D. K. Lewis, M . Sarr, and M . Keil
436
Cyclopentene Decomposition in Shock Waves David K. Lewis,* Michael Sarr, and Mark Keil Department of Chemistry, Coigate University, Hamiiton, New York 13346 (Received August 29, 1973) Publication costs assisted by Colgate University
The rate of thermal dehydrogenation of cyclopentene was studied in a single-pulse shock tube. Reactant mixtures consisted of 0.25 and 1.00% cyclopentene in argon, and total gas pressures were about 1 atm. Reaction temperatures were determined uia the comparative rate technique, using the decomposition of tert-butyl alcohol as the internal standard reaction. Over the temperature range 1020-1189°K the data support a first-order rate equation, with rate constants in units of sec-l given by log k(C5Hg C5H6 Hz) = 13.35 - [60.0 X 103/4.58T"K]. The reaction was essentially free of complications over this temperature range, but significant concentrations of side products were formed a t higher temperatures. A separate study of the products formed from cyclopentadiene decomposition suggests that these side products were produced primarily from cyclopentene. Possible mechanisms are discussed.
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Introduction This paper reports a shock tube study of the homogeneous gas-phase thermal reactions of cyclopentene. The dehydrogenation of cyclopentene to form cyclopentadiene has been studied extensively in static systems over the temperature range 690-800"K.1-4 The experimental conditions and results of three rate studies and a measurement of the equilibrium constant are summarized in Table I. The reaction was found to be unimolecular over the pressure range studied, and as can be seen from the table, there is good agreement on the Arrhenius parameters, despite the relatively narrow temperature range covered. However, there is some disagreement concerning the extent of ring fragmentation that takes place concurrently with dehydrogenation. Vanas and Walters,l who obtained their rate data from pressure measurements, believed that their products consisted of only cyclopentadiene and hydrogen up to at least 25% conversion of cyclopentene. Shoemaker and Garland5 recommend this reaction as excellent for study in an undergraduate physical chemistry laboratory experiment because it is supposedly free of complications up to 50% conversion. However, Tanji, et ~ l . and , ~ Knecht3 found considerable amounts of ethene and propene in their reaction products. The latter observed that, at the quarter time of the reaction, these side products accounted for 5-6% of the loss of cyclopentene. Grant and Walsh4 also found substantial quantities of ethene and propene plus cyclopentane and smaller quantities of methane and other hydrocarbons after 50% conversion of cyclopentene a t 774°K. With this background of studies, it appeared worthwhile to extend the cyclopentene dehydrogenation rate measurements to higher temperatures and also to determine extents of ring fragmentation, under the strictly homogeneous conditions attainable in a shock tube. In this study, we have made dehydrogenation and ring fragmentation rate measurements uia the comparative rate technique developed by Tsang,6 using the decomposition of tert-butyl alcohol to isobutene and water as the internal standard reaction, and also via the absolute rate method in which reaction temperatures are deduced from incident shock speeds. Measurements have also been made of the extent of ring fragmentation of cyclopentadiene in the same temperature range. The Journal of Physical Chemistry. Vol. 78, No. 4, 1974
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Experimental Section Materials. The cyclopentene and tert-butyl alcohol were both Reagent Grade from Analabs and Brothers Chemical Co., respectively. Cyclopentadiene was prepared from the dimer (Matheson Coleman and Bell Practical grade, >95%) by distillation at atmospheric pressure. All reagents were subjected to a purification process in uacuo involving three consecutive freeze-pump-thaw-distill cycles. At each distillation, the first and last 10-20'70 fractions were discarded. Gas chromatographic analysis of the purified reagents showed no detectable impurities (estimated limit for detection -0.1%). Nmr analysis of the cyclopentadiene at the time of gas sample preparation showed no detectable dimer. Matheson TJltra-High Purity argon (99.999%) was used as the diluent gas for the reactant samples. Linde helium was used as the driver gas. Matheson ethene (CP grade, 99.5%), propene (CP grade, 99%), allene (97%), isobutene (CP grade, 99%), and other hydrocarbons were used as obtained from the tanks for preparation of gas chromatograph calibration samples. Apparatus. A 10-cm j.d. stainless steel single-pulse shock tube was used in this study. This apparatus and details of its operation have been described p r e v i ~ u s l y . ~ Methods of reagent purification and sample preparation were also described in that earlier report. Reaction times behind reflected shocks ranged from 800 to 900 psec. Analyses. Reactant and product samples were analyzed on a Varian Aerograph 1440 chromatograph with hydrogen flame detector. A 1.8 m x 0.32-cm column of 20% DEGS on 60-80 mesh Chromosorb W a t 40" was used for the relative rate experiments run during 7/71. This was replaced by a 3.0 m X 0.32 cm column of 20% polypropylene glycol saturated with silver nitrate on 80-100 mesh Chromosorb W a t 50", for the relative rate experiments run during 8/72 and for all experiments from which extents of side product formation were determined. Peak areas were determined by the "cut and weigh" method. Multiple analyses were run on the products of each shock, and calculated rate constants were averaged. Gas chromatograph calibrations were checked before and after each day's experiments and at intervals during the day. Conditions. Two series of comparative rate experiments were run on mixtures containing 0.25% cyclopentene,
437
Cyclopentene Decomposition in Shock Waves TABLE I: Summary of Previous Studies of Cyclopentene Dehydrogenation T range, Ref
1 2 3 4
OK
730-800 690-800 750-800 774.4
P range, Torr
A,
39-249 12-30 4-34
58.8 1013.345 9 . 9 1012.77 5 7 . 7
...
E aI sec-1
kcal/mol
1013.04
...
...
K,atm
...
... 0 :424 (k0. 059)
0.25% tert-butyl alcohol and 99.50% argon, and another series was run on a mixture containing 1.00% cyclopentene, 0.25% tert-butyl alcohol and 98.75% argon. These series are referred to hereafter as sets A, B, and C, respectively, Three other groups of experiments were run on 0.25% and 1.00% mixtures of cyclopentene (sets D and E) and a 0.25% mixture of cyclopentadiene (set F) in argon. Experimental conditions for all sets are summarized in Table 11. Calculations. Reflected shock temperatures in the comparative rate experiments (sets A, B, and C) were calculated from measured extents of tert-butyl alcohol decomposition to isobutene and water, based upon the previously reported parameters for that r e a ~ t i o n log , ~ k = 14.6 [66.2 X 103/4.58T"K]. For absolute rate experiments (sets D, E, and F), temperatures were computed from measured incident shock velocities on the basis of ideal onedimensional treatment of the shock process. Thermodynamic data for cyclopentene and cyclopentadiene were calculated from vibrational assignments and structural data listed by Furuyama, Golden, and Benson.8 Rate constants for cyclopentene dehydrogenation and tert-butyl alcohol decomposition were calculated from the integrated form of the rate expression for an irreversible first-order reaction. At these low sample pressures, negligible error is introduced by the assumption of irreversibility up to large extents of reaction. For example, although C5Hs Hz has a n equilibrium conthe reaction stant K , = 0.424 a t 774"K,4 the value becomes K , = 100 at 1200°K (standard states are 1 atm). Thus equilibrium will be achieved when about 99.998% of the cyclopentene has been converted to the diene, under the experimental conditions of this study.
+
+
Results The cyclopentadiene dehydrogenation was observed to proceed cleanly up to 1150"K, and the tert-butyl alcohol decomposition has been previously shown to be well behaved up to that temperature.' The rate constants for formation of cyclopentadiene and isobutene in experiments of sets A-C are listed in Table I11 and are shown together in the comparative rate plot, Figure 1. All rate constants are in units of sec-l. The line, resulting from linear leastsquares treatment of the data up to 1189"K, has a slope of 0.906 and an intercept of 0.51. Based on the Arrhenius parameters for tert-butyl alcohol decomposition7 listed earlier, rate constants for cyclopentene dehydrogenation are given by log h(CjH8 + CLH, Hz) = 13.35(+0.27) -
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[60,000(* 1350)/4.58T0K] (1020-1189°K) (1) The low-temperature limit of this study, 1020"K, was determined by limitations in the product gas analysis; we were not able to obtain reproducible measurements of very small cyclopentadiene peaks because of overlap with the large reactant peak. The upper limit of 1189°K was
Figure 1. Relative rate plot: rate constants for formation of cyclopentadiene v s . rate constants for formation of isobutene: 0 , set A; 0 , set 6;A , set C. See Table I for experimental conditions.
chosen because significant concentrations of side products were produced by both reactants above that temperature. The three experiments run at higher temperatures are included in Figure 1 to show that the influence of side reactions on the two primary reactions is not extreme up to 60% conversion of cyclopentene and 70% conversion of tert-butyl alcohol. The uncertainties given in eq 1 represent one standard deviation in the least-squares analysis and do not reflect the total uncertainty in these values. The uncertainty in the parameters for tert-butyl alcohol should also be considered as should the uncertainties in the parameters for cyclohexene decyclization, the reaction with respect to which tert-butyl alcohol decomposition was ~ o m p a r e d . ~ However, it is reasonable to expect that random errors will cancel to some extent, so a realistic estimate of uncertainties in the parameters in eq 1 is about twice the magnitudes listed. The products of cyclopentene ring fragmentation were monitored in the experiments of sets D and E. The major side products detected over the range 1170-1490°K were ethene, propene, allene, and 1,3-butadiene, in a fairly constant ratio 6:3:2:3. Methane, acetylene, and a lesser amount of ethane were also formed. The methane:ethene ratio rose from Ih:6 at T 1300°K to 6:6 at 1450°K. The acety1ene:ethene ratio also rose rapidly with temperature; the observed concentrations of acetylene can be attributed to dehydrogenation of ethene at these temperature^.^ Total amounts of these six side products and amounts of cyclopentene and cyclopentadiene detected in product gas samples, as a function of reflected shock temperature, are shown in Figure 2 . These values are for an initial 0.25% cyclopentene in argon sample (set D). Also included are predicted concentration US. temperature curves for cyclopentene and cyclopentadiene, based on the Arrhenius parameters determined for cyclopentene dehydrogenation (eq 1) and assuming no loss of either compound due to side reactions. With reference to Figure 2, two conclusions can be drawn: (1) under the homogeneous conditions in the shock tube, side reactions do not account for more than 2% of the loss of reactant up to about 1250", a t which temperature the dehydrogenation goes 50% to completion in the 800 psec reaction time; ( 2 ) there is an apparent drop in temperature of the reacting gas in experiments run The Journal of Physical Chemistry. Voi. 78. No. 4. 1974
D. K. Lewis, M. Sarr, and M. Keil
438
TABLE 11: S u m m a r y of Experimental Conditionso Set
A B C D E F a
Symbol (Figure 1) 0
0 A
Date
7/71 8/72 8/72 7/71,6/72 6/72 7/71
Initial pressure range: 20-40 Torr; reaction pressures
0.25 0.25 1.oo 0.25 1 .oo
...
= 1 atm; reaction dwell times =
... ... ... ... ...
70 C4HoOH
T range, OK
0.25 0.25 0.25
1121-1258 1020-1189 1028-1178 1080-1490 1325-1450 1120-1460
... ... ...
0.25 SO0 psec.
TABLE 111: R a t e C o n s t a n t s for Formation of Cyclopentadiene and I s o b u t e n e in Relative R a t e Experiments, Sets A, B and Ca Set A (*)6.36, 6.59; (*)5.72, 6.06; 5.09, 5.10; 4.85, 4.95; 4.80, 4.81; 4.95, 4.75; 4.51,4.31; 4.39,4.24; 4.65, 4.28; 4.13, 3.92; 3.95, 3.65; (*)6.93, 7.26 Set B 0.72, 0.70; 2.17, 1.94; 1.86, 1.31; 1.73, 1.07; 4.66,4.69; 5.40, 5.33; 3.84, 3.86 Set C 1.96, 1.61; 2.05, 1.79; 1.55, 0.95; 2.87, 2.38; 1.49, 1.23; 4.94,4.89; 4.25, 4.16; 3.74, 3.71; 4.97,5.07 a Number pairs are In k(CsHe), In k(C4HeOH). Experiments noted with an asterisk (*) were not included in the least-squares reduction. Dwell time for all experiments, 800 psec.
at temperatures above 1200°K. Temperature values of 1350 and 1450°K calculated from shock speed measurements appear to be 50 and loo", respectively, higher than the actual effective gas temperatures determined from measured extents of cyclopentene dehydrogenation and the Arrhenius parameters in eq 1. This discrepancy is probably due largely to endothermicity of the reactions occurring a t these temperatures, although unimolecular falloff and/or systematic experimental errors may also be contributing factors. One may speculate that the dehydrogenation reaction is not entirely within the high-pressure regime under conditions of the experiments of set D above 1200"K, but it is difficult to attribute the entire factor of 5 decrease in the rate constant at 1450°K to unimolecular falloff. Recall that there was no observable dependence of the rate constant upon reactant concentration up to 1189°K in the experiments of sets A-C. It is also unlikely that this discrepancy arises solely from unreliable shock speed measurements; in sets A-C, temperatures determined from shock speed measurements generally agreed with the relative rate values within experimental error (k25"K). The ring fragmentation experiments on the 1.00% cyclopentene mixture (set E) gave very similar results in terms of identities and ratios of concentrations of species formed. Although there was greater scatter in total amounts of fragmentation for different experiments at a given temperature, a comparison with the 0.25% cyclopentene experiments indicated that the rate of ethene formation shows about first-order dependence on cyclopentene concentration. The extent of cyclopentadiene ring fragmentation over the temperature range 1120-1460°K was investigated in the experiments of set F. The C1-C4 products and their ratios of concentrations were similar to those found when cyclopentene was shocked to the same temperatures, except that only very small quantities of 1,3-butadiene were produced and that there were small quantities of an additional unidentified (probably CS) alkene. However, at a given temperature, the total concentrations of all these products were only about one-third as great as concentraThe Journal of Physical Chemistry. Vol. 78, No. 4 . 1974
T
OK
Percentage of reactant and products (as analyzed by reflected shock temperature (evaluated from shock 0 for experiments of speeds), after residence time T ( ~ 8 0 psec) set D (initial composition 0.25% cyclopentene in argon): 0 , cyclopentene: 0 , cyclopentadiene; 8 , sum of all C1-C4 side products. Solid lines, drawn through experimental points: dotted lines, expected cyclopentene and cyclopentadiene concentration after T vs. reflected shock temperature, assuming no formation of side products or cooling of gas due to endothermicity of reactions. Figure 2. vpc) vs.
tions produced by shocking cyclopentene to the same temperature. There was no measurable cyclopentane formed from either cyclopentene or cyclopentadiene at any temperature. This is not surprising as the low sample partial pressures resulted in a very low rate of biomolecular hydrogenation.
Discussion The Arrhenius parameters deduced for the cyclopentene dehydrogenation from this relative rate study are in excellent agreement with those reported from the lower temperature s t u d i e ~ , l - but ~ particularly with the highest . ~ kinetic values of E, and A, reported by Tanji, e t ~ 1 Thus parameters of this reaction are well established over the range 690-1190°K. There is ample evidence that the dehydrogenation is a concerted process. The Woodward-Hoffman ruleslO predict that concerted 1,4 elimination should be allowed, whereas 1,2 elimination should not, and Baldwinll has reported, from experiments on monodeuterated cyclopentene at 823"K, that 1,4 elimination is favored over 1,2 elimination by a ratio of 6:l. Recently, Anet and Leyendecker,12 studying the reverse process, reported that reaction of deuterium with cyclopentadiene at 823°K results in a predominantly suprafacial 1,4 molecular addition. Isotopic labeling to check the mechanism was not performed in the
439
Cyclopentene Decomposition in Shock Waves
present study, but the consistency of the Arrhenius parameters with the low-temperature work, the freedom from side reactions below 1150"K, and the absence of cyclopentane at all temperatures are strong arguments that the concerted process is maintained to high temperatures. Side product formation was more extensive from mixtures initially containing cyclopentene than from mixtures containing cyclopentadiene, at comparable reflected shock temperatures. The difference in rate of ring fragmentation is somewhat greater than the apparent 3:1 ratio because the cyclopentene mixtures were cooled more than the cyclopentadiene mixtures by the endothermic dehydrogenation process. Tanji, et al., reported finding that most side products in their samples were produced from cyclopentene. Comparing ring fragmentation of tetrahydrothiophene (thiocyclopentane) and thiophene (thiocyclopentadiene), Bauer, et a1.,I3 found the rate of the former to be considerably greater than the rate of the latter a t comparable temperatures. However, in this study, because of the rapid depletion of cyclopentene by dehydrogenation a t high temperatures, and the resulting presence of molecular hydrogen, the source of the side products cannot be determined with certainty. Cyclopentane can be ruled out as the major contributor because the rates of collisions between cyclopentene and the liberated hydrogen were far too loyv for enough cyclopentane to have been formed under our experimental conditions. The complete absence of cyclopentane in the product samples and the apparently first-order kinetics for side product formation support this argument. While the results of this study seem to rule out cyclopentadiene as the major source, they do not exclude the possibility that the side products are formed from nascent cyclopentadiene. Newly formed product molecules, certainly highly excited, ought to undergo ring fragmentation more rapidly than thermalized cyclopentadiene. This possibility ought to be subject to experimental verification; a manyfold increase in pressure should result in more rapid thermalization of the nascent product and a subsequent reduction in side products formed from it. Also selective pumping of cyclopentadiene vibrations with a tunable laser might give information on the tendency of these excited molecules to fragment. Experimental limitations precluded our testing these hypotheses. Weighing the available evidence, we feel that the most likely source of the bulk of the side products was cyclopentene. If cyclopentene ring fragmentation is the source of most of the side products, then there are two possible initial steps by which this fragmentation can occur: (a) bimolecular hydrogen atom transfer to form a cyclopentyl and a cyclopentenyl radical and (b) C-C bond rupture to form a biradical alkene. The results of earlier studiesl4-I6 suggest that if process a were operative, the cyclic monoradicals would quickly decyclize as follows
--+
0
--+
C,H, -I- H,C-CH-CH, C,H,
+
Hk=C=CH,
Subsequent hydrogen abstraction reactions would result in formation of stable ethene, propene and allene, in the ratio 2:l:l. While this is not far removed from the experimental ratio of these constituents, this process does not explain the presence of considerable amounts of 1,3-butadiene. Also, the hydrogen atom transfer step ought to be rate limiting. This would lead to second-order kinetics for side product formation contrary to experiment. Finally, the C-H bond energy is considerably higher than the C-C bond energy in cyclopentene, so the formation of the biradical ought to be favored energetically over the hydrogen atom transfer process. Assuming that the side products are formed directly from cyclopentene, we conclude that ring fragmentation via biradical formation is probably the dominant side reaction.
Acknowledgments. The authors wish to thank the Cabot Corporation for its donation of the shock tube and other equipment to Colgate University. Summer research fellowships were provided by the Sloan Foundation and the Carter-Wallace Foundation via science support grants to Colgate University. The authors also wish to thank the Colgate Research Council for support. Portions of this work were presented at the 163rd National Meeting of the American Chemical Society, Boston, Mass., April 1972. References and Notes (1) D. W. Vanas and W. D. Walters, J. Amer. Chem. Soc., 70, 4035 (1948), (2) H. Tanji, M. Uchiyama, A. Amano, and H. Tokuhisa, J. Chem. SOC. Jap., Ind. Chem. Sect., 70, 307 (1967). D . A. Knecht. Ph.D. Thesis. Universitv of Rochester, 1968. C. J. Grant and R. Walsh, Chem. Commun., 667 (1969). D. P. Shoemaker and C. W. Gariand, "Experiments in Physical Chemistry," 2nd ed, McGraw-Hill, New York, N. Y., 1967, pp 228236. W. Tsang, J. Chem. Phys., 40, 1171 (1964); 41, 2467 (1964); 42, 1805 (1965). D. Lewis, M. Keii, and M . Sarr, submitted for publication in J. Amer. Chem. SOC. S. Furuyama, D . M. Golden, and S. W. Benson, J, Chem. Thermodyn.. 2, 161 (1970). G. 6. Skinner, R. C. Sweet, and S. K. Davis, J. Phys. Chem., 75, 1 11971)
R.,- 6 . ' Woodward and R. Hoffman, "The Conservation of Orbital Symmetry, Verlag Chemie, Weinheim, West Germany, 1970, p 141. J. E. Baldwin, Tetrahedron Left., 2953 (1966). F. A. L. Anet and F. Leyendecker, J. Amer. Chem. SOC., 95, 156 (1973). S. H. Bauer, R. A. Fifer, P. Jeffers, A. Lifshitz, S. Matsuda, G. E. Millward, R. Moreau, ,and 6. P. Yadava, "SO*-Its Odessy in a Combustion Chamber, paper presented at 165th National Meeting of the American Chemical Society, Dallas, Tex., April 1973. H. E. Gunning and R. L. Stock, Can. J. Chem., 42, 357 (1964). A. S. Gordon, Can. J. Chem., 43, 570 (1965). T. F. Palmer and F. P. L o s i n g , Can. d. Chem., 43, 565 (1965).
The Journal of Physical Chemistry, Voi. 78, No. 4 . 1974
