High-Energy 38CI-for-CISubstitution For e-fold attenuation, that is [scavenger] = C3,, we have eq 2.
1
_ _ - 1= k,-C,,/kl, [ solvent] 0.31
C , , = 1.77k~,,[solvent] / k e . Hereafter we ignore the numerical factor. A somewhat different approximation was used previously in whlch the rate was integrated over the lifetime of the electron relative to localization, not relative to recombination, as stated erroneously. (10) C. D. Jonah, J. R. Miller, and M. S.Matheson, J . Phys. Chem., 81, 1618 (1977).
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(11) J. W. Hunt and J. W. Chase, Can. J . Chem., 55, 2080 (1977); J. Phys. Chem., 79, 2835 (1975). (12) J. H. Baxendale and P. H. G. Sharpe, Int. J. Radiat. Phys. Chem., 8, 621 (1976). (13) K. Y. Lamand J. W. Hunt, Int. J. Rad& Phys. Chem., 7, 317 (1975). (14) I. A. Taub and K. Eiben, J. Chem. Phys., 49, 2499 (1968). (15) L. Sanche and G. L. Schultz, J. Chem. Phys., 58, 479 (1973). (16) K. Hiraoka and W. H. Hamill, J. Chem. Phys., 57, 3870 (1972). (17) D. Chipman, J . Phys. Chem., 82, 1080 (1978). (18) R. L. Bush and K. Funabashi, J. Chem. Soc., Faraday Trans. 2, 73, 274 (1977). (19) D. Raiem, W. H. Hamill, and K. Funabashi, J. Chem. Phys., 67, 5404 (1977).
On the Stereochemical Course of the High-Energy Chlorine-38-for-Chlorine Substitution in cis/trans-1,2-Dichlorohexafluorocyclobutanes in Solutions’ Tobias R. Acciani and Hans J. Ache* Depadment of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 2406 1 (Received January 23, 1978)
The stereochemistry of the high-energy chlorine-38-for-chlorinesubstitution process was studied in the cis/trans isomers of 1,2-dichlorohexafluorocyclobutanes.The experimental results indicate that the substitution occurs mostly under retention (>81%) in the presence of hydrogen containing solvents, while in the neat systems and in n-perfluoroheptane solution the yields of inverted product drastically increase. The results are explained in terms of a modified solvent-solute interaction model, which also considers the possible interaction of the reactive molecules making up the solvent environment with the reacting intermediates forming the substituted products. Alternatively the observed results could be interpreted by assuming a solvent effect on the energy distribution of the reacting 38Cl,resulting in changes in the relative importance of the reaction channels leading to substitution via retention and inversion, respectively.
Introduction The stereochemical course of the halogen-for-halogen substitution initiated by translationally excited species produced as a result of nuclear processes, such as 37Cl( n , ~ ) ~ ~has C lbeen , the subject of several investigations.2-11 Gas phase hot atom substitution reactions in diastereomeric molecules show generally a very high degree of stereospecificity with greater than 90% retention of initial configuration. In contrast, the gas-to-liquid phase transition is usually accompanied by a loss of stereospecificitya2+ Due to the difficulties involved in the absolute radiochemical yield determination in the presence of solvents, the study of the stereochemical course proved to be also a convenient tool to investigate the effects of solvents on the reaction mechanism. The first experiments in this direction by Wai and Rowland with the diastereomers of 2,3-dichlorobutane, neat, and in the presence of relatively large amounts of butadiene or bromine, seemed to indicate an increase in the retention of configuration of the 38Clf o r 4 1 substitution product. The authors explained this observation in terms of the elimination of spurious radical reactions by these additivesS3i4 A more comprehensive investigation was carried out by Vasaros, Machulla, and Stocklin2 who studied the stereochemistry of the 38C1-for-C1substitution reaction in the same compounds. They related the observed solvent dependence to the relative abundance of the conformational isomers, which varies as a function of nature and concentration. This in turn, according to these authors, leads to changes in the relative probability for front or back side attack in the various conformers, resulting in sub0022-3654/78/2082-1465$0 1.OO/O
stitution via retention or inversion of configuration, respectively. The effect of the solvent on the stereochemistry of 38C1-for-C1substitution, however, had to be recently reinterpreted. Experiments in our own 1aboratorylO showed that even in those cases where a change of concentration or nature of solvent does not induce a corresponding change in the relative conformer abundance, a distinct solvent effect on the stereochemical course of the substitution process can be observed. This became quite obvious in the case of the diastereomeric 2,4-dichloropentane, where only one major conformer is present, whose concentration is only negligibly affected by the nature or concentration of the solvent. It appeared that the dielectric properties of the solvent, probably causing differences in the solute-solvent interactions in the various solutions, control to a great degree the substitution mechanism.1° In the present work we have investigated the dependence of cis/trans isomerization upon 38C1-for-C1substitution in cis- and trans-1,2-dichlorohexafluorocyclobutane as a function of the nature of the solvent in solutions. cis-1,2-Dichlorohexafluorocyclobutane has only one conformer, so that whatever solvent effects are observed can be directly related to the physical properties of the solvent. Furthermore the possibility of self-scavenging,12which may have caused deviations from the correlation between the retention inversion ratios and the dielectric properties, has been investigated. The possible solvent effect on the energy distribution of the reacting 38Clwill be discussed and its consequences as to the relative importance of substitution via retention 0 1978 American Chemical Society
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CIS -
or inversion will be considered. Experimental Section A. Materials and Purification. A mixture of cis- and trans-1,2-dichlorohexafluorocyclobutanewas obtained from Columbia Organic Chemical Go. The mixture was purified and separated on two different columns in order to obtain the isomers with purities of greater than 99%. The first separation was carried out on a glass column (15.3 m long and 6 mm i.d.) filled with 30% isoquinoline on Chromosorb P-NAW, 60-80 mesh, at 35 "C and a helium flow of 30 mL/min. This separation resulted in products of about 95% purity. Better than 99% purity was obtained by separating the prepurified substances on a glass column (23 m long and 4 mm i.d.) containing 30% dibutylphtalate on Chromosorb P-NAW, 60-80 mesh at 70 "C and a helium flow of 40 mL/min. All solvents were either certified ACS spectroanalyzed from Fisher Scientific Co. or Spectrograde distilled in glass from Burdick and Jackson Laboratories Inc. All solvents were dried and tested gas chromatographically for purity. Iodine with a purity of greater than 98% was obtained from Aldrich Chemical Co. and used as such. B. Sample Preparation and Irradiation. Samples were prepared by placing 2.5 mg of I,, as scavenger, and 14 mg of either cis- or trans-1,2-dichlorohexafluorocyclobutane and the appropriate amount of solvent into a quartz ampoule. The contents of the ampoule were degassed by the usual freeze-thaw technique under vacuum and sealed. The irradiation with thermal neutrons was carried out at the VPI and SU nuclear reactor with a neutron flux of 1.3 X lo1' n cm-2 s-l for 4 min at 40 "C. C. Sample Analysis. The irradiated samples were cooled to liquid nitrogen temperature, opened, and 50 pL of ether was added. They were subsequently treated with 20 pL of 10% aqueous solution of a 1:lmixture of Na2S03 and Na2C03. The organic layer was washed with HzO, separated, dried with Na2S04, and analyzed by gas chromatography, The column used in this study was the 30% dibutylphthalate column under the conditions as described in A. D. Radioactivity Assay. The chlorine-38 labeled products separated by gas chromatography were adsorbed on charcoal directly from the effluent gas stream. The radioactivity of the products was subsequently measured in a well-type scintillation counter. Appropriate decay corrections were made and the ratio of retention to inversion was determined by direct comparison of the radioactivity incorporated in the compound formed following chlorine-38-for-chlorine substitution under retention of configuration to that observed in the compound formed under inversion of configuration. E. N M R Spectra. The 19Fspectra were measured at 94 MHz on a Jeol high resolution standard and lock signal source. The sweep time was 250 s, and the sweep width was 1080 or 2700 Hz for trans and cis isomers, respectively. The 19F NMR spectra of the two isomers show quite distinct features, each consisting of a single line plus a symmetrical AB quartet. The single line originates from the F nuclei of the -CFCl groups and the quartet is caused by C-F, C-F resonances. The quartets were analyzed as usual as AB spectra.l* F. Conformational Analysis. The most stable conformers of the cis and trans isomers of 1,2-dichlorohexafluorocyclobutane are shown in Figure 1. From this figure it can clearly be seen that the cis isomer should have only one stable conformer, namely the axial-equatorial arrangement of the two chlorine atoms, while in the trans
AXIAL-EQUATORIAL
TRANS
ce
i EQUATORIAL-CL
de
'
AXIAL-ce
Figure 1. The most stable conformers of cis- and trans-1,Bdichlorohexafluorocyclobutane.
TABLE I: I9F NMR of cis- and trans-1, 2-Dichlorohexafluorocyclobutanea J'4B , b
ppm Pure trans 206.8 90% trans-heptane 206.8 90% trans-pentanol 206.8 Pure cis 206.8 90% cis-heptane 206.8 90% cis-pentanol 206.8
A6AByd
AB,' P P ~ P P ~ 38.71, 38.21, 38.31, 42.40, 41.70, 42.00,
34.79 34.34 34.44 30.75 30.15 30.43
3.92 3.86 3.87 11.65 11.55 11.57
33.65 32.95 32.95 24.25 23.70 23.80
a Hexafluorobenzene was employed as an internal standard. Experimental coupling constant of AB quartet. Change in chemical Chemical shift of AB quartet. shift of AB quartet. e Chemical shift of the single line.
TABLE 11: Percent Retention of Configuration Following 3aC1-for-C1Substitution in cis- and trans-l,2-Dichlorohexafluorocyclobutane in the Presence of Additives (80 mol X ) % retention
Additive
Cis
n-Heptane Cyclohexane n-Pentanol Methanol Cyclohexanone None n-Perfluorohep tane
88 c 3 88i 3 85i 3 82i 3 76 i 2 55+ 2
Trans 84 c 87 * 85i 81 i
3 3 3 3
77i 2 72 i 2
isomer the chlorine atoms can be either in the equatorial or axial position (assuming a puckered ring structure). It has, however, to be kept in mind that conclusive information as to whether the two trans conformers are in reality sufficiently different can only be obtained from a more detailed 13CNMR spectra analysis which has not yet been carried out. It may very well be that the vicinal incorporation of two chlorine atoms into perfluorocyclobutane does not lead to appreciable ring deformation, i.e., nonplanarity and the two trans conformers may be indeed practically indistinguishable. In any case, the data in Table I where the pertinent parameters extracted from the 19FNMR spectra of cis-and trans-1,2-dichlorohexafluorobutaneare listed give no indication for a change in the conformer population of the trans species upon solvent change. Results and Discussion In a series of experiments the relative yields of the products formed as a result of the 3sCl-for-C1replacement under retention or inversion of configuration in the cis and
High-Energy 38CI-for-CI Substitution
trans isomers of 1,2-dichlorohexafluorocyclobutanewere determined as a function of the nature of the solvent present. The results, which are listed in Table 11, generally show a high retention of configuration. Only in the neat material and in the presence of 80% n-perfluoroheptane are drastic increases in the formation of the inverted product observed. In contrast to the previous results in the 2,3dichlorobutane or 2,4-dichloropentane systemdo hardly any difference (within the experimental error) can be noticed between the results with 1,2-dichlorohexafluorocyclobutane obtained in presence of an alcohol, such as pentanol or methanol, with dielectric constants of 13.9 and 32.6, respectively, and a hydrocarbon such as n-heptane or cyclohexane with considerably smaller dielectric constants of 1.9 and 2.0, respectively. In the previous work with 2,3-dichlorobutane and 2,4dichloropentane Wu and Achelo explained the observed correlation between retention/inversion ratios and the dielectric properties, or more precisely the parameter ( E - l)/(2t 1) in terms of solute-solvent interactions. Within the framework of the radical-radical recombination model, these authorslo suggest that strong solvent-solute interaction prevents the intermediate organic radical from obtaining planarity before recombination occurs and thus recombination occurs mainly by forming the product which was obtained in the initial replacement step. Thus in the present investigation, one could postulate that the time scale for the recombination of the chlorohexafluorocyclobutane radical with 38Clin the solvent cage is short as compared with the time required for the radical to achieve planarity, which is essential for racemization to occur.1o This explanation tacitly assumes that the initial attack of the 38Cl leads to the replacement of C1 by 38Cl under retention of configuration. The fact that no dependence on the dielectric constant of the various hydrogen containing solvents is observed in the present investigation could be rationalized by assuming that the activation energy required for achieving planarity is considerably greater in the case of the chlorohexafluorocyclobutane radical than in the chlorobutane or chloropentane radical. Such assumption does not seem to be unreasonable in view of the more constrained cyclobutane ring system. While these explanations may rationalize the results obtained in the hydrogen containing solvents the question arises what causes the increase of inversion in the neat 1,2-dichlorohexafluorobutaneand even more so in the presence of n-perfluoroheptane. If solute-solvent interaction would be the sole parameter which controls the stereochemical course of the reaction then the reaction in n-perfluoroheptane which has a dielectric constant 1.77 should give similar results as observed in n-heptane or cyclohexane solutions. In this context it seems important to point out that similar anomalities, Le., deviations from the correlation between retention/inversion ratios and dielectric constants have been observed in the previous work with 2,3-dichlorobutane, 2,4-dichloropentane,1° and 1,2-dichloro1,2-difl~oroethane’~ with solvents such as bromine, acetonitrile, and in the latter system also with n-perfluoroheptane. A common feature of these solvents is that they are expected to be very reactive (Big)or very unreactive (nperfluoroheptane) in reactions with chlorine atoms in comparison with hydrogen containing compounds. An estimate of the relative reactivity may be obtained from
+
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978 1467
the enthalpies for Br, H, and F abstraction by 38Cl,from the corresponding compounds. The bond dissociation energies involved are as follows:16 Br-Br, 46.08 kcal/mol; F-C, 107.0 kcal/mol; H-C, 102 kcal/mol; NC, 204.0 kcal/mol; ClBr, 53.0 kcal/mol; ClF, 61.4 kcal/mol; HC1, 103 kcal/mol; ClN, -65 kcal/mol (NBr = 67 kcal/mol, NF = 62 kcal/mol). From these data the heat of the abstraction reaction can be estimated. The abstraction reaction of Br by C1. from liquid bromine is exothermic (7 kcal/mol), F by C1- from perfluoroheptane is endothermic (45.6 kcal/mol), N by C1. from acetonitrile is endothermic (139 kcal/mol), and H by C1 from a typical hydrocarbon is exothermic (1kcal/mol).17 In view of these results it seems rather tempting to explain these observations in terms of “self-scavenging” of the %C1 by the very reactive Br2 or by a lack of self-scavenging by the much less reactive perfluoroheptane while the lack of scavenging of the acetonitrile may be understood in terms of a solvent cage oriented in such a way that the more negative CN group forms the inner wall of the cage thus preventing the 38Clfrom colliding with hydrogen atoms. One would have to assume two competing reactions: W l t X-Y 38Clt R.
-+
+
38Cl-X
Re ”CI
self-scavenging
(1)
recombination
(2)
If reaction 1 is relatively fast as in Br2 only those 38Cl atoms can recombine which are still in the immediate vicinity of the organic radical, that means recombination would have to occur on a short time scale, i.e., before the organic radical can achieve planarity, and the initial configuration is retained. In the case of an unreactive cage wall, e.g., n-perfluoroheptane, reaction 1 is slow, and a longer time, allowing relaxation of the radical, is available for recombination, the yield of inverted product increases, as observed. For all the hydrogen containing solvents used in these investigations the self-scavenging process would be approximately the same and whatever solvent dependence is observed should therefore be caused solely by solvent-solute interactions. This latter solvent effect will be most noticeable if the relaxation time of the radical is of the same order as the time for 38C1-radical recombination, as in 2,3-dichlorobutane, 2,4-dichloropentane, and to a lesser degree in 1,2-dichlorodifluoroethane.On the other hand if the relaxation time of the radical as formed by the breakup of the 38Clsubstituted 1,2-dichlorohexafluorocyclobutane is considerably longer, the subsequent recombination leads mostly to retention in the presence of hydrogen-containing solvents. It should be noted, however, that recent work by Root et al.13 on the hot 18Freactions in liquid fluoroethane/H2S mixtures has provided no evidence for the interference of an efficient hydrogen abstraction (18F H2S H18F) with primary caging. Thus the problem of “self-scavenging” of trapped species in solvent cages by reactive solvent molecules requires definitely more studies. In view of this still somewhat inconclusive evidence it seems also appropriate to consider other alternatives which could explain the results observed in these experiments. In the previous investigations Ache et al.1° considered also a one-step mechanism including two reaction channels, one leading to the retained, the other to the inverted product, each one showing different deexcitation-stabilization characteristics. In this case it was argued that an increased solutesolvent interaction could lead to enhanced energy transfer and by making certain assumptions about the energy dependence of the reaction channels for front
+
-
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vs. back side attack, leading subsequently to retained or inverted product, respectively, the observed results could be rationalized. Such a mechanism would, however, again be at variance with the fact that the replacement of the strongly interacting methanol, with a dielectric constant of 32.6, which should be very effective in transferring energy and thus deexcitation-stabilizing the reactive product, with nheptane, which has a considerably smaller dielectric constant of 1.9 and which therefore should be much less effective in the deexcitation-stabilization process, results in no significant change in the retention to inversion ratios. On the other hand as discussed above in the presence of n-perfluoroheptane as solvent, with a similar small dielectric constant, the retention to inversion ratio is considerably lower. A possible explanation for this phenomena could be that the replacement of the hydrogen bearing solvent by n-perfluoroheptane significantly alters the energy spectrum of the 38Cl atoms reacting to give 38C1-for-C1substitution.18 By analogy to available experimental results obtained with energetic chlorine and fluorine atoms one could expect that the major reaction pathway of these “hot” C1 atoms would be energetic hydrogen abstraction reactions, C1 to HC1, in the hydrogen containing solvents. The steady state hot atom kinetic theorylg would then suggest that gross changes in the effective total hot reactivities of the solutions would likely accompany the replacement of the 1,2-dichlorohexafluorocyclobutaneby RH or Br2 in solutions, but that this would not occur with n-CTF16. The Brz and n-C7FI6anomalies could then be indicative of such reactivity changes as well as of detectable cross section energy range differences for the inverted and retained C1-for-C1substitution channels. The dominant presence of a relatively inert additive would unambiguously and perhaps drastically lower the average energy of the substituting hot chlorine atoms thus indicating that the inverting pathway becomes more likely at reduced energies. Such cross section behavior was in fact predicted by the classical trajectory calculations of Bunker and Valencich for the energetic 3H + CH4 system.20 Several studies to further assess the detailed mechanism of the energetic 38C1-for-C1substitution reaction are presently being carried out at this laboratory.
These studies will also take into consideration the possibility that the substitution may be the result of a concerted action consisting of the motion of the incoming chlorine, the outgoing chlorine, and the rest of the molecule. In such a case, one can visualize the effect of the solvent by the tendency of the strongly interacting or polar solvent of pulling away the chlorine which is to be replaced from the neighboring carbon atom thus allowing the chlorine-38 to make the attack from the front side resulting in retention of configuration. Whereas in a more inert medium no such movement occurs and a greater fraction of the substitution process occurs via back-side attack. This assumption has its parallel in a variety of classical substitution reactions in ordinary thermal organic chemistry and may resemble the basic features of the previously postulated “caged complex”.2~8~21
References and Notes Work supported by the US. Department of Energy, Dvisloll of Basic Energy Sciences. L. Vasaros, H. J. Machuila, and G. Stocklin, J. Phys. Chem., 78, 501 (1972). C. M. Wai, C. T. Ting, and F. S. Rowiand, J. Am. Chem. Soc., 88, 2525 (1964). R. S. Rowhnd, C. M. Wai, C. T. Ting, and G. Miller In “Chemical Effects of Nuclear Transformations”, Vol. 1, IAEA, Vienna, 1965, p 333. C. M. Wai and F. S. Rowiand, J. Phys. Chem., 71, 2752 (1967). C. M. Wai and F. S. Rowiand, J . Phys. Chem., 74, 434 (1970). G. F. Paiino and F. S. Rowiand, Radiochim. Acfa, 15, 57 (1971). H. J. Machulia and G. Stockiin, J . Phys. Chem., 78, 658 (1974). Y. Y. Su and H. J. Ache, J. Phys. Chem., 80, 659 (1976). J. Wu and H. J. Ache, J . Am. Chem. Soc., 99, 6021 (1977). D. J. Stevens and L. D. Spicer, J. Am. Chem. SOC.,submitted for publication. For a recent discussion of scavenger effects in hot atom reactions, especially in connection with caging, see ref 13. R. G. Manning and J. W. Root, J . Phys. Chem., 81, 2576 (1977). B. Atkinson and M. Stedman, J. Chem. Soc., 512 (1962). T. R. Acclani, Y. Y. Su, E. P. Rack, and H. J. Ache, J. Phys. Chem., 82, 975 (1978). R. C. Weast, Ed., “Handbook of Chemistry and Physics”, 51st ed., The Chemical Rubber Co., Cleveland, Ohio, 1971. A comparison of the activation energies of these reactions would be more meaningful. Unfortunately most of these data are not available. The authors thank J. W. Root for suggesting this akernatlve to explain the observed anomalies in presence of Br, or n-C,F,. E. R. Grant, D. F. Feng, J. Kelzer, K. D. Knierim, and J. W. Root, ACS Symp. Ser., No. 68, 314 (1978). D. L. Bunke and T. Valencich. Chem. Phvs. Lett.. 20. 50 (1973). K. C. To, M. E. Berg, W. M. Grauer, and E. P. Rack, i,Phys. Chem:, 80, 1411 (1976).
