongly, all of our CD-argon mixtures were quite dilute. We used 10, 20, 50, and 100 ppm of CD4, all of these mixtures being made by dilution of more concentrated mixtures. In each experiment the Lyman-a intensity dropped suddenly when the reflected shock wave passed the observation station, because of the absorption due to the CD,. This sudden change was followed by slower changes due to the appearance of D atoms; thus the two effects could be distinguished. The CD4in the 100-ppm mixtures adsorbed about 20% of the radiation at our experimental pressure of 3 atm, a figure in reasonable agreement with data reported by Ditchburn14 for CH4 at room temperature. We also found, as we had earlier, that a few percent of the radiation from the lamp was not absorbed even by a high concentration of D atoms and therefore was either H Lyman-a or some other type of radiation. A correction was made for this radiation by an adjustment in the base line of our light intensity curve. We have assumed the mechanism of CD4decomposition that has been established for CH4 by recent w ~ r k : ~ ! ' J ~ J ~ CD4 -+ CD3 + D
(1)
followed by D
+ CD4
+
CD3
+ D2
(2)
Model calculations show that under our low concentration conditions these are the only reactions of significance, the radical combination process 2CD3 --* C @ 3
t 3)
and subsequent steps leading to CzD4 and CzD2occurring too slowly to affect the results. After an initial period, a steady state is reached in which D atoms produced in reaction 1 are used by reaction 2, so that [Dl = kl/k2 where kl and kz are rate constants for the reactions. In analyzing the absorption curves, we assumed that the absorption coefficient of CD3was the same as that of CD4, so that, in the absence of reaction products other than CD3 and D, the molecular absorption remained constant throughout our experiments. Since only a small fraction of the CD4 dissociated in our higher concentration experiments, this assumption would not lead to a significant error unless the absorption coefficient of CD3 is much larger than that of CD4 (an unlikely possibility). At our experimental conditions, reaction 1 is approaching the second-order limit, so that in a large excess of argon it may be written CD4
+ Ar
-
CD3
+ D + Ar
(la)
and kl = kl,[Ar]. In principle, kl may be obtained from the initial slope of a graph of [D] vs. time, whereas k2 may be found from kl and the steady-state value of [D]. In practical terms, the steady-state value of [D] was often too large to be measured accurately; therefore we did not obtain k2 from all of our experiments. Data were taken by using two of the light sources described in ref 17: source A, with a mixture of 1% D2 in helium at 3.0 torr in the discharge, and source B, with a mixture of 0.170D2 in helium at 3.0 torr. Source A is less sensitive than source
T, K
P , , atm
k,,
k,,, mol-' cm3ss-'
10 ppm of CD, in Ar, Light Source 3.0 torr of 1%D, in He 1988 3.3 1.432 6.936 2088 3.3 2.532 1.337 2146 3.2 4.132 2.237 2146 3.2 5.932 3.237 2177 3.1 8.032 4.637 2250 3.1 1.333 7.537 2326 3.1 2.633 1.638 2372 3.0 2.533 1.738 2372 3.0 2.233 1.539 2444 2.9 5.433 3.739
k , , mol-' cm3s-'
A, 4.5312 1.1313 2,0313 7.7312 1.8313 1.8313 1.7313 1.5313 1.1313 2.5313
10 ppm of CD, in Ar, Light Source B, 3.0 torr of 0.1%D, in He 1847 3.3 6.331 2.936 1903 3.3 1.032 4.836 1944 3.2 1.532 7.536 1987 3.1 2.232 1.237 2032 3.0 4.632 2.637 2146 3.1 8.332 4.737 2146 2.9 1.133 6.837 2218 2.9 1.533 9.637 20 ppm of CD, in Ar, Light Source B, 3.0 torr of 0.1%D, in He 1847 3.4 3.131 1.436 1911 3.3 8.831 4.236 1945 3.2 1.432 6.836 2014 3.2 2.632 1.337 2079 3.1 4.332 2.437 2167 3.1 8.532 4.937 2229 3.1 2.033 1.238 50 ppm of CD, in Ar, Light Source B, 3.0 torr of 0.1%D, in He 1778 3.1 1.631 7.835 1878 3.3 4.631 2.136 3.9312 1944 2.8 8.531 4.836 7.5312 2032 3.3 2.432 1.237 2069 3.1 2.932 1.637 2088 3.3 3.432 1.837 2107 3.0 2.932 2.337 2127 3.0 4.832 2.837 2166 3.1 6.832 3.937 100 ppm of CD, in Ar, Light Source B, 3.0 torr of 0.1% D, in He 1786 3.5 1.331 5.435 1823 3.4 2.031 8.635 5.7312 1847 3.4 3.131 1.436 4.5312 1971 3.3 4.131 1.936 5.0312 1962 3.4 9.831 4.736 1962 3.2 1.132 5.436 2033 3.1 2.232 1.237 2041 3.3 2.432 1.237 2127 3.2 4.032 2.237
B, so that measurements could be made at higher deuterium concentrations. One set of experimental data is shown in Figure 1. The rate constants are listed in Table I. Arrhenius equations were calculated by least squares for each of the reactions. These are kl = 2.5 X lo1'. exp(-74600/RT) s-l; kl, = 5.4 X 1015exp(-80300/RT) mol-' cm3 s-1; kz = 2.1 x 1015 exp(-223OO/RT) mol-l cm3 s-l, where the activation energies are in calories. The standard deviations of the points from the equations are 0.12 for log hl and 0.14 for log k2. Discussion The unimolecular dissociation of methane has been treated by Placzek, Rabinovitch, Whitten, and Tschuikow-Roux,18using the RRKM method. Placzek's Model
Pyrolysis of CD, behind Shock Waves
12
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The Journal of Physical Chemistry, Vol. 84, No. 9, 1980 941
Steady Value ---__ oaQv" -6.5
0 0
10/
l
0
I 100
O
o
I 200
I
300
TIME, microseconds
1.5
Flgure 1. Measured D concentrations for 50 ppm of CD, in Ar, 1944 K, 2.8 atm P,, by using light source B.
4 was found by Chen, Back, and Backg to correlate their falloff data at 1000-1100 K very well and also to match the single-pulse shock tube data and some data on the reverse reaction at lower temperatures. However, Chen, Back, and Back did not obtain agreement with the data of Hartig, Troe, and Wagner at higher temperatures. It is now clear from the additional shock tube data at high temperaturesw and also from our own work that rate constants for CHI and CD4dissociation at pressures of a few atmospheres with high argon dilution are definitely lower than a simple extrapolation of the lower-temperature data would suggest. The reason is probably related to the increasing energy demands of the reaction at high temperature and the limited capability of the argon to provide that energy by collision. The available data are not sufficient to entirely clarify the energy-transfer processes, but it is possible to interpret them with a simple energy-transfer model. Tardy and Rabinovitchlg showed that, for several molecules, a "quasiuniversal" curve relates the collisional efficiency, p, with the quantity ( M )(/E + ) ,where ( M )is the average amount of energy transferred per collision, and (E+)is the average excess energy of the reacting molecules in the low pressure limit. The quantity /3 can be understood as the ratio of the number of CHI-CHI collisions to the number of Ar-CH4 collisions needed to produce a certain number of reactive molecules. From a limited number of studies it appears that the average energy transfer between an inert gas molecule and a molecule comparable in complexity to methane is about 1kcal/mo1,20v21and there is an indication that the amount of energy transferred decreases with increasing temperature.22 The quantity (E') is not strongly dependent on the model of the activated complex. We obtained values ranging from 1 kcal/mol at 500 K to 3 kcal/mol at 1300 K to 7 kcal/mol at 2500 K. With a stepladder energytransfer model, these lead to ,& of 0.25 at 500 K, 0.04 at 1300 K, and 0.01 at 2500 K. Similarly small values of /3 have been obtained by using a different method of calculation for related reactions in the same temperature range by Luther and T r ~ e . ~ ~ With these low values of 0,the effect of a few percent of methane in the mixture (with efficiency of 1) can be substantial. If we write a reaction
CD4
+ M-
3
CD3 + D
+M
(1b)
2.0
2.5
0
Log P ,torr
Flgure 2. Data of Chen, Back, and Back' for the dissociation of CH, at 1038 K: (0)experimental points: (--) RRKM calculation, Chen, Back, and Back model; (--) RRKM calculation, Hase model. Units of k , , s-1.
where [MI is the total concentration of molecules and x is the mole fraction of CD4,then in the low-pressure limit klb
= kla(1 - x
+ x/P)
and if x = 0.1 and p = 0.01, then k l b = 10.9kla, a very substantial change. Such changes cany over to calculations in first-order form, although they become smaller as one moves from the second-order region into the falloff region of higher pressure and/or lower temperature. We repeated Chen, Back, and Back's calculations with two minor modifications: (a) We used the Bayer and Swinehart algorithmZ4J5for evaluating densities and numbers of states. This change caused a reduction in the calculated rate constants by 10-15% at 1100-1200 K. (b) We decreased the energy of reaction at T = 0 (E,) from 103.0 to 102.7 kcal, a value that is in better agreement with that recently reported by Baghal-Vayjooee, Colussi, and BensonqZ6This raised the calculated rate constants by about the same amount. The new calculations matched the data of Chen, Back, and Back quite well, as shown in Figure 2. We then applied this model with allowance for the lower energy transfer of argon to the shock tube data by using a stepladder energy-transfer model with ( A E ) = 1.0 kcal/mol at all temperatures (results shown in Figures 3-5). We should note that, where literature data were given in second-order form, they have been converted to first order by multiplying by [MI. If there was a systematic change of [MI with temperature, the graph reflects this fact. The effect is particularly noticeable for the data of Tabayashi and Bauer shown in Figure 4, in which [MI decreased by a factor of more than 2 from the lowest to the highest temperatures. This effect was of course included in calculations made to simulate the data. For CD4we took the ratio of the vibrational frequencies of the complex to the frequencies of the corresponding vibrations in the molecule to be the same as for CHI, leading to values of 2150 (l), 400 (l), 2380 (2), 1050 (2), and 205 (2) cm-', where the numbers in parentheses are the multiplicities, while Eo is increased to 107.1 kcal by the difference in the zero-point energies of the vibrations. The ratio of the moment of inertia of the complex to that
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The Journal of Physical Chemistty, Vol. 84,No. 9, 1980
I
I
I
4.5
5.0
5.5
Chiang et al.
1
I
I
4.0
4.5
1 0 ~,K1 ~
Flgure 3. Comparison of experiment and calculation for first-order dissociation constant of CD4. Experimental points: (A) 10 ppm of CD, source A; (0)10 ppm of CD4, source B; (V) 20 ppm of CD4, source B; (0) 50 ppm, source B; (0) 100 ppm of CD,, source B. (-) Least-squares llne through data; (- -) RRKM calculation, Chen, Back, and Back model; (--) RRKM calculation, Hase model. Units of kl,
I
5.0
Log P I torr Flgure 5. Comparisons of falloff data for CH, dissociation at 2200 K: (-) experimental (--) RRKM calculation, Chen, Back, and Back model; (--) RRKM calculation, Hase model. Units of k,,s-l.
S-1. 0
1
I
I
I
2
1
I
I
3
Log P, torr
-
Figure 6. Comparison of experiment and calculation for reactlon H f CH, CH4. Experimental points: (0)ref 27; (X) ref 28; (A)ref 29; (V)ref 30; (A)ref 31; (0) ref 31. (--) RRKM calculation, Chen, Back, and Back model; (--) RRKM calculation, Hase model. Units of k-', mol-' cm3 s-l.
well. This suggests that a more rigid model would be Howappropriate, coupled with a larger value of ever, the above calculation, combined with the equilibrium constant for the reaction, gives a low value, by a factor of 6, for k, for H + CH, CH4, for which a value is given by Cheng and Yeh.27 This is shown, along with other data for the combination reaction at lower pressures, in Figure 6. In view of the above considerations, we made a second calculation by using the approach suggested by Hase33*34 in which minimum-state density is used to define the activated complex. An important result of this approach is that Eo becomes less than the thermodynamic energy of reaction. In our calculation Eo became 101.4 kcal for CH4, a decrease of 1.3 kcal. At the same time we increased the bending frequencies of the complex from 580 to 850 cm-l, and from 280 to 410 cm-l, respectively, and decreased Z*/I to 2.40. For CD4, Eo became 105.8 kcal, with bending frequencies of 590 and 300 cm-l, and Is/Z became 2.31. To match the data of Chen, Back, and Back we used (AE) = 1.27 kcal, again with a stepladder model. The results
(a).
10'1~
,K
Flgure 4. Comparison of shock tube data on first-order rate constant for methane dissociation with RRKM calculations. HPSP, average RJ, pressures 3.2 atm, 5% CH, in argon average c~ncentration;~ average pressure 2 atm, 200 ppm or less of CH., in argon;' SR, 5 atm, 1 2 % CH, In argon;' TB, 0.4 to 1.6 atm, 10 and 20% CH4 in argon;' (-) data; (--) RRKM calculation, Chen, Back, and Back model, and also Hase model for HPSP, SR,and TB; (--) Hase model for RJ. Units of k,,s-'.
of the molecule, Z*/Z, decreased from 3.34 for methane to 3.22 for CD4. I t can be seen that the agreement between the experimental data and the calculations is moderately close. Our calculations came out higher than experimental values for two of the sets of CHI data shown in Figure 3 and lower than experimental values for the other two. However, in Figure 5, it is clear that the slope of the calculated falloff curve does not match that of the experimental one very
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The Journal of Physical Chemistfy, Vol. 84, No. 9, 1980 943
Pyrolysis of CD, behind Shock Waves I
I
I
I
5.5
6.0
13.!
N Y
13.( J
12.! 1
4.5
5.0
+
,K
-
10?T
+
-
Figure 7. Data on reactions H CH, H2 CH3 and D 4- CD4 D2 CD, at high temperatures. Experimental data for D: (0)10 ppm of CD4 in Ar, source A; (A)50 ppm of CD4 in Ar, source 8; (0) 100 ppm of CD4 in Ar, source B. (-) Least-squares lines through data: CBS, this research; RJ, ref 6,for H 4- CH,; FJ, ref 35,for H 4- CH,. (--) ACT calculations: H, for H 4- CH4; D, for D 4- CD4. Units of /cp, mol-' cm3 s-'.
+
of these calculations are also shown in Figures 2-6. The differences between the two sets of calculations are quite small, but there seems to be improved agreement of the Hase calculation with the data of Roth and Just and with the room temperature data. We should point out that in Figure 6 the pressures shown for the low-pressure data have been adjusted by multiplication by the /3 appropriate to the Hase calculations, since all were obtained with a weak collider carrier gas (usually He). At 308 K, /3 is 0.63. For the Chen, Back, and Back model calculation, with a lower ( AE) assumed, /3 is 0.50. Accordingly, the pressures of the low-pressure data should be shifted 0.1 unit of log P to the left for comparison with the CBB curve. As T B point out, the agreement of their data (when plotted in second-orderform) with those of RJ implies that /3 for argon should be close to 1,since otherwise the use of 10-20% methane by T B should lead to higher rate constants than RJ would obtain from mixtures with less than 0.03% methane. This result is hard to accept, partly because of the other evidence that /3 for argon becomes small at high temperatures and partly because we have had no success in developing an RRKM model that matches all of the data nearly as well as the two we have described if we take /3 to be 1. We suspect that a resolution lies in a number of small adjustments: /3 may be 0.03 rather than 0.01 at 2500 K; the RJ data may be a little too high, whereas the T B data may be a little too low. We consider, on the basis of comparisons of our experimental results with those for methane and with our calculations, that the experimental activation energies given above are probably on the low side. We recommend instead Arrhenius equations based on our Hase calculations, shown in Figure 3. These are k l = 1.4 X loll. exp(-81000/RT) s-l and kl, = 2.1 X 10l6exp(-84900/RT) mol-I cm3 5-l for the temperature range 1800-2500 K. Our analysis of the data cannot distinguish between different models of energy transfer. If the Hase model calculations are combined with an exponential energytransfer model with (AE)of 1.0 kcal/mol, the result is almost the same as the stepladder calculation we have
shown. On the other hand, introduction of the energytransfer coefficient /3 has provided a clear, semiquantitative explanation of why the rate constants for CHI and CD, decomposition in the high-temperature shock tube experiments are lower than might otherwise be expected. We note that TB report RRKM calculations which give klH/klD i= 0.5 at 2400 K. However, this calculation assumed an Eo difference of only 1.7 kcal, based on experimental measurements, whereas our calculation of the zero-point energy differences leads to a difference of 4.4 kcal. Since this difference can be calculated precisely once models of the complexes are chosen, it should probably be used in self-consistent calculations. At 2500 K we obtain klH/klD = 2.8 at the high-pressure limit and 1.0 at the low-pressure limit. Under our experimental conditions (and also those of T B and RJ)we calculate klH/klD = 1.5. With a ratio this close to 1, it is not surprising that T B did not observe a significant isotope effect in their experiments. D CD4 CD, + D2. This reaction has not been studied at high temperatures, but the hydrogen analogue was studied by Roth and Just6 and also by Fenimore and Jones (FJ),l in flames at 1225-1800 K. There have also been several low-temperature studies of the reaction of H with CH4 which have been summarized by Clark and Dove.32 The latter authors made an activated complex theory calculation which correlated the low-temperature data and Fenimore and Jones' data quite well. We have repeated their calculation at high temperatures and also made comparable calculations for D CD4. These curves are shown in Figure 7 along with our data and with the curves determined by Fenimore and Jones and Roth and Just. Again, the agreement between observation and calculation is quite good. (It may be noted that the scale of Figure 7 is larger than that of Figure 4, the standard deviation of the points from a smooth curve being essentially the same for both.) For these reactions the isotope effect approaches (2.014/ 1.008)1/2(essentially 2ll2) at high temperatures and is already 1.50 at 2500 K. The calculated Arrhenius activation energy in the 2000-2400 K range is 19.7 kcal for H CH4and 20.2 kcal for D CD4,the latter value being in good agreement with our experimentalvalue of 22.3 kcal.
+
-+
+
+
+
Acknowledgment. This research was supported by the United States Department of Energy under Contract EY76-S-02-2944. The authors thank Mr. John Dryden for assistance with the figures.
References and Notes (1) G.B. Skinner and R. A. Ruehrwein, J. phys. Chem.,63, 1736 (1959). (2) V. Kevorkian, C. E. Heath, and M. Boudatt, J. Phy.9. Chem., 64, 964 (1960). (3) G. I. Kozlov and V. G. Knorre, Combust. Flame, 6, 253 (1960). (4) H. B. Palmer and T. J. Hirt, J. Phys. Chem., 67, 709 (1963).
(5) R. Hartig, J. Troe, and H. G. Wagner, Symp. (Int.)Combust., [ h o c . ] , 13. 147 11971). P. Roth and T. Just, Ber. Busenges. Phys. Chem., 79, 682 (1975). W. M. Heffington, G. E. Parks, K. G. P. Sulzmann, and S.S. Penner, Symp. (Int.) Combust., [Proc.], 16, 997 (1977). K. Tabayashi and S. H. Bauer, Combust. Flame, 34, 63 (1979). C. 4. Chen, M. H. Back, and R. A. Back, Can. J . Chem., 53, 3580
(1975). (10) C. 4. Chiang, A. Lifshitz, G. B. Skinner, and D. R. Wood, J . Chem.
Phys., 70, 5614 (1979). (11) D. Appel and J. P. Appleton, Symp. (Int.) Combust., [Proc.], 75, 701 (1974). (12) G.B.'Skinner, R. C. Sweet, and S. K. Davis, J. Phys. Chem., 75, l(1971). (13)G.6.Skinner, Int. J. Chem. Kinet., 9, 863 (1977). (14) R. W. Ditchburn, Proc. R . SOC. London, Ser. A , 229, 44 (1955). (15) T. Yano and K. Kuratani, Bull. Chem. Soc. Jpn., 41, 799 (1968). (16)T. Yano, Bull. Chem. Soc. Jpn., 46, 1619 (1973). (17) A. Lifshitz, G.B. Skinner, and D. R. Wood, J. Chem. Phys., 70, 5607 (1979).
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(18) D. W. Placzek, 8. S.Rabinovitch, G. 2. Whitten, and E. TschuikowRoux, J. Chem. Phys., 43, 4071 (1965). (19) (a) D. C. Tardy and B. S.Rabinovitch, J. Phys. Chem., 74, 3151 (1970); (b) ibid., 48, 1282 (1968). (20) Y. N. Lin and B. S. Rabinovitch, J. Phys. Chem., 74, 3151 (1970). (21) F. M. Wang and B. S. Rabinovitch, J. Phys. Chem., 78, 863 (1974). (22) I. E. Klein, B. S. Rabinovitch, and K. H. Jung, J . Chem. Phys., 67, 3833 (1977). (23) K. Luther and J. Troe, Symp. (Int.)Combust., [Prm.],17, 535 (1976). (24) T. Bayer and D. F. Swinehart, Commun. ACM, 16, 379 (1973). (25) S. E. Stein and B. S.Rabinovitch. J. Chem. phvs.. 58. 2438 11973). (26) M. H. BaghaCVayjooee,A. J. Colussi, and S.'W.' Benson, j . A i , Chem. Soc., 100, 3214 (1978). (27) J. -T. Cheng and C. Yeh, J . Phys. Chem., 61, 1982 (1977).
(28) J. M. Brown, P. B. Coates, and B. A. Thrush, Chem. Commun., 843 (1966). (29) A. F. Dodonov, G. K. Lavrovskaya,and V. L. Tal'rose, Kinet. Katal., I O , 477 (1969). (30) M. P. Halstead, D. A. Leathard, R. M. Marshall, and J. H. Purnell, Proc. R . Sac. London, Ser. A , 316, 575 (1970). (31) L. Teng and W. E. Jones, J. Chem. Soc., Faraday Trans. 1 , 68, 1267 (1972). (32) J. V. Michael, D. T. Osborne, and G. N. Seuss, J . Chem. Phys., 58, 2800 (1973). (33) W. L. Hase, J . Chem. Phys., 57, 730 (1972). (34) W. L. Hase, J . Chem. Phys., 64, 2442 (1976). (35) C. P. Fenimoreand G. W. Jones, J. Phys. Chem., 65, 2200 (1961). (36) T. C. Clark and J. E. Dove, Can. J. Chem., 51, 2147 (1973).
Effects of Polar ,l Substituents ? in the Gas-Phase Pyrolysis of Ethyl Acetate Esters Gabriel Chuchani,* Ignacio Marfin, Jos6 A. HernHndez A.,' Alexandra Rotinov, German Fraile, Centro de QGmica, Instituto Venezolano de Investigaciones Ciensficas, Apartado 1827, Caracas, Venezuela
and David B. Bigley University Chemlcal Laboratory, Canterbury, Kent, CT2 7".
England (Received October 15, 1979)
Publication costs assisted by the Instituto Venezolano de Investigaciones Ciensficas, Caracas, Venezuela
The rates of the gas phase pyrolysis of six P-substituted ethyl acetates were studied in a static system over the temperature range 319-450 "C and the pressure range 63-207 mmHg. In seasoned vessels the reactions are homogeneous, follow a first-order rate law, and are unimolecular. The temperature dependence of the rate constants is given by the following Arrhenius equations for the compounds indicated: 2-(dimethy1amino)ethyl acetate, log k(s-l) = (13.90 f 0.30) - (220.4 f 3.8) kJ.mol-' (2.303RT)-'; 2-methoxyethyl acetate, log k(s-') = (12.04 f 0.24) - (203.7 f 2.9) kJemol-l (2.303RT)-'; 2-(methy1thio)ethyl acetate, log k(s-') = (11.27 f 0.39) (179.0 f 4.6) kJ-mol-' (2.303RT)-'; 2-chloroethyl acetate, log k(s-') = (12.14 f 0.66) - (202.0 f 8.4) kJ.mol-l (2.303RT)-'; 2-fluoroethyl acetate, log k(s-') = (12.68 f 0.60) - (211.2 f 7.1) kJ.mol-' (2.303RT)-'; 2-cyanoethyl acetate, log k ( d ) = (11.51 f 0.13) - (171.9 f 1.7) kJ.mol-l (2.303R7'-'. The effect of substituents in the gas-phase elimination of @-substitutedethyl acetates may be grouped in three types. The linear correlation of several -I electron-withdrawing groups along strong u bonds is presented and discussed. A small amount of anchimeric assistance is proposed in the pyrolysis of the 2-(methy1thio)ethylacetate. The experimental data are consistent with the transition state where the C,-0 bond polarization is the rate-determining process.
Introduction In recent years, the mechanism for the gas-phase pyrolysis of esters has been frequently described in terms of a semipolar six-membered cyclic transition ~ t a t erather ~-~ than a concerted,8i ~ n - p a i r , ~or, ' concerted ~ heterolysis'l state. The semipolar transition state of these reactions led to the successful use of structure-reactivity relationships, especially with substituents in the benzene ring remote from the reaction ~ e n t e r . ~ f ~ JVery ~ - ' *few aliphatic series have been analyzed in this way, but of particular relevance here is the gas-phase elimination of P-substituted ethyl acetates.lg Plotting the log k / k o vs. E, values (Taft equation)20gave an approximately straight line (r = 0.913), indicating that steric acceleration is important in determining the rate. The overall interpretation required electronic effects as well: (a) Alkyl groups and several polar substituents insulated by at least three methylene groups from the C,-0 bond enhance the rate of reaction. This must be due to a steric effect. (b) The few reported polar substituents-CH30, CH3CH20, and C6H60-at the 0 carbod0J6produce a significant decrease in rate relative to ethyl acetate. It was proposed that this deactivation was polar in origin, but further work was recommended. (c) Finally, the CH&O group in 2-acetylethyl acetate16 0022-3654/80/2084-0944$0 1.OO/O
dramatically increases its rate of elimination, as a result of the -M effect. We felt it useful to examine the effect of some other @-polarsubstituents in these acetates and to correlate it by a linear free energy relationship, if possible, in order to provide further understanding on the mechanism of ester pyrolysis. Moreover, since in chloride pyrolysis the neighboring CH,S and (CH&N groups accelerate the eliminations by a factor of 558 and 560 relative to ethyl chloride,21i22respectively, the purpose of this paper is also to extend the study to acetates. To this end we have used the series 2-methoxy-, 2-(dimethylamino)-,2-(methylthiol-, 2-chloro-, 2-fluoro-, and 2-cyanoethyl acetates. The first of these acetates has already been examined in the flow systemlo but only cursorily in a static system.16 Neighboring-group participation does not occur in solution reactions of these esters, except for the second-order rate constants for acid h y d r o l y s i ~ . ~ ~
Experimental Section 2-Methoxyethyl Acetate. Acetyl chloride was added to ethylene glycol monoethyl ether in pyridine.% The product (bp 144.5-145 "C) was distilled several times, and the fraction of 98.8% purity as determined by GLC (FFAP 0 1980 American Chemical Society
