Reactions of Chemically Activated Cycloalkyl Radicals
191
TABLE IV: Wavelength Dependence of Product Yields C'2F4,
A,,,
nm 405 366 334 325 313
Torr
0.11 20.31 4.4 High
C3F6)
C3F6)P~0
Phillips This work k i o / k 9
> 0.01 -0.03 1.2 1.5
0.4 2.2
15 4
0.3 1.4 10 27.7
pear to be too high compared to our measured values. There are other minor discrepancies but it is probably not worthwhile to devote more space to them. The analogy between cyclobutanone and PFCB in their photochemical transformation mechanism is very impressive.
Acknowledgment. This research has been supported by the Office of Naval Research (Contract No. N00014-69A0200-9005). We thank Dr. D. C. England (E. I. du Pont de
Neumours and Co.) for the generous gift of the perfluorocyclobutanone sample and also for pointing out the extreme reactivity of difluoroketene. We also thank Professor Alan S. Rogers (Texas A&M University) for providing us with the thermochemical information. References a n d Notes ( 1 ) R. S. Lewis and E. K. C. Lee, J. Chem. Phys., 61, 3434 (1974). (2) J. C. Hemrninger and E. K. C. Lee, J. Chem. Phys., 56,5284 (1972). (3) N. E. Lee and E. K. C. Lee, J. Chem. Phys., 50,2094 (1969). (4) H. 0. Denschlag and E. K. C. Lee, J. Amer. Chem. SOC., 90, 3628 (1968). (5) D. Phillips, J. Phys. Chem., 70, 1235 (1966). (6) (a) R. B. Cundall, F. J. Fletcher, and D.G. Miine, Trans. Faraday SOC., 60, 1146 (1964);(b) W. A. Noyes, Jr., and K. E. AI-Ani, Chem. Rev., 74, 29 (1974). (7) At 5 Torr, 14.7-cm path length, and 366 nm where the extinction coefficient is highest, only 1 1 % of the actinic light is absorbed. (8) See, for example, R. G. Shortridge, Jr., and E. K. C. Lee, J. Amer. Chem. SOC., 92, 2228 (1970). (9) T. H. McGee, J. Phys. Chem., 72, 1621 (1968). ( I O ) E. K. C. Lee in "Excited State Chemistry," J. N. Pitts, Jr., Ed., Gordon and Breach, New York, N.Y., 1970, p 59. ( 1 1 ) D. C. England and C. G. Kaspan, J. Org. Chem., 33,816 (1968). (12) A. M. Halpern and W. R. Ware, J. Chem. Phys., 53, 1969 (1970). (13) A. Gandini, D. A. Whytock. and K. 0. Kutschke, Roc. Roy. SOC., Ser. A, 300, 529 (1968). (14) A. S. Rogers, private communication.
Ring Opening and Isomerization of a Series of Chemically Activated Cycloalkyl RadicaIs1a S. E. Steinlb and B. S. Rablnovltch* Department of Chemistry, University of Washington, Seattle, Washington 98 195 (Received August 16, 1974) Pobllcation costs assisted by the Office of Naval Research
Unimolecular reactions of a series of chemically activated (-40 kcal mol-l) cycloalkyl radicals, vibrationally excited to the region of -45 kcal mol-l, were studied in the gas phase. Cyclopropylethyl, cyclobutyl, cyclopentyl, and cyclohexyl radicals were produced by H-atom addition to the appropriate olefin. Straight chain ring-opened products were observed for the first three of these radicals, while cyclopentylmethyl was the major product of the fourth. Critical threshold energies, Eo, for these reactions have been deduced. Eo for cyclopropylethyl ring opening is -16 kcal mol-l, which is less than that for the other reactions by a t least 10 kcal mol-I; this indicates that for this reaction the ring strain in the activated complex is much less than that in the molecule and the findings can be related to studies on homoallylic rearrangements. For the other cyclic radicals, Eo is much closer to values found for corresponding acyclic processes, Le., 31-33 kcal
Introduction The elucidation of the role of ring strain energy in the ring opening of cyclic radicals as well as its reverse, the intramolecular addition of a radical function to a double bond, has received considerable attention in recent years.2 Gaseous c y c l o ~ e n t y l methylcyclobutyl-1,3a ,~ and, perhaps, cyclobuty14 radicals have been found to undergo ring opening with a threshold energy, Eo 30-35 kcal mol-l. In addition, the thermal A factor for the ring opening of the cyclopentyl radical has been calculated to be 101*.O from a comparison of the rates of ring rupture and ring formation a t two different levels of vibrational e ~ c i t a t i o n Thermal .~~
-
-
studies of the ring opening of cyclopropyl radi~al58,~ gave E, 20 kcal mol-' which indicates that the activated complex for this reaction is loosened and is considerably less strained than the molecule. Due to inconsistencies in the cyclobutyl ~ o r k it , is~uncertain ~ ~ ~ whether this result represents a break from the cyclobutyl behavior. Gordon3c has postulated that cyclopentyl may decompose directly to ethylene and propyl radical. No quantitative results for the ring opening of radicals larger than the cyclopentyl radical have been reported. In order to clarify the effect of ring size on kinetic parameters for ring opening, we have examined a series of The Journalof Physical Chemistry, Vol. 79, No. 3, 1975
192
chemically activated cyclic radicals having ring sizes from three to six members. Radicals were produced by H-atom addition to the appropriate cyclic olefin. The ring opening of activated sec- cyclopropylethyl radical (SCE.) has been studied. This was the only radical investigated in the study in which the unljaired electron was not on the ring, and an activated complex somewhat different from that for a substituted cyclopropyl radical could result. This system is relevant to the kinetics and mechanism of homoallylic rearrangement. Ring opening of chemically activated cyclobutyl (CB.) and cyclopentyl (CPn.) radicals was studied in order to clarify the nature of these reactions. The unimolecular reaction of activated cyclohexyl radicals (CHx.) has been investigated quantitatively. The larger size of this radical may result in an increased probability for a new reaction path, the intramolecular ring contraction to a five-membered ring.zc During the late stages of the work, a preprint of ref 3a was received in which the chemically activated ring opening of CPn- and methylcyclobutyl-1 radicals was described.
Experimental Section Materials. Ultrapure Airco Hz was further purified by passage through silica gel a t liquid nitrogen temperature and through a column containing BTS catalyst. Vinylcyclopropane, cyclobutene, cyclopentene, and cyclohexene were obtained from Chemical Samples Co. and were further purified by gas-liquid-phase chromatography. High-purity Phillips ethylene was similarly purified. Apparatus a n d Procedure. This chemical activation technique has been used in the past for a large number of studiesU6Experiments were made with a conventional high vacuum Pyrex vacuum system fitted with Teflon stopcocks containing Viton 0 rings. Three different size reaction vessels were used: a Pyrex 12-1. vessel fitted with a quartz window for low pressures ( < l o Torr), a 500-cc quartz vessel for medium pressures (10 to 100 Torr) and a 50-cc quartz vessel for high pressures (>lo0 Torr). A G.E. 8-W germicidal lamp fitted with a filter of variable intensity was used for the Hg photosensitization of Hz,Pressures were measured with a Hg manometer, and the vacuum was checked periodically with Pirani and thermocouple gauges. In all runs, the ratio of Hz to substrate was greater than 50 to 1. Reaction mixtures were made up in the following way. A measured amount of olefin was frozen into the reaction vessel which was then pressurized with Hz;in some runs, a mixture of olefin and Hz was made up and then was transferred to the reactor. Reaction times varied from 10 min to 2 hr. The percentage reaction varied from 0.2 to 1.5%. After low pressure runs, the reaction vessel was pressurized with H2 to facilitate collection by pumping through a glass wool packed trap a t -195O. The condensate was then transferred to the injection trap of the chromatograph in a slow flow of Hz. Analysis. The products were separated on a 100-ft. SCOT squalane column and were identified by calibration with authentic compounds. The identity of some peaks was checked with a AgNO3-glycol column in series with the squalane column to characterize the olefinic nature of the unknown. A flame ionization detector was used. Peak areas were proportional within 5%to their concentration corrected for carbon number.
Results sec-Cyclopropylethyl Radical. Hydrogen atoms were The Journal of Physical Chemistry, Vol. 79, No. 3, 1976
S.E. Stein and B. S.Rabinovitch
found to give 97.5% terminal addition to vinylcyclopropane:
H
+
P C = C
-
AHI", = -39.0 kcal mol-'
[>-CC*
(1)
Vibrational deexcitation (stabilization) by a bath molecule ( M E H2) can occur
[>-CC*
+
M
-% F C C
(s)
(2)
Exothermic ring opening may also occur
D Cc* cc=ccC.* A H " , = -5 kcal mol-' (3) This ring-opening reaction was found to be so fast that SlD could be measured at the highest pressures only (Table I). No clear dependence of S/D on pressure could be measured. Aside from the reverse of reaction 3, the excited pent-2-en-5-yl radical may undergo the following unimolecular decomposition and isomerization or collisional stabilization cc=ccc.*
-
%
[9] ---t
c=c=ccc*
(4c)
cc..;c-cc* % cc..=c..w (Sac) (4d Because of the low activation energy of reaction 3 (relative to the ring-opening activation energies for most @C-C bond rupture in radicals), it was necessary to work at relatively high pressure (p > 200 Torr) before the apparent stabilization of SCE. became measurable. At these pressures, the amounts of Dz and D4 were negligible relative to SBaand S5b. It is uncertain how much S Bwas ~ produced by reaction 4f (see below). Runs were carried out with ethyl radical-getting in reaction mixtures having ethylene: c- C3H&=C ratios of 2 8:l. The results are shown in Table I. Ethyl radicals were produced by H 3. C=c -+E.* (Sa)
cc.* 2% cc.
(5b)
The reverse of (5a) is negligible a t the pressures used. Ethyl-getting was used in order to check the results in selfgetted systems as well as to eliminate decane or higher products which were difficult to handle. Ethyl-getting was preferable to self-getting in this system, also, because S/D was 119601. --(18) J. H. Georgakakos, E. S. Rabinovitch, and E. J. McAlduff, J. Chem. Pbys., 52, 2143 (1970). (19) S. W, Benson, "Thermochemical Kinetics," Wiley, New York, N.Y., 1967. (20) S. Arai, S. Sata, and S. Shida, J. Chem. Phys., 33, 1277 (1960). (21) R. C. Lamb, P. W. Ayers, and M. K. Toney, J. Amer. Chem Soc, 85, 3483 (1963). (22) C. W. Larson, Ph.D. Thesis, University of Washington, Seattle, Washington, 1970. (23) J. 0. Hlrschfelder, C. F. Curtiss, and R. E. Bird, "Molecular Theory of Gases and Liqulds," Wiley, New York, N.Y., 1967. (24) F. D, Rossini, "Selected Values of Physical and Thermodynamic Properties of Hydrocarbons," Carnegie, Pittsburgh, Pa., 1953. (25) Secondary and primary C-H dissociation energies are as used in similar work, see, e.g., ref 22. C-H dissociation energies for the cyclic hydrocarbons are assumed to be as follows: Doo(cyciic paraffin C-Hj = D o0 (sec-acyclic C-H) [ERs(cyciic olefin) - ERs(cyciic paraffin)]/2. --I
+
