574
Unimolecular Reactions Subsequent to Recoil Tritium Reactions with Dichloromethane. Formation of Monochlorocarbene’ Yi-Noo Tang and F. S. Rowland2 Contribution from the Departments of Chemistry, University of Kansas, Lawrence, Kansas, and University of California, Irvine, California. Received October 7 , 1967
Abstract: The reactions of recoil tritium atoms with dichloromethane produce CHTC12*and CH2TCl*by substitution of T for H and T for C1, respectively. Unimolecular decomposition of CHTC12* proceeds largely through elimination of hydrogen chloride, with the formation of monochlorocarbene in the singlet state. The latter was
detected through its addition to ethylene to form cyclopropyl-t chloride in both the presence and absence of O2as scavenger. The excited CH2TCl*molecules decompose by C-Cl bond breakage with no evidence for carbene formation as a competitive process. The relative yields of the primary hot reactions with CHIClzare in the ratio HT:CHTCh:CHzTCl = 100:51 =t5 : 2 5 f 2. These values were determined in scavenged liquid-phase experiments and are very different from previously measured values because of extensive unimolecular decomposition of the substitution products in the gas phase. T* + CHzC12 +H T
+ CHCI, T* + C H G +CHTClz + H T* + CHzClz +TC1 + CHZCI T* + CHzC12 CHzTCl* + C1
E
nergetic tritium atoms from nuclear recoil are able to react with most organic compounds to form various radioactively labeled compounds, many of which possess considerable excitation energy. 3 , 4 Study of the replacement of H by T in cyclobutane, as in reaction 1, has indicated that the median excitation enT* + R H e R T * + H
(1)
ergy of the resulting c-C4H7T molecule is about 5 ev and is therefore large enough to produce secondary reaction by paths characteristic of unimolecular decompositions through thermal e ~ c i t a t i o n . ~Investigation of the decomposition induced by such primary reactions has disclosed several kinds of secondary reaction: cyclobutane decomposes to two molecules of ethylene;5 methyl chloride splits the C-CI bond to give CH2T and C1;e and ethyl chloride undergoes the molecular elimination of HCI.? These earlier studies have encouraged the use of recoil tritium substitution reactions as a means for investigating the unimolecular decompositions of other molecules,8 and these techniques have been applied here to the labeled species formed in hot tritium reactions with dichloromethane. The four hot tritium reactions expected with dichloromethane from earlier studies of the alkyl halidesgv10 are the abstraction of, and substitution for, atoms of either hydrogen or chlorine, as given in eq 2-5. The species of prime interest in this system for a study (1) This research was supported by AEC Contracts No. AT-(I 1-1)-407 with the University of Kansas and AT-(ll-1)-34, Agreement No. 126, with the University of California. Part of the work was submitted by Y.-N. Tang in partial fulfillment of the Ph.D. requirements of the University of Kansas. Presented in part at the 150th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 1965. ( 2 ) Author to whom inquiries should be addressed at the University of California. (3) F. Schmidt-Bleek and F. S . Rowland, A n g e w . Chem. Intern. Ed. E n d , 3, 769 (1964). (4) R. Wolfgang, Progr. Reaction Kinetics, 3,97 (1965). ( 5 ) E. K. C. Lee and F. S . Rowland, J . Am. Chem. Soc., 85, 897 (1963). (6) Y.-N. Tang, E. K. C. Lee, and F. S . Rowland, ibid., 86, 1280 (1964). (7) Y.-N. Tang and F. S . Rowland, ibid., 87, 3304 (1965). (8) F. S . Rowland, E. K. C. Lee, F. Schmidt-Bleek, and Y.-N. Tang, “Symposium on the Kinetics of Pyrolytic Reactions,” Ottawa, Canada, 1964, pp Q-1-Q-8. (9) R. Odum and R. Wolfgang, J . Am. Chem. Soc., 83,4668 (1961). (10) R. Odum and R. Wolfgang, ibid., 85. 1050 (1963).
Journal of the American Chemical Society
(2) (3) (4) (5)
of unimolecular reaction mechanisms are the two molecules formed by the substitution reactions, CHTC12* and CHZTCI*. The latter has previously been formed in the methyl chloride system, and a comparison of the decompositions of the same molecule formed in two different systems is therefore possible. Additional information was also expected from the study of excited dichloromethane-t, whose unimolecular reactions were not completely understood. The specific possibility that CHTC12* might decompose by the elimination of HCl,” as in reaction 6, was considered because of the earlier suggestion of this reaction in pyrolytic decomposition,12 and the well-known liquid-phase formation of dichloromethyllithium from dichloromethane reactions with organolithium compounds. 13-l5 Confirmation that this reaction indeed occurs has been published in a brief communication. l e CHTC12* +CTCl
+ HCI
(6)
The study of recoil tritium reactions with dichloromethane is also of considerable interest for the understanding of the primary reactions of the energetic tritium atoms themselves. The series of experiments carried out in the gas phase for a variety of halocarbons had previously shown that the ease of replacement of halogen atoms, as measured by the appearance of the T-for-X product, varied substantially with the nature of the other substituents adjacent to the original studies from our laboratory C-X b ~ n d . ~ ~Previous ~O of reactions with methyl6 and ethyl chloride? have indicated that a complete study of the pressure and phase dependence of product yields is usually nec(11) The alternate isotopic reaction of elimination to TC1 would also be expected, but the TCI so formed would be indistinguishable experimentally from that formed in reaction 4. (12) A. E. Shilov and R. D. Sabirova, Dokl. A k a d . Nauk S S S R , 114. 1058 (1957). (13) G.L. Closs and L. E. Closs, J . A m . Chem. Soc., 82, 5723 (1960). (14) G.L. Closs and G.M. Schwartz, ibid., 82, 5729 (1960). (15) G.L. Closs, ibid., 84, 809 (1962). (16) Y.-N. Tang and F. S . Rowland, ibid., 87, 1625 (1965).
90:3 1 January 31, 1968
575
essary in order to make a reasonable estimate of the original primary yield prior to decomposition of some of the excited molecules. The current experiments with CH2C12furnish the primary yield data needed for comparison with the earlier alkyl halide data.
Experimental Section General Procedure. These experiments were carried out by the usual techniques employed in studying the chemical reactions of tritium atoms produced by neutron irradiation of He3 in the gas phase and LiB in heterogeneous liquid-solid systems. 3 , 6 , 7 Gas samples containing CH2C1? up to its room-temperature vapor pressure (about 0.5 atm), He3, and other additives were sealed in Pyrex 1720 bulbs of about 12-ml volume. Liquid samples were condensed onto lithium fluoride at the bottom of capillary tubes, The neutron irradiations were carried out either at the TRIGA reactor of the Omaha Veterans Administration Hospital, or at the University of Kansas reactor, each with a nominal neutral flux of 10” neutrons/(cm2 sec). The gas samples were usually irradiated for 1 hr, while the liquid samples wereirradiated for 3 hr. The analysis of the samples after irradiation was performed by radio gas chromatography. 17,18 These techniques, except for the particular separation columns employed, have been previously described for methyl chloride6 and for many other systems, Accurate measurement of the labile tritium (including TCI, and atoms bonded to 0, I, Br, etc.) was not possible with these chromatographic techniques, since isotopic exchange would have occurred in handling or on the columns. Accordingly, this tritium was removed from the gas sample by isotopic exchange on a cotton plug in the flow system during transfer of the sample for gas chromatography. The gas chromatographic columns employed in the analysis, with the order of separation observed on each, are described elsewhere. l 9 Cyclopropyl-f chloride and CHTCICH=CH, are not separated on the 50-ft tri-a-tolyl phosphate column ordinarily used, but were readily separated with the addition, in series with the TTP column, of a 20-ft AgN03-ethylene glycol column. Quenching of Proportional Counter Action by Halocarbons. Dichloromethane, like many other alkyl halides, has an electron affinity sufficiently high to initiate quenching action within the proportional counter. Comparison of the macroscopic CH,CI, content of each aliquot with a calibration curve for its quenching action under our operating conditions showed an over-all correction for CHTCI, yield less than the statistical error of counting.19 Chemicals. Spectroquality dichloromethane was obtained from Matheson Coleman and Bell, checked by gas chromatography for purity, and used without further processing. Ethylene was obtained from the Matheson Co., with an impurity level less than 0.5%. Other chemicals such as cyclobutane, He3, LiF, O,, 12, and Bracame from the same sources as described earlier.5-7
Results Variation of Product Yields with Pressure. Absolute Yields. Several different methods have been employed in earlier studies for the comparison of results at different pressures, including (a) absolute measurement of the total radioactivity among the various products of reaction with cyclobutane ; (b) comparison of relative yields in binary mixtures with cyclobutane or with methane; and (c) intramolecular comparison us. the yield of a product for which the pressure dependence is known or can be reasonably ass~med.~-7 Any one of these methods would be potentially satisfactory for establishing the dependence of product yields upon pressure in a system under investigation. However, the heterogeneous nature of the samples used in liquid-phase irradiations makes impossible the accurate measurement of absolute yields in such experiments. The experimental conditions for neutron (17) R. Wolfgang and F. S. Rowland, Anal. Chem., 30,903 (1958). (18) J. K. Lee, E. K. C. Lee, B. Musgrave, Y.-N. Tang, J. W. Root, and F. S. Rowland, ibid., 34,741 (1962). Tang, Ph.D. Thesis, University of Kansas, 1964. (19) Y.-N.
irradiation limit the gas-phase measurements to roughly the pressure range from 10 to 35 cm for CH2ClZ,corresponding to a change of only about 10% in the yield of CHTC12, as established in measurements described below. Moreover, accurate absolute measurements in the gas phase require accurate knowledge of the stopping powers of the components in order to correct for recoil loss to the walls at the lower pressures;Zo the uncertainty involved is easily of the same order of magnitude as the expected change in yield with pressure. For these reasons, we have not used method a to establish the dependence of product yields on pressure in the CH2Ch system. Absolute measurements of yield were performed with Br2-scavenged gaseous samples of CHzClZ at approximately 0.5 atm pressure, using pure propane samples as monitors for the neutron flux. The absolute yields measured in these experiments were (in per cent of total tritium formed): HT, 18.7 f 1.8; CHzTCl, 1.6 f 0.2; CHTCh, 2.8 f 0.4; CH2TBr, 2.9 0.4. These results are in excellent agreement with the published results of previous experiments by Odum and Wolfgang. l o Competition Reactions with Cyclobutane. The yields from recoil tritium reactions with CHzClz and cyclobutane in direct competition with one another are shown in Table I. Comparison with the sum of the yields of c-C4H7Tand C2H3T (shown to be pressure independent in experiments with cyclobutane alone) indicates that the yield of CHTClz is higher in the liquid phase than in the gas phase by more than a factor of 2.z1 The variation of yield with pressure anticipated for the small available range of gas pressures was not very great and falls within the relatively large limits of error for such binary mixture experiments.21 Pressure Dependence Studies. Halogen-Scavenged Samples. In earlier studies, the yield of H T from recoil tritium reactions was shown to be virtually independent of pressure for cyclobutane, methyl chloride, and ethyl chloride, and was therefore a logical choice as an intramolecular standard for experiments with CH,Cl,. The results from a series of gas- and liquidphase experiments with Brz as the scavenger molecule are shown in Table 11. The mole fraction of Brz is approximately the same in all samples, and variations in reactivity with mole fraction should be unimportant. Table I11 contains the data for a comparable series of Iz-scavenged gas-phase experiments, and includes the earlier data of Odum and Wolfgang for comparison. lo All data in both Tables I1 and I11 have been expressed with the yield of H T = 100 as the relative base. The agreement between the two sets of data is quite satisfactory, except for the yields of CHTClX. We believe that the combined process of scavenging of the original radical, and preservation of the molecule through the analytical procedure, is more efficient for CHTClBr than for CHTClI, and we have based our later quantitative estimates on the Brz-scavenged data of Table 11.
*
(20) See, for example, J. W. Root, Ph.D. Thesis, University of Kansas, 1964. (21) These experiments were not conducted at constant mole fraction, or over a range of mole fractions of the cyclobutane-dichloromethane mixtures. However, numerous experiments in other systems imply that variations in relative specific yields with mole fraction for these two species would be much smaller than the differences observed between gas and liquid phases: J. W. Root and F. S. Rowland, J . Chem. Phys., 38, 2030 (1963); see also ref 20.
Tang, Rowland / Formation of Monochlorocarbene
576 Table I. Product Yields from Recoil Tritium Reactions with Gaseous and Liquid Mixtures of CH2CIZand Cyclobutane Sample composition (CHzClz c-CaHs) Scavenger Tritium source Mole ratio” CHzCh/c-CaHs Re1 activities CHTClz c-C~H~T c-C4H7Tb/(c-C4H7T CZHIT) Substitutionc probability per C-H bond
+
+
5.9 cm 0.9 0 2 1.5 cm He3
27.6 cm 1.9 0 2 1.5 cm He3
Liquid Brz LiF
Liquid
2.96 f 0.20
0.71 rt 0.04
1.35 f 0.07
0.62 f 0.03
1435 Z+ 50 2815 f 60
1790 f 50 17,540 f 150
6210 =t 90 22,500 f 160
11,700 f 120 86,400 f 300
0.47
0.54
0.82
0.82
0.33 =t 0.03
0.33 i 0.02
0.68 f 0.04
0.72 =t 0.03
I2
Li F
Ratios in pure cyclobutane at same total pressure, from ref 5.
0 Measured by thermal conductivity. GH7T C~H~T)/~(C-C~H~).
+
(CHTC~Z)/~(CHZCIZ) f (c-
Table 11. Variation of Product Yields with Total Pressure in Recoil Tritium Reactions with Bromine-Scavenged CHZCI2 Gas pressure, cm CHzCIz 7.4 He 3 1.9 Brz 1.2 Total pressure, cm 11. 5 Product HT 100 CHTClz 14.2 CHzTC1 6.7 CHzTBr 15.3 CHTClBr 16.9 a
21.6 1.9 3.0 26.5
11.5 1.9 1.7 15.1 f 0.4 Z!= 0 . 2 f 0.4 f 0.6
100 15.3 7.3 16.3 16.8
Z+ 0.3
f 0.2 f 0.3 f 0.5
100 15.8 7.7 16.3 17.4
29.7 1.9 4.1 35.7
f 0.3 Z!= 0 . 2 f 0.3 =t 0.5
100 15.5 8.1 15.9 16.2
32.5 2.0 4.2 38.7
(-14) LiF -2, Liquid
100 100 15.2 f 0 . 4 30.3 8.7 i 0 . 2 23.0 15.5 f 0.3 3.4 ... 11.0
Z!= 0.4
Z+ 0 . 2
f 0.3 rt 0.5
(-18) LiF (-2.5) Liquid =t 0.6
f 0.4 =t 0 . 2 f 0.5
100 27.3 21.5 2.6 10.3
f 0.6 =t0.4 f 0.2 f 0.5
Lost in handling.
Table 111. Variation of Product Yields with Total Pressure in Recoil Tritium Reactions with Iodine-Scavenged CH2Cl2 Gas pressure, cm CHzCh He 3 I2 Total pressure, cm Product* HT CHTClz CHzTCI CHzTl CHTCII CH3T a
Not measured.
b
14.8 1.5 Yes 16.3
8.1 1.5 Yes 9.6 100 13.9 6.9 15.3 6.4
