NOTES
2016
Retro-Diels-Alder Reactions. 11.
Table I : Polymer from Radiolysis of 570
Gas-Phase Thermal Decomposition of
Benzene-C14 in Tritiated Cyclohexane
Bicyclo[2.2.l]hept-2-ene Sample size, g. T o t a l dose, e.v./g. X T o t a l polymer, mg. Carbon-14 activity in polymer, mpc. Benzene content, mg. Cyclohexane content, mg. Tritium activity in polymer, mpc. Tritium equivalent of cyclohexane content, mpc. Excess tritium activity relative to benzene content, pc./mole of CsR8 Specific tritium activity in cyclohexane, pc./mole of H
36.0 1.93 6.90 27.3 2.85 4.05 110.0 42.00 1860 73.00
20.4 4 11 7.05 37 5 3 90 3 15 124 8 32 8 1840 73.0
The number of excess hydrogen atoms per benzene residue in the polymer, as calculated froin the tritium activity, is unreasonably large. The magnitude of this value must represent not only an isotope effect favoring addition of tritium a t o m over protium atoms,11n12but must also be caused by the addition of more than one hydrogen atom per benzene residue. Abnormal tritium isotope effects have already been observed for addition of tritium and hydrogen atoms to alicyclic olefins,l3 but the absolute magnitudes are still in doubt; thus, we cannot estimate the number of hydrogen atoms per benzene residue. The observation of cyclohexadienyl radicals in the irradiated solid benzene has been verified by electron spin resonance spectroscopy,14 and hydrogen addition products, such as cyclohexadiene, are reported in the radiolysis of benzene in the liquid phase.15t16 Our observation of the abnormal tritium isotope effect in the formation of the benzene polyiner provides an unequivocal evidence that hydrogen atom addition reactions rather than bimolecular processes resulting in the hydrogen abstraction by an excited benzene molecule are involved since the latter process would be expected to show the normal isotope effect of a hydrogen transfer reaction. l7
(11) J. Bigeleisen and M. R’olfsberg, “Advances in Chemical Physics,” Vol. 1, Interscience Publishers, Inc., New York. X . Y . ,1961, p. 15. (12) E. G . Spittler. et al., J . Phys. Chem., 67,2235 (1963). (13) J. Y . Yaug and J. G. Burr, P u r e A p p l . Chem., in press. (14) H . Fischer, Kolloid-2.. 180, 53 (1962). (15) T. Gauman. Hela. C h i m . A c t a , 46, 2873 11963). (16) 3.1. K. Eberhardt, J . Phys. Chem.. 6 7 , 2856 (1963). (17) H . S. .Johnston in “Advances in Chemical Physics,” Vol. 3, Interscience Publishers, Inc., New York. K. Y.. 1961.
T h e Journal of Physical Chemistry
by William C. Herndon, William B. Cooper, J r . , and Melissa J. Chambers2 Department of Chemistry, University of M i s s i s s i p p i , Universitu, M i s s i s s i p p i (Received J a n u a r y 27, 2964)
I n general, Diels-Alder adducts tend to yield their component dienes and dienophiles upon the application of heat. The kinetics of such processes can, in principle, reveal information about the mechanisms of the respective Diels-Alder reactions. However, only a few quantitative studies of reverse Diels-Alder reactions have been r e p ~ r t e d l , ~ d Jand ” , ~ the evidence has been adduced in favor of both one-step and two-step mechanisms. This paper will report kinetic data for the gas-phase thermal decomposition of bicyclo [2.2.lIhept-2-ene. Possibly related reactions which have been previously investigated are the thermal decomposition of dicyclopentadiene,4a,o bicyclo [2.2.1]hepta-2,5-diene,1t4fcyclohexene, 4d,h and 4-vinylcyclohexene. 4i We do not consider the thermal decomposition of adducts containing carbonyl groups to be closely related reactions since the transition states and intermediates (if any exist) for such reactions should be quite polar by comparison.
Experimental Chenzicals. Bicyclo [2.2.l]hept-2-ene was obtained from K and K Laboratories, Inc., and Chemical Procurement Laboratories, Inc. A simple vacuum distil(1) Paper I in this series is W. C. Herndon and L. L. Lowry, J . Am. Chem. SOC.,86, 1922 (1964). (2) Undergraduate research stipends for W. B. C. and >I. J. C . and
other financial support (Grant No. GP247) from the National Science Foundation are gratefully acknowledged. (3) Some recent papers concerning the mechaniem of the DielsAlder reaction are: (a) J. A. Berson and A. Remanick, J . Am. Chem. Soc., 83, 4947 (1961); (b) M . G. Ettlinger and E. S. Lewis, T e x a s J . Sci., 14, 58 (1962); (c) C. Walling and H. J. Schugar, J . Am. C h e m . SOC.,85, 607 (1963); (d) S. ‘Seltzer, ibid., 85,1360 (1963); (e) M. J. Goldstein and G. L. Thayer, Jr., ibid., 85, 2673 (1963). (4) (a) B. S. Khambata and S. Wassermann, J . Chem. SOC.,375 (1939); (b) G . B. Kistiakowsky and J. R. Lacher, J . Am. Chem. Soc., 58, 123 (1936); (e) J. Harkness, G. B. Kistiakowsky, and W. H . Mears, J . Chem. Phys., 5, 682 (1937); (d) L. Kuchler, T r a n s . Faradau SOC.,3 5 , 874 (1939); (e) J. 4.Berson and W. A. Llueller, J . Am. Chem. SOC.,83, 4940 (1961); (f) J. H. Birely and J. P. Chesick, J . P h y s . Chem., 46, 568 (1962); (9) J. E. Baldwin and J. D . Roberts, J . Am. Chem. Soe., 85, 115 (1963); (h) S. R. Smith and A. S.Gordon, J . Phys. Chem., 65, 1124 (1961); (i) T. F. Doumani, It. F. Deering. and A. C. McKinnis. I n d . Eng. Chem.. 39, 89 (1947).
NOTES
lation of the commercial bicyclohepteiie gave material which was essentially free from impurities. The major impurity in the commercial material, present to the extent of less than 5%, was identified as nortricyclene. Sortricyclene was prepared from the bicyclo12.2.11hept-2-ene for the purpose of comparison by the method of S ~ h l e y e r . ~Cyclopentadiene was prepared by cracking the dimer which was obtained from K and K Laboratories, Inc. Ethylene was purchased from the RIatheson Co., Inc. Kinetic Procedure. Part of these kinetic experiments was conducted a t atmospheric pressure using a stirred flow reactor Eiystern which has been described in detail previously.1*6 Briefly, the stirred flow system employed an inert carrier gas, prepurified nitrogen, which was allowed to pass over the crystalline bicycloheptene. The gas stream, now containing bicycloheptene, then passed into a spherical reactor, volume 35.1 ml., immersed in a molten lead constant temperature bath and from there through a rising soap film flow meter. The gas stream could be sam-. pled both before and after the reactor and the gas samples were analyzed by gas chromatography. I n experiments a t higher temperatures (faster rates) the spherical reactor was simply replaced by a tube with a volume of 0.43 ml. The temperature in the tube was determined by moving a chromel-alumel thermocouple through the tube. The temperature profile in the tube indicated practically perfect eyui-. librium with the constant temperature bath. At each temperature three gas samples were analyzed a t each of three to five different flow rates, utilizing Wilkens Instrument Co. “Aerograplh” Model A-90-P thermal conductivity and “Hy-Fi” hydrogen flame detector gas chromatographs, and areas were de-. termined with a disk integrator.
Results and Discussion One would expect that the thermal decomposition of bicyclo [2.2.1Ihept-2-ene would yield cyclopenta-. diene and ethylene (reaction 1). This expectation was confirmed by comparison of the retention times of
ethylene and cyclopentadiene with those of the products of the reaction. No other products could be detected a t the extreme limit of gas chromatographic analysis, even at the highest temperature investigated. The ethylene to cyelopentadiene ratio mas invariable a t all flow rates and temperatures. Therefore, other nonvolatile products such as cyclopeiitadiene polymers
2017
are not produced. We estimate that other products, if formed, are present to the extent of less than O.Olg-/,. First-order rate constants were calculated from data obtained over the temperature range 304-398’ in the stirred flow reactor and over the temperature range 357-443’ in the tubular reactor. The rate constants were calculated from k = (U/V)(CPD,’BCH) in stirred reactor experiments and IC = (U/V)(ln [l CPD/BCH]) in tubular reactor experiments. In these equations IC is the first-order rate constant, li the flow rate of gas, I/ the volume of the reactor, and CPD/ BCH is the ratio of cyclopentadiene to bicycloheptene in the exit gas stream. When flo~7rates were varied by a factor of 10 at the same temperature, calculated rate constants varied by less than 4%, and this close correspondence supports the assumption of first-order kinetics for the decomposition. The rate constants are tabulated in Table I and a
+
Table I : Decomposition of Bicyclo[2.2.l]hept-2-ene -----Stirred Temp., ‘ C .
3 04 310 320 327 330 340 344 349 362 374 398
reactor---Rate constant sec.-l
0 0 0 0 0
0 0
0 0 0 0
00376 00579 0114 0154 0213 0403 0361 0578 121 206 508
_--
Tubular reactor----Temp., “ C . Rate constant, 8eo.-1
357 375 378 379 398 400 418 438 440 443
0.0883 0.289 0,238 0.301 0.78‘7 0.845
2.02 4.66 5.01 4.85
plot of log k US. the reciprocal of the absolute temperature is shown in Fig. 1. Using all 21 sets of data, one finds that log k = (13.78 i 0.19) - (42,750 + 560/2.3BT). The constants in this equation were calculated by the method of least squares and the limits of error are standard deviations. Either group of dat’a alone gives the same Arrhenius equation within its respective limits of error. Other carrier gases were used a t 349O, and the rate constant given in Table I for that! t’eniperature is the average of experiment’s using nitrogen, helium, carbon dioxide, and steam as carrier gases. One other variable, the surface to volume ratio, was 42 cm.-I in the tubular reactor and wa,s 1.4 cm.-l in the stirred flow reactor. (5) P. von R. Schleyer, J . Am. Chem. Soc., 80, 1700 (1958). (6) W. C. Herndon, M. B. Henly, and J. Xi. Sullivan, J . Phgs. Chem., 67, 2843 (1963).
Volume 68, h’umber Y
J u l y , 1904
NOTES
2018
normal for a unimolecular decomposition and is quite similar to those of other retro-Diels-Alder reactions. * I n fact, this thermal decomposition is so “normal” hat very little mechanistic delineation is possible. (7) N. E. Duncan and G. J. J a m , J . Chem. P h y s . , 20, 1644 (1952). (8) C. Walling, “Free Radicals in Solution,” John Wiley and Sons, Inc., New York, N. Y., 1957, Chapter 2.
Free-Radical Substitution in Aliphatic Compounds. VI.
The Reaction of Fluorine
Atoms with Carbon Tetrachloride1
by D. T. Clark and J. JI. Tedder Department of Chemistry, T h e University, Shefield 10, England (Received January $1, 1964)
I
I
1.4
1.5 1.6 I/T x io3
1.7
Figure 1. Arrhenius plot for bicyclo[2.2.1] hept-2-ene pyrolysis, 0 (stirred flow reactor), 0 (tubular flow reactor).
The activation energy found in this reaction (42.8 kcal.) is intermediate between that found for dicyclopentadiene (34.2 kcal.)*” and that found for bicyclo[2.2.l]hepta-2,5-diene (50.2 kcal.).’ It differs considerably from those reported for cyclohexene pyrolysis (57.s4d or 67.64h kcal.) and from that calculated for 4-vinylcyclohexene pyrolysis (-62 kcal.) Semiquantitatively, the values for the bicyclic compounds support a rate-determining step in which only one bond i s broken. Bond breaking in dicyclopentadiene procluces two allyl radicals, in bicyclohepteiie an allyl and alkyl radical, and in bicycloheptadiene an allyl and irinyl radical. The activation eiiergies for these processes8 should, therefore, lie in the order which is t‘ound; however, the established order does not eliminate the possibility of a mechanism in which two bonds are broken simultaneously. The pre-exponential factor for the bicyclo [2.2.1]hept-Bene pyrolysis ‘is of the magnitude considered
.’
The Journal of Physical Chemistry
Previous work in this laboratory has been devoted to the study of the abstraction of hydrogen from substituted aliphatic hydrocarbons by halogen atoms2 or trichloromethyl radicals. A fairly complete picture of this process has been d e v e l ~ p e d . ~The present investigation is the first of a series in which the abstraction of halogen atoms from aliphatic compounds will be studied. The fluorination of carbon tetrachloride has been studied qualitatively by Ruff5 and by Simons.6 Ruff found little reaction a t room temperature; and when the carbon tetrachloride was refluxed, explosions occurred. Both Ruff and (subsequently) Simons carried out the heterogeneous fluorination of carbon tetrachloride, bubbling the gaseous fluorine through refluxing carbon tetrachloride and adding various “catalysts” such as iodine, arsenic, or bromine. Simons also studied the fluorination of difluorodichloromethane in the gas phase, again in the presence of “catalysts.” The notable feature of this work was that difluorodichloromethane was much less reactive than carbon ~
~~
~~
~
(1) This research was supported in part by Aeronautical Systems Division, AfSC. through the European Office, Aerospace Research, U.S.A.F., Grant No. AF.EOARDC-61-7. (2) P. C. Anson, P. S. Fredricks, and J. M. Tedder, J . C h e m . SOC., 918 (1959): P. S. Fredrioks and J. M. Tedder, ibid., 144 (1960); ibid., 3520 (1961); I. Galiba, J. M. Tedder, and R. A. Watson, ibid., in press. (3) B. P. McGrath and J. M .Tedder, Bull. SOC.Chim. Belges, 71, 772 (1962). (4) J. M. Tedder, Quart. Rea. (London), 14, 336 (1960). (5) 0. Ruff and R. Keim, 2. anorg. allgem. Chem., 201, 245 (1931). (6) J. H. Simons, R. L. Bond, and R. E. McArthur, J . Am. C h e m . SOC.,62, 3477 (1940).
