1876
J. Phys. Chem. 1980, 84, 1876-1881
Thermal Chlorine-38 Reactions with Propyne F. S. C. Lee and
F. S. Rowland*
Department of Chemistty, University of California, Irvine, California 927 17 (Received January IO, 1980) Publication costs assisted by the Division of Basic Energy Sciences, Department of Energy
Thermal 38Clatoms are formed by moderation in collision with CClF3of the extra kinetic energy given to the recoil atoms from the 37Cl(n,y)38C1 nuclear reaction. These thermal 38Clatoms react with propyne at 25 "C almost entirely by addition, with less than 5% reacting by H-atom abstraction. Terminal addition is favored over central addition by a factor of 8 f 1 to 1. After stabilization by collision, the terminal CH3C=CH3%1 radicals react stereoselectivelywith HI to form almost exclusively c~s-CH~CH=CH~~C~. Addition to propyne radicals by 38Clis 1.9 f 0.2 times as rapid at 25 "C as reaction with HI to form H3Vl. Half of the C3H438C1* formed by thermal 38Claddition to propyne are collisionally stabilized by CClF3at a pressure of 250 f 50 torr. This half-stabilizationpressure is approximately equal to that found for C3HBBC1*radicals from 38cl plus propene and a factor of 3 slower than for CzH438C1*from 38Clplus CzH4.
Introduction The gas-phase reaction of thermal chlorine atoms with asymmetric alkynes provides an opportunity for study of the selectivity of attack of halogen atoms on the central and terminal carbon atoms of such molecules, as in eq 1 and 2. Such studies are particularly suitable for radio-
+ 38Cl + CH,C=CH
88C1 CH3C=CH
-
-
CH3C38C1=CH*
(1)
CH3C=CH3%1*
(2)
active tracer investigation using 38Clatoms thermalized by multiple collisions with CC1F3 after formation by exposure of gaseous CClF3to a thermal neutron flux in which the 37Cl(n,y)38C1 nuclear reaction can take place.lt2 In this system, only those reactions are recorded in which the initial attack on the propyne substrate is made by the radioactive 38Cl atom since the analytical technique of radio gas chromatography employed here only registers products containing radioactivity. Free radicals formed by 3sCl addition to propyne are subsequently converted by reaction with a suitable scavenger molecule (such as HI or HzS) into stable products measurable after gas chromatographic separation. Comparison of the selectivity of reaction with propyne can then be made with the addition of atomic chlorine to alkenes3 and with the addition of other halogen atoms to both alkenes and a l k y n e ~ . ~ -Al ~ series of such investigations can help to bring more complete understanding of the factors governing selectivity of halogen reactions with simple r-bonding systems. The detailed evaluation of the reactive fates for C1 atoms with propyne must include the hydrogen abstraction of reaction 3 and the competition between loss of 3sClfrom the excited radicals in reaction 4 and their collisional stabilization in reaction 5. The C3H438C1*radicals whose 38Cl+ CH3C=CH
-
+ CHzCrCH C3H4+ 38Cl C3H438C1 +M
H3%l
+ -
C3H438C1* C3H438C1* M
(3)
(4) (5)
excess energy has been removed in reaction 5 can then be converted into stable measurable products by reactions 6-8. The stabilized radical from reaction 1 is converted CH3(FC1=CH
+ HI
-
CH3C3'C1=CH2
+I
(6)
CH3C=CH38C1
-
+ HI
-
cis-l-[38Cl]chloropropene tI (7)
trans-l-[38C1]chlor~pr~pene tI
(8)
into 2- [38Cl]chloropropeneby reaction 6, whereas stereochemical information about the CH3C=CHC1 radical can be obtained from the relative yields of ~is-l-[~~Cl]chloropropene and trans-1- [38Cl]chloropropenefrom reactions 7 and 8. The only other 38Clreaction with more than a minor percentage yield in the CC1F3/propyne/HI system is the thermal reaction with HI (reaction 9). 3 ~ 1 HI +
-
~ 3 8 c 1 +I
(9)
Experimental Section Sample Preparation. The samples for irradiation were prepared with a standard grease-free vacuum line equipped with either Kern or Kontes Teflon stopcocks, following methods which have been described in detail elsewhere.2 The CC1F3 and HI were obtained in lecture bottles from Matheson in >99% and >98% purity, respectively, and were used without further purification. Propyne (Matheson, 96%) showed measurable, volatile impurities and was purified by preparative gas chromatography. All of the compounds were thoroughly degassed at -196 "C on the vacuum line before filling of the irradiation ampules. The sample ampules were made from Pyrex 1720 glass tubing, with volumes in the range 9-12 cm3. Neutron Irradiation. The formation of thermal 38Cl atoms by thermal neutron irradiation of CC1F3has been described previously.1+2All of the experiments in this series have involved gas-phase mixtures containing mole fractions of CClF, > 0.85 and of propyne < 0.06, reducing the translationally energetic reactions of 38Clwith propyne to negligible percentages. Less than 5% of the 38Clatoms form a stable chemical bond as a consequence of a translationally hot collision with CClF,, and the remaining 95% of the 38Clatoms become thermalized by multiple nonreactive collisions with the CC1F3serving as the moderator gas.'yZ The radicals formed by 38Clatom addition t o propyne are converted to stable molecules by subsequent abstraction of hydrogen from HI. The reactor power levels for the thermal neutron irradiations have been minimized, as well as the irradiation times (- 15 kW for 2 min), to reduce any radiation damage effects in the samples, following procedures worked out for the nuclear
0022-3654/80/2084-1876$01.00/00 1980 American Chemical Society
Thermal Chlorine-38 Reactions with Propyne
The Journal of Physical Chemistry, Vol. 84, No. 15, 1980
1877
COUNTS reaction 37Cl(n,y)38C1 with other a-bonded substrates.'V2 5,000 l2 We have estimated that the y-radiation effects accompanying these nuclear reactor irradiations can cause macroscopic decomposition of about 0.0190 of the CClF, and about 1% of Ihe prcspyne under typical experimental 4'000 COLUMN SWITCH conditions. All neutron irradiations were carried out in the Lazy Susan facility of the TRIGA nuclear reactor of the University of California, Irvine. The neutron fluxes were monitored through the observation of radioactive 4 1 ~ formed from the " A r ( n , ~ ) ~ l Anuclear r reaction on small 2 ,000 T W - I -%I - PRO2 % I - PROPENE concentrations of Ar included in each ~amp1e.l~ PE:NE 1 After irradiation each sample was held briefly in the I,000 reactor area for radiation-safety purposes while short-lived it radioactivities induced in the glass ampule largely decayed away. The total elapsed time from the end of the nuclear reactor irradiation to injection onto the analytical gas RETENTION TIME IMIN.) chromatographic column usually was about 20 min. Figure 1. Radio gas chromatogram of 38CI atoms with propyne) In Radio Gas Chromatographic Analysis. The primary CCIF,/propyne/HI mixtures. The measurements began with a 2 4 4 analytical task in these experiments is the radio gas PCA column in series after a 3 5 4 TTP column. The PCA column was chromatographic separation of 2-[38C1]chlor~pr~pene and withdrawn at the 19-min mark after emergence of the "Ar peak. The the two isomeric 1-[38C1]chloropropenes. The standard TTP column was operated at 50 OC with a flow rate of 36 mL/min. radio gas chromatographic analytical arrangement employed am external gas-flow proportional counter and has The radio gas chromatographic analysis of a typical been described elsewhereaZNo direct measure was made sample is illustrated in Figure 1. The data are shown as of HSC1 jfrom reactions 3 or 9, but the former can be shown recorded and are corrected later for the progressively larger not to be the major reaction with propyne by the obserfractions of radioactive decay of 38Clwith increasing revation of' as much as '75% of the total 38Clradioactivity tention times prior to emergence of the chromatographic in the form of organically bound products. Hydrogen peaks. The retention time of 80 min for t r a n ~ - l - [ ~ ~ C l ] iodide was irreversibly absorbed from the gas flow stream chloropropene corresponds to 23% survival of the 3L'Cl with a 3-in. "stripper" column of K4Fe(CN)6.3H20.The present in this chemical form at the time of gas chromaorganic compounds were found not to be affected, even in tographic injection, and only about 15% of that present tracer concentrations, by the presence or absence of this at the end of the neutron irradiation. The largest perstripper column in the analytical system. Authentic samcentage yield among the 38Clcompounds is obviously that ples of these three chloropropenes,and of 3-chloropropene and the radio gas chromafor cis-l-[38Cl]chloropropene, as well, were available for calibration of various gas chrotogram is quite clean in the (&-(& region, with no further matographic columns. radioactivity peaks being observed for an additional 2 h Consecutive measurements of all of the %C1radioactive after the period illustrated in Figure 1. A yield of about peaks from a single chromatographic aliquot are made 0.5% was regularly observed corresponding to C2Hb3%l, difficult by the wide range of boiling points involved, tovarying little with any changes in the experimental pagether with the 37.3-min half-life of %C1. Most radioactive rameters. The origin of this peak has not been establisbed samples were analyzed with a 35-ft tritolyl phosphate but is believed to be largely from CzH4 present as a trace (TTP) column plus a 24-ft propylene carbonate-on-aluimpurity (as little as 0.01% is sufficient).lV2 In several mina (PCA) column in series. The TTP columns (several samples, a very small radioactivity peak (0.03-0.10%) was were used with slightly varying characteristics) were observed corresponding to the retention time for 3operated at a constant temperature in the range from chloropropene. In the rest of the samples, however, this 50-65 "C, while the PCA column was maintained at room yield was below the level of detection (99% formation of the cisterrhinal/central ratio (isomerization to the more favorable 1-bromopropene in both the gas and liquid phases.21 On CH3&CH38C1 radical) at lower HI concentrations. the other hand, the thermal addition of 18Fto propyne Within the reproducibility of the experiments here, no shows a preference for terminal addition over central adtrend toward a higher terminal/central ratio can be seen dition of only a factor of 3, and the cis-l-[’sF]fluoropropene in Figure 3, and therefore no indication of a 1,2-chlorine and trans-1-[lsF]fluoropropene isomers are formed in a shift in the C3H?T1 radicals formed by 38Claddition to ratio of approximately 2/1, in good agreement with the propyne. The absence of a 1,2-chlorineshift here could expectations from thermochemical e q ~ i l i b r i a . ~ ~ , ~ ~ well be a corollary of a higher barrier associated with viThe overwhelming tendency to form the cis-2-[38C1]nylic radicals. chloropropene following terminal addition of 38Clto proFormation of trans-l-[38Cl]chloropropene. The obpyne indicates a very strong preference for formation of served yields of trans-1- [38Cl]chloropropeneare always the cis radical precursor, with the unpaired electron orbital small and remain approximately constant (1.2 i 0.5%) trans to the C1 atom. The absence of any similar strong preference following terminal addition of 18Fto propyne throughout all of the variable changes in Table 11. The absence of any systematic variation in these yields makes implies further that this special radical stability is charit unlikely that the observed trans-l-[38C1]chl~r~pr~pene acteristic not of halogens in general but of chlorine and is formed solely by the thermal 38Cl addition/HI-scahigher members of the series in these methylhalovinyl venging mechanism of reactions 2, 5, and 8. With yields radicals. Free radical additions of CC13, CH,COS, and CZH,S to substituted alkynes have been studied by only in the 1% range, it is possible that nonthermal 38Cl mechanisms play an important role in the formation of and have been found to give varying Kampmeier et alaM-% degrees of stereospecificity. The addition of ethyl mertrans-l-[38Cl]chloropropene, and even the existence of the straightforward addition/scavenging mechanism is not captan to ethoxyacetylene leads to the cis adduct by a firmly established for the trans compound. It is worth factor of more than 100 over the trans,26exhibiting stenoting that the yield of the trans component was essenreospecificity comparable to that observed here for C1 plus CH3C=CH. In the case of l-ethoxy-2-(ethylthio)ethene, tially unaffected in the two samples of Table I1 whose the trans product is thermodynamically more stable, and yields of the cis compound and of 2- [3sCl]chloropropene were strongly affected by impurities present in the HI used yet the cis product is essentially the only one formed. In as the scavenger molecule. This observation certainly our system, the cis-1-chloropropene is energetically more supports some nonstandard, possibly nonthermal mechastable and is essentially the only terminal addition product formed. nism for formation of the trans product. In any event, the terminal addition of thermal 38Clto propyne is obviously The degree of stereospecificity of radical addition of strongly stereospecific toward formation of the cis isomer alkynes can be analyzed in terms of the relative rates of of l-[38Cl]chloropropene,with at least 95% cis stereointerconversion between the isomeric cis/trans vinyl radspecificity and possibly 98-100% as the true situation for icals and the rate of reaction of the vinyl radicals with the thermal 38Cl atoms. radical scavenging agent. Our observation of almost complete stereospecificity in the formation of c i ~ - l - [ ~ ~ C l ] The thermochemistry of the cis and trans isomers of chloropropene requires both (a) a very strong preference 1-chloropropenehas been studied through the equilibrium for initial formation of the cis radical and (b) isomerization isomerization from 3-chloropropene over the temperature of 1-methyl-Bchlorovinylradicals very much slower than ranges 573-652 K (using Iz as catalyst)16a1’and 42&520 K abstraction of H from HI. The addition of C2H6Sradicals (using HBr as the catalyst).18 From these data, an equito an alkoxyethyne has been similarly interpreted because librium cis/trans ratio of about 4 to 5 can be estimated of its strong cis stereospecificity,26whereas the additions for 298 K. A direct study of the equilibrium has also been of CC13 and CH3COS radicals to alkylethynes have been carried out over the temperature range 293-673 K, with cited as examples of rapid isomerization of the intermean equilibrium cis/trans ratio of about 2 for 298 K.l9 diate vinyl radicals prior to conversion to the final stable Although the agreement among all of these results is only produ~ts.~~~~~ semiquantitative, it is certain that the observed ratios of cis-1-[38C1]chloropropene/ trans-1- [38C1]chloropropene in The differentiation between the stereospecificities of these experiments are not the consequence of thermoterminal addition to propyne of fluorine and chlorine atchemical equilibrium among the products. oms suggests the presence of d-orbital interactions between C1 (or Br) and the unpaired electron orbitals on the adOur gas-phase observations have a parallel in the liqjacent carbon atoms, with the consequence that the C1 and uid-phase studies of Poutsma,20who found a 30/1 prefthe unpaired electron are held trans to one another. One erence for trans-1,2-dichloro-l-butene over cis-1,2-diparticular form of such an interaction could be a “bridged chloro-1-butene in the chlorination of 1-butyne. With radical in which the C1 atom is partially bonded to the terminal addition of the first chlorine atom to 1-butyne, adjacent carbon atom as well.27However, with our present the intermediate radical CH3CH2C=CHC1 in Poutsma’s state of knowledge in this system, there is no necessity to experiments is directly comparable to the CH3C=CHC1 postulate an interaction that strong or that definite; it is radical so preferentially favored in the present experisufficient simply for a strong interaction of an unspecified ments. The current experiments are more explicit in denature to produce the preference observed in the formation fining the intermediate, since the order of addition here of the cis product. More extensive experiments with a must be 3sCl in the terminal position and then H at the
The Journal of Physical Chemistry, Voi. 84, No. 15, 1980 1881
Thermal Chlorine-38 Reactions with Propyne
TABLE 111: FCelative Reactivities of Various Olefins and Alkynes towar’d Addition of Chlorine Atoms (38Cl) reaction 3*c1t HI -+ ~ 3 5 t~ I 1 3 ~ 1 + C,H, -+ c,~,38ci* 38Cl+ C,H, -+ C,H,38C1* 38Cl+ CH,CH-CH, + C3H,”C1* 38Clt CH3C=CH .+ C3H,35C1*
rela.tive rate (1.0) 1.7 1.2 t 1.6 i 1.9 f
* 0.1 0.1 0.1 0.2
ref
2,3 4 2 3 this work
variety of alkynes may help to clarify further the characteristicti of these interactions between the chlorine atom and the neighboring unpaired electron orbital. Relative Rates of Addition to Alkenes and Alkynes. The relative rates of the addition reaction of thermal 38Cl to various substrates have been separately measured in competition with the abstraction reaction from HI, as with k,/k, in these experiments. Very little variation in the overall rate of addition has been found for ethylene, propene, acet,ylene, or propyne, as summarized in Table 111. In both the propene and propyne systems, a strong preference exists for addition to the terminal end of the molecule (>85% in each case). Nevertheless, the overall rates of addit ion to the molecules appear to be relatively unaffected by these intramolecular preferences for reaction. Poutsma has studied the relative addition rates in the liquid phase at 0 “C for several C4alkenes and alkynes and of those addition rates were within generally found that +dl a factor of 2 of each other.1° One example of a larger intermolecular selectivity is that of CClZ=CClz, for which Knox has reported a relative addition rate in the gas phase a t 310 K about 8 times slower than that for ethylene. Poutsma has also reported that addition in liquid-phase experiments is about 11times slower to CC1,=CC12 than for the corresponding C1-atom additions to cis-2-butene, trans-:!-butene, and 1-butene. However, recent experiments in this laboratory indicate that the intermolecular preference in1 the gas phase for reaction with C2H4 rather than CzCl4 is considerably less than estimated by Knox.28 The rates of the reverse reactions to the addition of 38Cl have been estimated for C2H4, CH3CH=CH2, and CH3Cd!H. In each case, decomposition of the excited radical is thermochemically barely possible because the radical possesses an excitation energy corresponding to the strength of the C-CL bond just formed and will need just that amount of energy to rupture the C-C1 bond again. The pressure for which the decomposition of C2H438Cl*has a probability of 50% is 800 f 120 torr, while the pl12values for c3H438c1*and C!3H638Cl*are in the 250 torr and 200 torr range, respectively. These results are consistent with the expectation that the larger radicals will decompose somewhat more slowly because of the additional vibrational degrees of freedom present in the molecules.29~30 Abstraction Reaction. The successful competition of chlorine-atom addition with abstraction by chlorine has been known for many years through the observation that the vapor-phase chlorination of propene produces 1,2-dichloropropane at room temperature, shifting to allyl chloride formation in the temperature range 200-600 0C.10,31The addition process is faster at all temperatures but is reversible at higher temperatures, allowing the reaction t o shift toward the slower but irreversible abstraction reaction leaving an allylic radical which is subsequently converted to allyl chloride. The extrapolation
of Figure 5 to an intercept very near zero implies that essentially all of the available thermal 38Clatoms would react by addition to propyne in a system (experimentally not feasible) containing only traces of HI scavenger. The abstraction of H by thermal C1 is very rapid for C2H6(k =6X cm3 molecule-l s-l) and molecules containing other C-H bonds weaker than the 102 kcal/mol bond strength in HCl.a2,33Since the C-H bonds in the CHS group of propyne have bond energies of W1cm3molecule-l s-l. The absence of any appreciable abstraction from propyne by 38Cl is not necessarily inconsistent with the CzH6abstraction results, needing only that the addition reaction proceed about 10-20 times faster, i.e., reaction on nearly every collision. Measurements to date indicate that C1-atom additions do proceed with very high efficiency, and the relative rate data of Table I11 demonstrate that the initial addition step is comparably rapid with all of those substrate molecules. Molecular beam studies have also demonstrated very high collision cross sections (20-35 A) with atomic chlorine and several different bromoolefins as the reactant^.^^^^^ Acknowledgment. This research has been supported by Department of Energy Contract No. DE-AT-03-76ER 70126.
References and Notes (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
(13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (28) (27) (28) (29) (30)
F. S.C. Lee and F. S.Rowland, J. Phys. Chem., 81, 1229 (1977). F. S. C. Lee, Ph.D. Thesis, University of California, Irvine, 1975. F. S.C. Lee and F. S.Rowland, J. fhys. Chem., 81, 1222 (1977). F. S.C. Lee and F. S.Rowland, J. Phys. Chem., 81, 1235 (1977). P. B. Ayscough, A. J. Cocker, F. S. Dainton, and S.Hirst, Trans. Faraday Soc., 58, 318 (1962). J. H. Knox and J. Riddick, Trans. Faraday Soc., 62, 1190 (1966). M. H. J. Wijnen, J . Am. Chem. Soc., 83, 3014 (1961). R. J. Cvetanovic, Adv. Photochem., 1, 115 (1963). M. L. Poutsma, J . Am. Chem. Soc., 87, 2161 (1965). M. L. Poutsma, Science, 157, 997 (1967). M. L. Poutsma, Methods Free-Radical Chem., 1, 79 (1970). L. Bertrand, J. A. Franklin, P. Gokifinger, and G. Huybrechts, J. fhys. Chem., 72, 3926 (1968). C. M. Wai and F. S.Rowland, J. Am. Chem. Soc., 90, 3638 (1966). J. M. Tedder and J. C. Walton, Adv. fhys. Org. Chem., 18, 51 (1978). W. J. Hehre and J. A. Pople, J . Am. Chem. Soc., 92, 2191 (1970). Z. B. Alfassi, 0. M. Goklen, and S.W. Benson, J. Chem. 7hemodyn., 5, 411 (1973). 2.B. Alfassi, D. M. Goklen, and S.W. Benson, Int. J. Chem. Kinet., 5, 155 (1973). P. L. Abell and P. K. Adolph, J. Chem. Thermodyn., 1, 333 (1969). I. Vussh and U. Zaved, Khim. Tekhnol., 14, 718 (1971). M. L. Poutsma and J. L. Kartch, Tetrahedron, 22, 2167 (1966). P. S.Skell and R. G. Allen, J. Am. Chem. Soc., 86, 1559 (1964). F. S.Rowland, F. Rust, and J. P. Frank in “FluorineContaInlng Free Radicals,” J. W. Root, Ed., American Chemical Society, Washington, D.C., 1978. C. Concannon and F. S. Rowland, unpublished experiments. P. S.Skell and K. J. Shea in “Free Radicals”, J. K. Kochi, Ed., Wiley, New York, 1973. J. A. Kampmeier and G. Chen, J. Am. Chem. Soc., 87,2608 (1965). R. M. Kopchik and J. A. Kampmeier, J. Am. Chem. Soc., 90, 2608 (1965). D. K. Sedegaertner, R. M. Kopchik, and J. A. Kampmeier, J. Am. Chem. Soc., 93, 6890 (1971). R. S. Iyer and F. S. Rowland, unwbllshed exoeriments. I. Orefand B. S.Rabinovitch, A&. Chem. Res., 12, 166 (1979). E. A. Hardwidge, B. S.Rabinovitch, and R. C. Ireton, J. Chem. fhys., 58. 340 - . - (19731. - - - -,H. P. A. Groll and G. Hearne, Ind. Eng. Chem., 31, 1530 (1939). 0.C. Fettis and J. H. Knox, frog. React. Kinet., 2, 3 (1964). NBS Spec. fubl(U.S.), No. 513, May 1978. J. T. Cheung, J. D. McDonaM, and D. R. Herschbach, J . Am. {?hem. Soc., 95, 7890 (1973). J. D. McDonald, Annu. Rev. fhys. Chem., 30, 29 (1979). -1
(31) (32) (33) (34) (35)
\
