Sept., 1962
THERMAL ISOMERIZATION OF VINYLCYCLOPROPANE
(i.e.> liquid-liquid) mixing process. However, this does not scem to be the case for the mixtures considered in the present work. Thus, we see from Fig. 1 and 2 that the curvature maximum appears to fall at solute moJe fractions of the order of 0.2 t,o 0.3. 'I'here is some doubt about the location of the enthalpy minima. However, they probably all occur a t Pignificantly higher solute concentrations. It should be noted also that for a given solute (e.g., strontium nitrate) the curvature maximum tends to shift to a higher solute concentration as the liquidl-liquid mixing enthalpy becomes more negative. Previously we attempted to relate the location of this maximum to the existence of a solid state double salt at the same cornposjiti~n.~In the light
1671
of the information now available we have some doubts about this relation. On the other hand, all t>hesystems with heat of dilution maxima appear to have solid sta,te double salts at some composition. Finally, we should like to mention that the magnitude of the heat of dilution of course is related to the limiting heat of solution and to the sizecharge parameter 6'. However, the extent and quantitative character of these correlations are very sensitive to the actual choice of solute concentration. Acknowledgments.-This work has been supported by the Office of Naval Research under Contract No. Nonr 2121(11) with the University of Chicago, and by the National Science Foundation under grant No. G 19513.
THE THERMAL ISQ&IERTZATIONOF VINYLCY CLOPROPANE BY C. A. WELLINGTON~ Department of Chemistry, University of Rochester, Rochester, N . Y. Received March 19. 1968
The gas phase thermal isomerization of vinylcyclopropane has been studied in a static system between 324.7 and 390.2'. It has been Found to be a first-order unimolecular process a t pressures above 8 mm., giving cyclopentene as the mx'or product (-96%) with small amounts (-1% each) of 1,4-pentadiene, cis- and trans-lJ3-pentadiene, but no isoprene. +he effect of small additions of nitric oxide and the effect of increasing the surface/volume ratio by a factor of 27 are discussed, the former having no significant effect while the latter increased the rate of reaction by a small extent. From experiments with initial pressures of 10-11 5 mm., the two most important processes, the over-all reaction and the formation of cyclopentene, were found to have activation energies of 50.0 f 0.3 and 49.7 f 0.3 Bcal./mole, respectively, and the rate constants (sec.-.l) could be expressed by k(over-all) = (5.3 f 0.1) X 10'8 exp( - 50,00O/RT) and k(cyc1opentene) = (4.09 f 0.05) X 1 0 1 8 exp( - 4 , 7 0 0 / R T ) . Under the same conditions, the activation energies (l;cal./mole) and rate constants (sec,-l) for the formation of the minor products were: lJ4-pentadiene, 57.3 f 1.0, k = (2.7 f 0.2) x 1014 exp( -57,300/RT). trans-l,S-pentadiene, 53.6 f 0.8, k = (1.01 i: 0.06) X 1018exp( -53,60O/ET); and cis-1,3-pentadiene1 56.2 & 0.8, k =' exp( -56,20O/RT). (8.0 f 0 5) X
Introduction Previou1.J work on the pyrolysis of vinylcyclopropane had indicated that the main product was cyclopentene. Vogel2 has stated that vinylcyclopropane and l-phenyl-l-vinylcyclopropane isomerize thermally into the corresponding cyclopentenes. He discusses the similarity between a double bond and a cyclopropane ring and he draws an analogy between the vinylcyclopropane isomerization and the reversible isomerization of cyclopropanecarboxaldehyde to 2,3-dihydrofuran. Furthermore, the pyrolysis in a flow system a t 500520' of a solution of 3 g. of a mixture of 68% vinylcyclopropane, 31% cyclopentene, and 0.3% 1,4pentadienc in 15 ml. of acetic acid gave 70% cyclopentene, 28% vinylcyclopropane, and 3% 1,4pentadiene. This indicated that the vinylcyclopropane had been converted to cyclopentene and perhaps to a little ll4-pentadiene. However, passage of vinylcyclopropane over kieselguhr a t 120-150 O produced piperylene, the catalyst IoRing its activity after one pass.4 Since in this latter experiment the reaction took place on the surface, it probably would not be of great signifi(1) Shell Foiindation Postdoctoral Research Fellow. ('2) E. Vogel, Angem. Chzm., 72, 4 (1960). (3) C . G. Overberger and A. E. Borohert,
J. Am. Chem. Soc., 82, 4896 (1960). (4) B. A. Kazanskii, M. P u . Lukina, and L. G. Cherkashine, Izvrst. Akad. Nnuk S.J.S.R. Ofdel. Khzm. N a u k , 553 (1959); Chena. Abstr., 63, 21701d (1959).
cance in the decomposition in the gas phase. Thus an investigation of the gas phase reaction was undertaken with a view to determining if the ring expansion reaction was a homogeneous gas phase process and determining the kinetic parameters of all the processes that occur. While this work was in progress, Flowers and Freys reported that vinylcyclopropane undergoes a first-order thermal isomerization to cyclopentene. Investigation a t four temperatures in the range 339391' gave a good Arrhenius plot from which they obtain k = exp(-49,600/RT) sec.-l. At 390.5' they report that 1% of the product was a mixture of 1,4-pentadiene, isoprene, and cis- and truns-ll3-pentadiene. Experimental Materials.-Vinylcyclopropane was obtained from the National Bureau of Standards, Washington 25, D.C., and the physical properties quoted for the sample were: m.p. ~ dZo0.72105 g./ml. The purity of -109.82'; n Z o1.4138; the sample was tested by gas chromatography using two different columns, diisodecyl phthalate on celite, and dimethyl sulfolane on firebrick. In both cases no peak other than that due to vinylcyclopropane could be detected, showing that the sample contained less than 1 part in 10,000 of impurity. Cyclopentene and 1,4-pentadiene were obtained from the Sational Bureau of Standards and were used without further purification. The impurity of each was checked by gas chromatography and found to conform to the quoted values of 0.034 f 0.021 and 0.07 f 0.05 mole %, respectively. (5)
PI.C. Flowers end H. M. Frey, J .
Chena. floc., 3547
11961).
C. A. WELLINGTON
1672
tmns-1,3-Pentadiene was obtained from Columbia Organic Chemicals Co. Inc., Columbia, S.C. I t T ? ( T ~found S by gas chromatography to contain no cis isomet, but about 47, cyclopentene as impurit,y. The sample was purified using a 2 m. silver nitrate/glycol chromatographic column a t room temperature. cis-1,3-Pentadiene was obtained from Phillips YOYO piperylene using a 2-m. silver nitrat,e/glycol chromatographic column a t low flow rate and room temperature. Isoprene was used only to determine its retcntion time under the various analyt,ical conditions used on the vapor phase chromatograph and a sample (Eastman fiodalr Co.) was distilled and dried. Apparatus and Procedure.-The apparatus has been described previously.6 In the apparatus the stopcocks in the vicinity of the reaction vessel were replaced by mercury cutoffs of t'he float valve type to a,void any absorption of the reactants and products in stopcock grease. The following procedure was used. In order that up to three experiments could be conducted a t the same initial pressure, vinylcyclopropane was first metered out into a nest of three sample tubes with equal ( 4 ~ 1 % volumes. ) The reactant vapor in one of the tubes was then condensed on a small cold finger in the mercury cut-off section in the immediate vicinity of the reaction vessel. The reaction was commenced by evaporating the reactant condensed on t'he cold finger by means of a bath of boiling water. The pressure in the reaction vessel was measured on a wide bore mercury manometer using a vernier t,elescope. The pressure was essentially constant during the course of the reaction, on occasion showing an increa~seof not more than 1 part in 3000. The reaction was st,opped by opening the contents of the reaction vessel t o a trap immersed in liquid nitrogen, where rapid condensation t,ook place. All the products were condensable, no permanent gas being produced during the reaction. The condensate then was transferred to a sample tube immersed in liquid nitrogen and kept therein until it was condensed into the ga,s sampling section of a gas chromatographic apparatus. Analysis of the Products.-Tm-o packed columns 'cyere used separately and successively to analyze the products. The mixture was first run through a 2-m. silver nitrate/ ethylene glycol column a t room temperature and a t low flow rat'e. By comparison with the retention times for the pure compounds under the same c,onditions, it was found that with this column it waa possible to detect and est'imate isoprene, truns-1,3-pentadiene, and cis-1,3-pentadiene, but 1,4pentadiene, vinylcyclopropane, and cyclopentene could not be separated from one another. When a 4 m. column of dimethyl sulfolane on firebrick was used, it was possible to separate and estimate 1,4-pentadiene, cyclopentene, and vinylcyclopropane from each other. However, cis- and Imns-1,3-pentadiene had retention times very similar to that of cgclopentene and could not be separated from the latter. The products of the reaction were analyzed as follows. The whole sample from the reaction vessel was introduced into the gas chromatograph and analyzed using the 2 m. silver nitrate/ethylene glycol column. Thus the amounts of cis- and trans-1,3-pentadiene and the total amount of 1,4-pentadiene, cyclopentene, and vinylcyclopropane were measured using a Perkin-Elmer printing integrator. The fraction containing 1,4-pentadiene, cyclopentene, and vinylcyclopropane was collected in a long, efficient, glass spiral immersed in liquid nitrogen and connected to the exit tube from the Perkin-Elmer vapor fractomet,er, Model 154D. Within the accuracy of analysis complete quantitative condensation of the fraction was achieved. This latter fraction then was reintroduced into the gas chromatograph. The three components could be separated and analyzed quantitatively by the use of the 4 m. dimethylsulfolane column a t medium flow rate. By this procedure no peak corresponding to that for isoprene was detected. Since the formation of isoprene has been reported previously,6 special steps were taken t o ascertain whether it was present in the products in this work. A4t393' a sample a t 12 mm. pressure was taken to a t least 90% decomposition. The products were run on the gas chromatograph and no peak corresponding to isoprene could be detected. In a second experiment the products of a decomposition were divided into two portions. The first, (6) C . ,4. Wellington and W. 4888 (1961).
n.
Vol. 66
portion wm analyzed in the usual way on the silver nitrate/ ethylene glycol column, while to the second portion a small amount of isoprene was added and then the mixture was analyzed in the same way as the first portion. On comparing the t w u chromatographs, the second showed the peak due to isoprene which was quite distinct and well separated from the rest of the products, with a retention time a little less than that of trans-l&pentadiene. The first, however, showed no trace of a corresponding isoprene peak. Thus, isoprene could not be detected in the products of the decomposition of vinylcyclopropane under the conditions used for the experiments in this work. However, peaks corresponding to the other compounds mentioned above always were present. Two other small peaks were often noted but these were not estimated, since each constituted an amount of less than 1 part in 10,000 of that of the products. One of these peaks, appearing a t long retention time, also appeared when a pure sample of cyclopentene was left in the reaction vessel a t the highest temperature employed in this work for many hours and then analyzed. Again its amount was negligible compared with that of the pure cyclopentene used. This peak was the only peak, other than that of the cyclopentene itself, that appeared in the analysis, showing that no vinylcyclopropane had been produced from cgclopentene. Thus, it seems that no equilibrium occurs betwepn vinyl cyclopropane and cyclopentene as exists between cyclopropane carboxaldehyde and 2,3-dihydrofuran ,277 The presence of 1,4-pentadiene and cyclopentene in the products was confirmed by collecting the fractions from the gas chromatographic analyses corresponding to each coinpound and taking an infrared spectrum of each, using a 1-m. gas cell. The two spectra obtained corresponded exactly to the respective spectra in the literatures and were quite different from those of the isomers of the two compounds. The isomerization of vinylcyclopropane can thus be represented by eq. 1
i+ 'v' __
-
I-+ - ~ _ _-_-_ "'/
I
+
96.02
1 . 6 4 (1)
e = =
=-E-
+ -=-
cis
1.16
truns
l . 18
The printing integrator and the gas chromatograph were calibrated for each of the products of the reaction, using standard mixtures of the pure compounds. The relative response of the intrunients compared with that for vinylcyclopropane were: cyclopentene, 1.00; 1,4-pentadiene, 0.98; trans-1,3-pentadiene, 1.01; cis-l&pentadiene, 1.01. With these values, the fractions of 1,4-pentadiene and cyclopentene in the mixture of these compounds and vinylcyclopropane were calculated from the results on the dimethyl sulfolane column. With this information, together with the results for the silver nitrate/ethylene glycol column, the fractions of cas- and trans-l,3-pentadiene in the products could be calculated. The over-all fraction of reaction then was computed and also the fraction of each product of the total products. The over-all rate constant and the relative rate constants for the formation of each product %-erecalculated. In many cases also, the over-all rate constant was obtained from results a t different durations (but all other conditions being the same) by plotting --log (1 - fraction reacted) against the time of reaction. This plot always produced a good straight line, showing the reaction was of first order (as indicated by Table I) andfrom the slope of the straight line the rate constant was obtained. In order that the system of analysis, which involved the condensation of a major part of the product and all of the unchanged reactant, might be well checked as to its accuracy, two identical experiments were performed. The products of the first experiment were analyzed by the same two-column sequential procedure already described and the (7) C. L. n'ilson, ibid.. 69, 3002 (1947).
Waiters, J . Am. Chem. SOC.,83,
(8) American Petroleum Institute, Catalog of Infrared Spectra Data (1959), No. 450, 362, and 456.
THERMAL ISOMERIZATIOK OF VISYLCYCLOPROPANE
Sept., 1902
1673 A
B
over-all rate constant calculated. The products of the second realation were analyned on the dimethyl sulfolane column only. I n this lattrr case small amounts of a s - and trans-l,3-pmtadienes contributed t o the cyclopentene peak. However, the response of the chromatograph was alniost the same (1%) tovard the 1,3-pentadienes as to cyclopentene, and so from the size of this composite peak and that of the peak for 1,4-pentadiene, the total amount of product was estimated and the rate constant calculated. The difference between the over-all rate constants from the two methods of analysis vvas only I part in 2,000, which qhowed that Chr method used for analvsis was adequate.
4.6
4.1
4.4
Results Order of the Reaction.--A number of experi- $3.6 ments a t five different temperatures in the range z 332.6 to 375" were conducted at different reaction durations at initial pressures of 4.6 to 12 mm. Some of the results of these experiments are given 3.1 in Table I where, in the fourth column, the values of the rate constants calculated assuming first-order kinetics are given. At each temperature the firstorder ratp constants are constant within experimental error a t increasing times of reaction, showing the decomposition to be first, order.
& bc
'
1 REACTION OB VINYLCYCLOPROPANE
3.9 'i
3.4
2.!1
TABLE
ORIIEROF Temp.
(0C.I
106 x k (over-all)
Duration (sec.)
% Reaction
3601 7200 1569 2826 1080 2160 300.0 1080.0 312.0 595.8 903.0
15.9 28.9 15.3 26.0 15.0 27.8 8.70 27.8 19.6 35.1 46.9
332.7 344.6 350.2 361.5 375.0
(see.
-1)
4.80 4.73 IO.59 10.66 15.01 15.06 30.5 30.2 69.8 70.1 70.0
1.55
1.60 1.65 1031~. Fig. l,-Temperature dependence of the first-order rate constants: ,4,over-all isomerization; B, formation of cyclopentene; 0, no additive; S O added; 0--,packed bulb.
+,
E
I
I
'1
6.8 6.4 6.0 5.6
5.2 -e
On varying the initial pressure of vinylcyclo- 2 4.8 propane from 24 to 0.14 mm. a t 381.5' the fall-off ' D in the first-order rate constant, as expected for a 5.2 unimolecular reaction, was observed and the re5.0 sults are given in Table 11. d
4.8
TABLE I1 FALL-OFF o r FIRST-ORDER RATECONSTANT Initial pressure (mm )
10' X k [over-all) (see
24 7 8 13 8 01 4 57 1.17 0 43 0.14
3 3 3 3 2 2 2
4.4
-1)
06 05 02 03 88 64 30
1.51
1.55
1.59 1.63 1.67 10"~. Fig. 2.-Temperature dependence of the first-order rate constants: C, formation of 1,4-pentadiene; I), formation of trans-1,3-pentadieiie; E, formation of czs-1,3-pentadiene; 0, no additive; f, KO added; 0--,packed bulb.
The fall-off curve of log(k/k,) against log po folIon ed closely that of methylcyclobutane. factors for each of the processes occurring in the Effect OF TemperaturelThe effect of tempera- reaction. The Brrhenius plots of (-log k ) us. 1/T ture from 324.7 to 390.2" on the reaction ~v-as are shown in Fig. 1 for the over-all isomerization studied at initial pressures in the range 10-11.5 mni. (plot A) and for the formation of cyclopentene in the unpacked reaction vessel in order to deter- (plot B), and in Fig. 2 for the formation of 1,4mine the activation energies and pre-exponential pentadiene (C) and cis- (E) and trans-pentadienes (9) Experiments ronductrd by W. D. Walters and A. Pataraechia in
thpr I,ahoratories,
(D). The Arrhenius plots for all five processes show good straight lines, the greater spread of
C. A. WELLIXCTON
1674
points for the latter three compounds being due to the difficulty of measuring accurately the very small amount of each formed in the reaction. A least squares analysis of each set of data was performed on an I.B.M. 650 computer to determine the activation energy. The activation energies (kcal./mole) were found to be: over-all process, 50.0 i= 0.3; formation of cyclopentene, 49.7 f 0.3; 1,4-pentadiene, 57.3 f 1.0; cis-1,3pentadiene 56.2 f 0.8; trans-1,3-pentadiene, 53.6 f 0.8. With these activation energies, the rate constants (sec.-I) with the corresponding standard deviations for the various processes could be expressed by: over-all process, (5.3 0.1) X 1013 exp( -50,000,lRT) ; formation of cyclopentene, (4.09 f 0.05) X 10l3 exp(-49,700/RT); 1,4pentadiene, (2.7 f 0.2) X 1014exp(-57,300/RT); cis-1,3-pentadiene, (8.0 f 0.5) X 1013 exp(-56,200/RT); trans-l,3-pentadiene, (1.01 i 0.06) X 1013exp(-53,600/RT). Effect of Nitric Oxide and Surface.-Since the parent compound was itself unsaturated, it seemed probable that if radicals were present during the reaction, addition to the unsaturated linkage would take place. However, no product of molecular weight higher than that of the parent compound was found on analyzing the products by gas chromatography and by mass spectrometry on a Consolidated 21/620 instrument. On addition of small and large amount,s of nitric oxide a t four different temperatures, the over-all rate was not affected. The relative rates of formation of the various products also were unaffected. The results are shown in Fig. 1 and 2. It was concluded that there was no effective radical participation in the reaction. On studying the reaction in a Pyrex reaction yessel packed with thin-walled Pyrex tubing (resulting in an increased surface to volume ratio of about 27), a significant increase in the over-all reaction rate was observed when no conditioning of the packed vessel was made. The increase in the over-all reaction rate did not seem to decrease significantly on doing four successive experiments and was not reproducible. However, after three decompositions of 3,4-dihydro-2Ei-pyran, which yield acrolein and ethylene, had been carried out in the vessel, it was found that the reaction rate of vinylcyclopropane was reproducible and only slightly greater than that in the unpacked vessel which previously had been used to study the decomposition of 3,4-dihydro-2H-pyran. The results are shown in Fig. 1and 2. It was concluded from these results that the contribution from a surface reaction in those experiments in the unpacked vessel was negligible. Discussion The foregoing results show that the isomerization of vinylcyclopropane is a unimolecular, first-order, essentially homogeneous process which is unaffected by additions of nitric oxide and yields mainly cyclo-
*
Yo]. 66
penteiie together with very small amounts of 1,4pentadiene and cis- and trans-l,3-pentadiene. It has been shown that cyclopentene does not form vinylcyclopropane under the conditions of this work and so the isomerization of vinylcyclopropane is not complicated by an equilibrium between vinylcyclopropane and cyclopentene as exists between cyclopropane carboxaldehyde and 2,3-dihydrofuran.' No isoprene could be found under the conditions of the present investigation. If isoprene is not a product, the minor products do not necessarily result from the usual rupture of the cyclopropane ring as has been ~uggested,~ but may result from a process involving a transition state in which the n-orbital of the vinyl group interacts with the partial delocalized electron cloud associated with the cyclopropyl ring in the vicinity of the two bonds of the ring adjacent to the vinyl gr0up.l This probably is not possible with the third bond which probably is too far removed from the n-bond. The possibility of this type of transition state may be the reason why this compound can isomerize at temperatures much lower than those associated with the isomerizations of cyclopropane and the alkyl cyclopropanes. The relative ease with which vinylcyclopropane isomerizes may be due to the fact that for the formation of the main product, cyclopentene, hydrogen migration is not necessary and moreover for those products where it is necessary, namely the pentadienes formed, the activation energies are significantly higher than that for cyclopentene formation. V'hile in the addition of methylene radicals to butadiene, Trotman-Dickenson, et reported no cyclopentene as a product, Freyl' has more recently reported that cyclopentene is formed from the decomposition of the excited vinylcyclopropane produced. The results of Frey show that, although the decomposition of the latter species also produces 1,bpentadiene and cis- and trans-l,3pentadiene, no isoprene is produced. This is consistent with the results of the present work and with the mechanism suggested above, and so the results of the decomposition of vinylcyclopropane may not be directly applicable to a consideration of the decomposition of cyclopropane and methylcyclopropane. NOTEADDEDIN PRoop.-The small entropy of activation of 0.28 cal./deg. mole a t 390" for the formation of cyclopentene indicates that the difference in rigidity between the normal and activated state3 of the molecule may be less than for the isomerization of cyclopropane. This is consistent with the mechanism for the isomerization suggested in the Discussion.
Acknowledgments.-The aut,hor wishes to thank Professor W. D. Walters for his great interest in this work and also Mr. Carl Whiternan, Jr., for making infrared measurements and least squares calculations. (IO) B. Grzybowska, J. IT. Knox, and A. F. Trotman-Dickenaon, J . Chen. Soc., 4404 (1981). (11) H. M. Fray, Tvans. Faraday Soc., 68, 518 (1952).
