THEGASEOUS PHOTOLYSIS OF THE METHYLENECYCLOALKANES
321
The Gaseous Photolysis of the Methylenecycloalkanes by R. K. Brinton Department oj Chemistry, University of California, Davis, Calijornia (Received March 1, 1867)
The direct and Hg(@PI) photolyses of methylenecyclopropane result in the production of ethylene and acetylene in about equal quantities. The reactions proceed cleanly with essentially no polymer formation, but small amounts of allene and hydrogen are also formed in addition to the main products. Behavior of the reactants in the presence of added oxygen indicates that the mechanism is intramolecular rather than the result of radical processes. The quantum yields of decomposition of the Hg(VP1) reaction decrease as the pressure of the methylenecyclopropane is increased, and the quantum-yield data are consistent with a deactivation-decomposition mechanism involving two excited states being deactivated by the methylenecyclopropane molecule. Methylenecyclobutane decomposes by the direct photolytic process, in a similar fashion, to produce ethylene and allene in equivalent quantities. The effect of added oxygen also suggests an intramolecular mechanism, but no quantitative quantum-yield data were obtained. Photolysis of the methylenecyclopentane, on the other hand, did not produce detectable decomposition products. Speculation concerning the details of ring scission and rearrangement for the two smaller homologs is made and possible explanations are given for the lack of reactivity of methylenecyclopentane.
A detailed study of the direct photolysis of the methylenecycloalkanes has not been made, although certain experimental evidence relevant to the problem has been obtained in connection with other studies.' Loeffler, Eberlin, and Pickett, in the course of their examination of the ultraviolet absorption spectra of small ring hydrocarbons, observed that methylenecyclobutane decomposed rather readily under irradiation and they were able to identify ethylene as one of the decomposition products. DeMarB, Strausz, and Gunning in a detailed study of the Hg(6T1)-sensitized decomposition of methylenecyclobutane vapor determined ethylene and allene to be formed in equal quantities along with smaller amounts of hydrogen, methylcyclobutane, isoprene, and 1,4-pentadiene, They discussed the mechanism in terms of a vibrationally excited triplet state subject to a stepwise deactivation or decomposition via two or more reaction modes. The decomposition of an activated methylenecyclopropane molecule formed by the addition of methylene to allene has been investigated in detail by FreyS2 The main products of this decomposition were ethylene and acetylene ("60%), butadiene-1,3 (=20%), dimethylacetylene (=lo%), and ethylacetylene (%lo%), although the relative amounts of these products were dependent on pressureand to a lesser extent on the mode of formation of the methylene group added to the allene molecule. Two investigations carried out independently by Chesicka and Blrandaur, Short, and Kellner4 were con-
cerned with the pyrolysis of methylenecyclobutane. Both studies indicated that the pyrolytic products were cleanly ethylene and allene and the activation energy of the first-order decomposition reaction was 63.3 and 61.5 kcal mole-', respectively. Recently a study made in this laboratory6 on the photolysis of the aliphatic allenimines indicated that the decomposition was, for the main part, intramolecular processes from one or more excited species. The present study was initiated to determine if the methylenecycloalkanes, with structures quite similar to the allenimines, would follow a similar type mechanism on direct photolysis. Evidence from Frey's investigation2 of the activated methylenecyclopropane molecule in which several paths are followed and the product distribution is somewhat variable would seem to show that the mechanism is quite complex. On the other hand, the Hg(63P~)-sensitized1b decomposition and the pyrolysis3p4of methylenecyclobutane follow a much simpler intramolecular mechanism to a large degree.
Experimental Section Direct photolyses in this investigation were carried out with a fused-silica cylindrical cell, 31 mm in diam(1) (a) B. B. Loeffler, E. E. Eberlin, and L. W. Piokett, J . Chem. Phys., 2 8 , 345 (1958); and (b) G. R. DeMarB, 0. I?. Strausz, and H. E. Gunning, Can. J. Chem., 44, 953 (1966). . (2) H.M.Frey, Trans. Faraday SOC.,57, 951 (1961). (3) J. P.Chesick, J . Phys. Chem., 65, 2170 (1961). (4) R. L.Brandaur, B. Short, and 8. M. E. Kellner, J. Am. Chem. Soc., 84, 3411 (1962). (5) R. K.Brinton, J . Phys. Chem., 68, 2652 (1964).
Volume 78,Number 1 January 1968
R. K. BRINTON
322 eter by 249 mm (189 cma). These irradiations were made with an end-on 1000-w water-cooled Hanovia ac hydrogen arc. No optical system or filter was used, but the design of the lamp produced a beam collimated, a t least to a limited degree. The absorption characteristics of methylenecyclopropane, given below, and the short-wavelength cutoff of fused silica limit the absorption to the region 1750-2450 A. Wavelength, A-
Absorption
2400
2360
2300
2260
2200
12
59
237
840
1135
Methylenecyclobutane and methylenecyclopentane absorb in a similar fashion. Mercury (63P1) sensitized photolyses were carried out by use of a low-pressure mercury arc of 30 cm in length, operated at a constant current of 100 ma ac. Light was taken from an end-on window, filtered through a 2-mm Vycor plate, and roughly collimated by use of a fused-silica lens and restrictive apertures. Light intensities were determined by using n-butane as an actinometer. Actinometer determinations were made after two or three methylenecyclopropane photolyses, and only small differencescould be detected in the lamp intensity. The quantum yield of hydrogen from n-butane at room temperature (25') and a pressure of 200 mm was taken as = 0.50, from the data of Bywater and Steacie.O Complete quenching was assumed for both the n-butane and methylenecyclopropane at all pressures. A few mercury-sensitized experiments were carried out with a helical Vycor tubing low-pressure mercury arc of about 1m in length, whose axis was placed parallel to the cell axis. Use of this intense source allowed rather high percentage decompositions in relatively short periods. The lamp was operated at 100 ma ac, and variation in the absorbed intensity was obtained by altering the distance between the lamp and the cell. In all sensitized experiments, a pool of mercury lay along the bottom of the absorption cell. Products from both the direct and sensitized experiments were separated by temperature fractionationusing Ward stills and a gas-buret collector in a greaseless vacuum system. The noncondensable gases were collected from a trap a t -210" and the C2H2-C2HrCaH4 fraction at - 155". In general, the noncondensable fraction was very small (0.14.2 rmole) and was contaminated by small amounts of residual gases, mainly Nz,from the system. Because of the small sample size, analysis was not made except for the few irradiations carried to substantial per cent decomposition. The size of this noncondensable fraction, however, fixes an upper limit for H2 production in the various photolyses. It was noted, in general, that its size was fairly constant and thus experiments with larger C2Hz-C2H4-CsH4 yields indicated smaller upper limits for H2 yield. The fraction separated a t - 155", containing mainly C2Hz The Journal of Physical Chemistry
and Ci", always had small amounts of methylenecyclopropane, C02 (not completely degassed from the starting material), and C3H4. The small amount of CaH4 (allene) was difficultly volatilized at 155" and thus a possibility existed for traces to remain behind in the residue. Thus, the yield of C3H4determined by analysis is of lower reproducibility than those of CZHZand CzK. Analysis of the light-gas fractions was made by a Consolidated 21-103 or 21-104 mass spectrometer. The residue containing the unreacted cycloalkane was analyzed by gas chromatography (2 m X 4 mm in diameter column of adiponitrile on firebrick, temperature 0"-ambient). Residue samples from photolyses of methylenecyclopentane were also chromatographed through a 500-ft, 0.03-in. i.d. stainless steel capillary column, coated with General Electric 96 (50) silicone fluid. Methylenecyclopropane was prepared by the method described by Anderson,? from a commercial sample of chloroisobutene. Final purification was made by a successive fractionation on three different preparative gas chromatographic columns. Elimination of some of the C4Hsisomers (especially bicyclobutane) was difficult. The final product contained less than a detectable 0.03% impurity. Methylenecyclopentane was obtained from the API; gas chromatograms made on several columns and under varying conditions did not reveal anything except trace impurities. Methylenecyclobutane was purified from commercially available samples (60430% purity). By use of successive fractionation on different gas chromatographic columns, a product of purity greater than 99.95% was obtained.
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Results Products produced in the methylenecycloalkane photolyses are summarized in Tables I and 11. In the case of methylenecyclopropane irradiated with the hydrogen arc, ethylene and acetylene are produced in closely comparable amounts; C2H2:CzH4 = 0.97 0.015 for MCP-1, 4, 5,6, a ratio differing from unity by no more than the experimental error of the determination, although the slightly lesser yield of CzHz may well be real and significant. For the series of mercury-sensitized experiments a t varying pressure, MCP-7,8,9,10, 11, 12, 13, CnH2:C2H4= 0.941 f 0.017, clearly demonstrating that the yield of CzH4 is greater than C2Hz under all of the pressure conditions tested. In addition, no systematic change in ratio seems to accompany the pressure variation, Oxygen added to the system in the direct photolysis (MCP-3) produced an appreciable yield of COz, COz:CzH4 = 1.38, but the C2H2:C2H4 ratio was (6) (a) 8. Bywater and E. W. R. Steacie, J . Chem. Phys., 19, 172 (1951); and (b) S. Bywater and E. W. R. Steacie, ibid., 19, 319 (1951). (7) B. C. Anderson, J . Org. Chem., 27, 2720 (1962).
THEGASEOTJE, PHOTOLYSIS
323
OF THE n4ETHYLENECYCLOALKANES
Table I : Methylenecyclopropane Photolysis Products Temp, OC
MCP-1
25
MCP-2 MCP-3' MCP-4 MCP-5 MCP-6 MCP-7 MCP-8 MCP-9 MCP-10 MCP-11 MCP-12 MCP-13
25 25 25 150 25 25 25 25 25 25 25 25
Mole cm-8 sec-1
x
1018.
p, mm
131 137 130 136 132 29.5 508 302 250 164 100 30.4 10.2
Deoomp,
Rate,a CZH4
--Relative CaHa
yields (CZH4) CaH8
-
1 O.O-
Arc
%
Hz helical 2537
0.10
8.4
0.98
0.024
