T H E
J O U R N A L
O F
PHYSICAL CHEMISTRY Registered i n U. S.Patent Office
(C Copyright, 1978, by the American Chemical Society
VOLUME 82, NUMBER 9
MAY 4,1978
Reactions of 4.5- and 6.0-eV Photochemically Produced Tritium Atoms with Fluoroform 6. K. Min, C.-T. Yeh, and Y.-N. Tang* Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received July 1, 1977; Revised Manuscript Received February 8, 1978) Publication costs assisted by the Robert A. Welch Foundation
Tritium atoms with initial energies of 4.5 and 6.0 eV have been formed by the vacuum UV photolysis of TBr. Reactions between CHF3and tritium atoms at these two energies, plus four lower ones, have been carried out. The threshold energies for the T-for-H and T-for-F substitution reactions in CHF3have been evaluated as 1.9 f 0.2 and 1.9 0.3 eV, respectively. The higher threshold and lower reactivity of T-for-F substitution in CHF3 in comDarison with those of CH,F indicate the absence of a possible inversion mechanism during the F substitution
*
in
CHF,.
Introduction The use of photochemical methods to form hydrogen atoms of certain unique energies and the study of their chemical reactions have been extremely successful during the past decade.'-3 Most often, the photodissociation of hydrogen halides, such as HBr and HI, is used to produce the high energy hydrogen atoms. Other molecules such as H2S, CH,SH, and H 2 0 have also been used. By photolyzing different compounds with radiation of different wavelengths, hydrogen atoms of various initial energies up to 3.2 eV have been conveniently formed in laboratories for a number of valuable studies in ,organic systems. Hydrogen atoms with all these energies can be used for studying the H-abstraction process while those in the electron volt range can also initiate H-substitution reactions. This photochemical method is especially useful in the determination of threshold energies for abstraction and substitution processes by hydrogen atomse3 Earlier, Kuppermann and White have obtained a threshold value of Eo = 0.33 f 0.02 eV for the reaction D + H2 HD + H,3a while Chou and Rowland have established the threshold energy for the reaction T + CD, CD3T + D as 1.5 eV.3b Very recently, Chou, Wilkey, and Rowland have studied the reactions of tritium atoms with CH3F, and have observed a lower threshold energy and a higher relative yield for F replacement than for H replacement
-
-
0022-3654/78/2082-097 l$Ol .OO/O
in this molecule.3c In the latter systems, high energy tritium atoms were obtained by the photolysis of TBr in a trace amount. It is only under such conditions that substitution processes can be s t ~ d i e d . ~ ~ ~ ~ In the present work, we have produced tritium atoms with much higher energies than those employed in previous studies by using the vacuum UV photolysis of TBr to yield tritium atoms with energies of 4.5 and 6.0 eV. We have carried out reactions between CHF, and tritium atoms at these two energies, plus four lower ones, and have evaluated the threshold energy for T-for-H and T-for-F substitution reactions in CHF,. Certain valuable mechanistic implications about hot tritium substitution can also be deduced from the present results.
Experimental Section General Procedure. The general procedure can be divided into three major steps.5 The first is the synthesis of TBr by passing a discharge through a mixture of T2 (from New England Nuclear) and Br2.435It is followed by the irradiation of a sample consisting of TBr, CHF, (from Matheson), and Br2 as a scavenger. The last step is the analysis of products utilizing standard radiogas chromatographic techniques.6 A total of six different wavelengths of radiation were employed for the photolysis. The light source, the filter, the window material, and the reaction cell material for 0 1978 American Chemical Society
972
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
B. K. Min, C.-T. Yeh, and Y.-N. Tang
TABLE I: Experimental Details for Producing Photochemical Hot Tritium Atoms Wavelength,
a
Light source
2531 2288
Low-Dressure Hg iamp Cd lamp
2139
Zn lamp
1849
Low-pressure Hg lamp Xe resonance lamp Kr resonance lamp
1470
1236
Filter
Window
Cell
100-A band filter Butene - 2 gas filter
these radiations are summarized in Table I. The expected energies of the tritium atoms formed from these photochemical dissociations are also included in the table. However, the translational energy of the CHF, molecules has been neglected during the calculation of these energies. T B r Synthesis. A TBr generator was constructed with a 50- or 500-mL Pyrex flask equipped with two tungsten electrodes. After discharge, the unreacted Tzwas pumped out a t liquid nitrogen or pentane-slush (-130 "C) temperatures under high vacuum. Repetitive T, elimination work is essential to acquire a low T,-containing TBr-Br2 mixture. After this procedure, known amounts of fluoroform were introduced together with this mixture into a photolysis cell. Before photolysis, portions of this sample were drawn for counting to calibrate the amount of T, remaining in the sample. In a typical run, the discharge time through the mixture of T2and Br, was about 30 min, and the amount of TBr synthesized was around mol. This transcribes into a pressure of about Torr in the reaction cell which is about lo6 times lower than the partial pressures of other components in the system. Irradiation of Samples. All the light employed for irradiation is essentially monoenergetic. The details of the four longer wavelength irradiations as indicated in Table I are essentially the same as those employed by Chou and Rowland in their previous ~ o r k s . ' ~ , ~ Xe and Kr resonance lamps with a titanium getter were fabricated with various kinds of wind0w.I Xe lamps were equipped with LiF or sapphire, while Kr lamps were provided with LiF or CaF, windows. LiF has the advantage of high transmission for both 1470- and 1236-A radiation, but it is easily deteriorated by moisture. Both sapphire and CaF, are less hygroscopic. Sapphire eliminates the 1296-Aline from a Xe lamp, but its transmission for 1470-A light is rather low. Similarly, CaF2 will eliminate the 1165-A line from the Kr lamp, but its transmission for 1236 A is not high. The monochromatic purity of the lamps was confirmed with a Jarrell Ash vacuum spectrometer. The irradiation time for the low-pressure Hg lamp was normally 10 min, while that for all the other lamps ranged from 30 to 1100 min. The vacuum UV lamps were operated with a Raytheon CMD-5 microwave generator which supplied 125 W full power to an Evanson cavity at 2450 MHz. Product Analysis. After photolysis, the reaction mixture was transferred from the cell into a sample injection loop and was assayed with a radiogas chromatographic setup equipped with a gas proportional counter. TBr and Br2 were absorbed in advance with a K,Fe(CN), column. The major tritiated products, HT, CHTF,, and CTF,, were separated with a 50-ft. PCA column (propylene carbonate coated on alumina) at 0 "C.
LiF, sapphire CaF,, LiF
Expected max T energy, eV
Vycor
1.1
Suprasil
1.6
Suprasil
1.9
Suprasil
2.8
Pyrex
4.5
Pyrex
6.0
Results and Discussion Relative Yields of Products f r o m Photochemical Tritium A t o m Reactions with CHF,. Fluoroform does not absorb light with a wavelength longer than 1200 A.8 The strong chemical bonds in this molecule render it as a good testing ground for the use of 4.5- and 6.0-eV tritium. In the present experiments, the photochemically produced tritium atoms might undergo either abstraction or substitution reactions with fluoroform as illustrated in reactions 1-4. The yields for all the expected products T* t CHF,
-+
HT t CF,
T* t CHF,-+ TF
T* +
CHF,
T* t CHF,
-+ --f
+ CHF,
(1)
(2)
CTF, t H
(3)
CHTF, t F
(4)
except TF have been measured. Since the experimental results in this work always indicate an H T yield which is much higher than those of the substitution products, it is necessary to ascertain the absence of any anomalous source of HT. (Some earlier work5 actually involved an anomalous H T yield. Those data have been discarded.) In the first place, the blank analysis of the reaction mixture before photolysis has been performed for every sample, and in most of the cases, the background correction for the Tz impurity was less than 10% of the observed total H T radioactivity. Secondly, photolysis of TBr in the presence of CF4 by the shorter wavelength radiations have been carried out, and the observed H T radioactivity was not more than the blank corrections. (CTF, was also not observed.) Thirdly, several TBr-Br,-CH3F photolyses at 1849 A have been performed, and the results agreed extremely well with those published by Chou and R~wland.~' Experiments have been carried out at each of the six photolysis wavelengths mainly as a function of Br, concentration. For tritium atoms with 1.1, 1.6, and 1.9 eV of energy, H T is always the sole observed product. The two substitution products, CTF, and CHTF,, are detected only when the tritium energy is 2.8 eV or above. Between the two, the data fluctuation of the CTF3 yields is much smaller than that of the CHTFp yields. In Figure 1, the dependence of the CTF3/HT ratio as a function of Br2 concentration for 2.8-, 4.5-, and 6.0-eV tritium atoms are illustrated. It can be seen that this product ratio is essentially independent of the Br, concentration, indicating that thermalized tritium atoms were efficiently scavenged by the bromine molecules for all these experiments performed. In addition, it has also been demonstrated that the CTF3/HT ratio is independent of a four-fold variation in photolysis time and a two-fold variation in total pressure. In Table 11, the relative yields of the tritiated products from photochemical tritium reactions with CHF3 are summarized. The yield of H T is arbitrarily normalized
Reactions of Tritium Atoms with Fluoroform
TABLE 11: -
Relative Yields of Products from Photochemical Tritium Atom Reactions with CHF,
TBr photolysis wavelength, A Initial tritium energy, eV
H abstraction HT T-for-F substitution CHTF, T-for-H substitution CTF, a
The Journal of Physical Chemistty, Vol. 82,No. 9, 1978 973
2537
2288
2139
1849
1410
1236
1.1
1.6
1.9
2.8
4.5
6.0
1000
1000
1000
1000
0 c 0.2
0 c 0.3
0 i 0.3
1.8
f.
0.1
0
0 c 0.3
0 r 0.3
4.1
i
0.5
i
0.2
1000
1000 3.7
i
Nuclear recoila
1.4
23.3 c 3.7
6.8
1000 f.
2.4
49.4 r 14.6
88 363
Data from ref 9.
Bc,
c
transitions involving the other two Q states are vigorously forbidden. For the three possible Q states, it was demonstrated from the dipole strength consideration that the contribution of 3Q(3~0+) in the Q N transition is less than
L
2 9 ~ 4
-
Both l Q ( l ~ and ) ,&(,a1)dissociate to give H and Br atoms in the lowest electronic states, Le., H(2Sl12)and m Br(2P32). However, both 3Q(3~0+) and T(,Z) dissociate to 2! ground state H atom and excited Br(2P1J . The electronic x excitation in Br(2Pl/z)is only 0.46 e d In the well-established case of 1849-A irradiation of TBr, the only II important available states are the Q states. Since the 3Q(3~0+) state is only populated to a very small extent, essentially all the dissociated H and Br atoms are therefore in their lowest state. As a result, the kinetic energy of tritium atoms from 1849-A irradiation is well defined.* In the case of 1470-A radiation, it is possible to reach the T(,Z) state, but this should be of minor importance. By examining the potential energy curves of HBr, it is obvious from the Frank-Condon principle that nearly all 4 the ground state HBr molecules which absorb 1470-A 0 2 .I .2 .3 .4 .5 radiation should reach the Q states. This means, like the 1849-A radiation, essentially all the resulting H and Br atoms are in their ground electronic states. With this Mole Fraction of Br2 information, it is straightforward to evaluate the kinetic energy of the resulting tritium atoms as 4.5 eV. Flgure 1. The dependence of CTF,/HT product ratio as a function of Certainty of the Initial Tritium Energies from 1236-A bromine concentration for 2.8-, 4 . 5 , and 6.0-eV photochemical tritium reactions. Radiation. The energy of the 1236-A radiation is higher than that of the V(lZ+) state, but the transition V(lZ+) cto 1000 for each of the six initial tritium energies. Data from the recoil tritium experiments are also i n c l ~ d e d . ~ J ~ N(lZ+) is not observed in the absorption spectra.12 (It is only detected in the emission spectra.) Therefore, the only I t is obvious that, at the energies covered by these exexcited states we have to consider for 1236-A radiation are periments, the relative yields of both CTF, and CHTFz again the Q and the T states. increase with increasing initial tritium energy, and that The Q states are known as the upper states of the the former always has a higher yield than that of the latter. continuous absorption of HBr which has a maximum a t Certainty of t h e Initial Tritium Energies from 1470-A 1850 A. The spectrum also contains a large number of Radiation. Before we start to discuss the experimental predissociations. It was believed that the Q states are also results in Table 11, we would like to examine the certainty responsible for the predissociations above 1190 A.13 The of the quoted initial tritium energies when TBr was predissociations a t a wavelength shorter than 1190 A are photodissociated by 1470- and 1236-p\ radiations. In presumably caused by the T(,Z+) state.13 calculating the initial tritium energy, the excess energy was From the above description, after the absorption of first evaluated by substracting D(T-Br) from the energy 1236-A radiation, the majority of the TBr molecules will of the absorbed radiation. The distribution of excess be in the Q states instead of the T states. Therefore, most energy between T and Br was calculated according to of them are likely to decompose to give T(2Sl/z) and conservation of energy and momentum principles. Minor Br(2P3 J . However, it is possible that a minor fraction of corrections for the translational and rotational energies of the T d r molecules might give Br(2P1/2).This means that TBr have also been p e r f ~ r m e d . ~ , ~ the majority of the tritium atoms being formed by 1236-A The electronic states of TBr are as fo1lows:ll radiation have 6.0 eV of initial kinetic energy while a small N 'E+ u2n4 fraction of them might possess 5.5 eV. The overall picture should still be consistent with the assumption that from Q 371, ln U 2 r r 3 U * 1236-A radiation we are basically dealing with tritium T 'E+ u7r4u* atoms with 6.0-eV initial energy. V ' x + u711T4a* Threshold Energies for t h e T-for-H and T-for-F Substitution Reactions i n CHF3 The zero yields of CTF, and The Q states (In,,r2,3n1,,rO+, 3n0-) are the lowest excited states. Among the Q states, the ' Q ( l r )3,Q ( 3 ~ l and ) , 3QCHTFz a t tritium energies of 1.9, 1.6, and 1.1eV indicate (,rO+) are involved in the Q N transitions, while that the threshold energy for the substitution reactions are *O 0
.I
.2
.3
-
.
6L
-
974
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
69
I-
0 RT= CTF3
50
-
0
40
-
B. K. Min, C.-T. Yeh, and Y.-N. Tang
1
RT=CH TF2
m
0 c
x 30 -
A
1-
=.20 I?
v
10
/
0 C
n
I
I
I
I
I
Initial T Energy (eV) Figure 2. Evaluation of the threshold energies for the T-for-H and T-for-F . substitution reactions in CHF,.
both above 1.9 eV. The relative yields of CTF, and CHTF2 from Table I1 together with their error ranges are plotted against initial tritium energies in Figure 2. The positive points for each molecule are connected separately and extrapolated to a zero relative yield. This extrapolation method is based on the presumption that the hydrogen abstraction reaction extends to a much lower energy range than the T-for-H or T-for-F substitution processes in CHF,, and therefore the CTF3/HT and CHTF2/HT ratios should approach zero as the reaction probability of the substitution reactions approaches zero. In Figure 2, a threshold of 1.9 f 0.2 eV is obtained for the T-for-H substitution, while a value of 1.9 f 0.3 eV can be evaluated for the T-for-F substitution threshold. The decreasing slope of the extrapolation follows the pattern established by the positive data points of the previous studies of the CH4 and CH3F s y ~ t e m s .The ~ ~ ,quoted ~ errors on the one hand reflect the possible deviation in extrapolation from the line shown in Figure 2 , and on the other hand account for the lowest detectable yields from the 1.9-eV tritium system. The T-for-F substitution threshold value contains more uncertainty because the CHTF2 yields are much lower and with larger errors. Previously, in addition to the 1.5-eV value observed for the onset of the T-for-D substitution in CD4,3the T-for-H displacement in n-butane, n-hexane, cyclopentane, and cyclohexane were all reported to have a threshold of 1.5 f 0.5 eV by the atomic beam m e t h ~ d . ' ~ ,The ' ~ present value of 1.9 eV for the T-for-H threshold in CHF, is actually very similar to 1.8-eV value evaluated for the corresponding threshold in CH3F.& The thresholds for the T-for-H substitution in these fluoromethanes are definitely higher than that in methane, because for 1.9-eV photochemical tritium reactions the latter system shows a definite positive yield while in the former systems the yield is essentially Such a higher threshold for the T-for-H substitution in fluoromethanes might be due to a slightly higher C-H bond strength which would increase the amount of energy required for the C-H bond rupture process, or due to the inductive effects of the F atoms in the molecules to lower the probability of C-T bond formation, or both. The C-H bond dissociation energy in CHF, has been measured as 106.7 f 0.5 kcal mol-' which is higher than those of the hydrocarbon^.'^,^' The operation of an electronegativity and an electron density effect for T-for-H substitution has also been recently estab-
lished.lsJg Mechanistic Implications when Compared with the CH3F Results. In the previous studies on the reactions of photochemically produced hot tritium atoms with CH,F, it was observed that the threshold for T-for-F substitution (1.3 eV) is lower than that for T-for-H substitution (1.8 eV), while the relative reactivity is always higher for the former process at every employed initial tritium energy.3c This observation is consistent with an explanation that involves an inversion mechanism for the near-threshold T-for-F substitution process. The F atoms can act as a leaving group along a linear T-C-F axis which undergoes Walden inversion during reaction. The reason for the different behavior of the two substitution processes is explained by Chou, Wiley, and Rowland as follows.3c "The necessarily concerted nature of such a substitution would be facilitated by the mobility of the three hydrogenic substituents. On the other hand, the substitution of H by T involves a presumably off-center heavy fluorine substituent which would be much less responsive in adjusting to changes in configuration." A recent trajectory calculation by Valencich and Bunker for the CH4 system has shown that substitution with inversion is indeed a feasible but minor process a t lower energieseZ0 In the present CHF, system, two major observations are apparently opposite to what has been indicated in the CH3F case. (i) The threshold for T-for-F substitution is about the same as that for T-for-H substitution. (ii) The relative reactivity for T-for-F substitution is much lower than that for T-for-H substitution a t all the employed initial tritium energies whenever their yields are measurable. However, these two seemingly opposite observations are actually in accord with the mechanistic explanation given above. The T-for-F substitution involves two heavy fluorine substituents which would be very difficult in adjusting to changes in configuration and therefore obscure the possibility of a Walden inversion process.21 With the absence of such an inversion mechanism, the remaining substitution-with-retention process is expected to be more efficient when the tritium atoms are reacting at a C-H bond than at a C-F bond from either the bond strength or the inertial considerations.'8,21 Alternatively, a direct comparison of the relative product yields in the CH3F and CHF, systems implies that the T-for-F substitution-with-retention process has a much higher threshold energy than the corresponding substitution-with-inversion process. The reaction cross section of the former is also much lower than that of the latter when the reactant energy is about 5 eV or below. Similarly, a direct comparison also reveals that for photochemical tritium, the T-for-H substitution in the CH3F system is more efficient than that in the CHF, system. For example, with 2.8-eV T atoms, the CH,TF/HT ratio is about 0.3 while the CTF3/HT ratio is about 0.05. This comparison at least qualitatively agrees with the nuclear recoil studies that in a normalized system the T-for-H substitution in CH3F is some 30% more efficient than the corresponding substitution in CHF, on a per bond base.'* However, the present results do indicate that the differences in reactivity of these two molecules does not depend on different threshold energies, but on a difference in the reaction cross sections a t various energies about the threshold. Acknowledgment. This research was supported by the Robert A. Welch Foundation. References and Notes (1) For the formation of photochemical hydrogen atoms, see: (a) R. A. Ogg, Jr., and R. A. Williams, Jr., J . Chem. Phys., 13, 586 (1945);
Solvent-Solute Interaction Effects on Stereochemical Reactions (b) R. M.Martin and J. E. Willard, /bid., 40, 2997 (1964); (c) K. H. Welge and F. Stuhl, ibid., 46, 2440 (1967); (d) B. deB Darwent, R. L. Wadlinger, and M. J. Ailard, J . Phys. Cbem., 71, 2346 (1967); (e) R. G. Gann and J. Dubrin, J . Chem. Pbys., 47, 1867 (1967); (f) G. P. Sturn, Jr., and J. M. White, J . Phys. Chem., 72, 3679 (1968); (9) C. C. Chou, J. G. Lo, and F. S.Rowland, J . Cbem. Phys., 60, 1208 (1974). (2) For the reactions of photochemical hydrogen atoms, see: (a) R. M. Martin and J. E. Willard, J . Chem. Phys., 40, 3007 (1964); (b) C. C. Chou and F. S. Rowland, J. Am. Chem. SOC.,88, 2612 (1966); J . Chem. Pbys., 46,812 (1967); 50, 5133 (1969); J. Phys. Chem., 75, 1283 (1971); (c) R. G. Gann and J. Dubrin, J . Chem. Phys., 50, 535 (1969); (d) C. C. Chou, T. Smail, and F. S.Rowland, J . Am. Cbem., Soc., 91, 3104 (1969); (e) R. G. Gann, W. M. Ollision, and J. Dubrin, ibid., 92, 450 (1970); (f) J. E. Nicholas, F. Bayrakceken, and R. D. Fink, J . Phys. Cbem., 75, 841 (1971); (9) S.W. Orchard, C. C. Chou, and F. S.Rowland, J. Chem. Phys., 60, 2567 (1974). (3) For the threshold energy studies, see: (a) A. Kupperman and J. M. White, J . Chem. Phys., 44, 4352 (1966); (b) C. C. Chou and F. S. Rowland, ibid., 50, 2763 (1969); (c) C. C. Chou, D. D. Wilkey, and F. S. Rowland, Chem. Phys. Lett., 20, 53 (1973). (4) C. C. Chou, Ph.D. Dissertation, University of California, Irvine, 1968. (5) B. K. Min, Ph.D. Dissertation, Texas A&M University, 1971.
The Journal of Pbysical Chemistry, Vol. 82, No. 9, 1978 975 (6) J. K. Lee, E. K. C. Lee, B. Musgrave, Y.-N. Tang, J. W. Root, and F. S.Rowland, Anal. Cbem., 34, 741 (1962). (7) Natl. Bur. Stand., Tech. Note, No. 496. (8) L. Edwards and J. W. Raymonda, J . Am. Cbem. SOC., 91, 5937 (1969). (9) R. Odum and R. Wolfgang, J. Am. Chem. SOC.,85, 1050 (1963). (10) T. Smail and F. S. Rowland, J . Phys. Chem., 74, 1859 (1970). (11) R. S.Mulliken, Phys. Rev., 50, 1017 (1936); 51, 310 (1937). (12) J. G. Stamper and R. F. Barrow, J . Phys. Chem., 65, 250 (1961). (13) R. F. Barrow and J. G. Stamper, Proc. R. SOC.London, Ser. A , 267, 277 (1961). (14) M. A. Menzinger and R. Wolfgang, J . Chem. Phys., 50, 2991 (1969). (15) R. L. LeRoy, A. J. Yencha, M. A. Menzinger, and R. Wolfgang, J . Chem. Pbys., 58, 1741 (1973). (16) J. C. Amphlett and E. Whittle, Trans. Faraday SOC.,64, 2130 (1968). (17) J. W. Coornber and E. Whittle, Trans. Faraday SOC.,62, 2183 (1966). (18) Y.-N. Tang, E. K. C. Lee, E. Tachikawa, and F. S.Rowland, J . Phys. Chem., 75, 1290 (1971). (19) S. H. Daniel and Y.-N. Tang, J . Phys. Chem., 75, 301 (1971). (20) T.Valencich and D. L. Bunker, Chem. Phys. Lett., 20, 50 (1973). (21) The ideal of a possible inertial effect in recoil tritium substitution reactions was first advocated by R. Wolfgang and co-workers. See, for example, ref 9.
Effects of Solvent-Solute Interactions on the Stereochemical Course in High Energy Chlorine-38-for-C hlorine Substitution in meso- and rac-l,2-Dichloro-l,2-difluoroethane in Solution‘ Tobias
R. Acciani, Yhg-yet Su, Hans J. Ache,*
Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 2406 1
and Edward P. Rack“ Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588 (Received December 1, 1977)
The stereochemistry of the chlorine-38-for-chlorinesubstitution was studied in diastereomeric 1,2-dichloro1,2-difluoroethanesin solutions. The experimental results are very similar to those previously observed in mesoand d,l-2,4-dichloropentanesolutions which by analogy suggest that the stereochemical course of the substitution process is in the present system also predominantly and directly controlled by the properties of the solvent molecules, most likely by the factors which govern the magnitude of intermolecular interaction between reactants and solvents. It appears that strong intermolecular interaction favors substitution via retention of configuration, whereas in solvents having a low dielectric constant the retention/inversion ratio decreases. These results seem further to suggest that if the reaction occurs via the previously postulated caged complex or excited intermediate that the primary attack by the energetic 38Clproceeds via both front and backside replacement.
Introduction A fundamental question in condensed phase high energy chemistry is whether reactions of halogens activated by nuclear transformations in organic media proceed by caging (radical or excited “caged complex”) or molecular mechanisms, or both. Two techniques which are having some success involve studies of (1) hot homolytic substitution reactions by nucleogenic halogen atoms (mainly fluorine and chlorine at asymmetric carbon atoms (generally diastereomeric haloalkanes) in gas and liquid systems2-10and (2) the effects of the gas to condensed state transition (density-variation technique on individual product yields) of high-energy halogen reactions with alkanes, haloalkanes, and unsaturated hydrocarbons.11-20 The various stereochemical experiments showed that the halogen for halogen exchange in the diastereomeric haloalkanes in the gas phase proceeded predominantly with retention of configuration and to a lesser extent in the liquid phase, supporting the Wolfgang “impact mode1”.21-22 The density variation technique in general demonstrates the characteristic sigmoid behavior of organic product 0022-3654/78/2082-0975$0 1.OO/O
yields with increasing density. The appearance of the enhancement region a t high densities has been interpreted as evidence for caging reactions. However, Root et al.12-15 pointed out that discretion must be employed in interpreting the significance of the enhancement effect at high densities since appreciable excitation-stabilization of excited products still occurs in the liquid phase. In an important suggestive study, Stocklin et de2 studied the effects of various solvents on the stereochemical course of hot chlorine-38 for chlorine substitution in liquid 2,3-dichlorobutane. The authors concluded that the solvents changed the rotational conformation population and, hence, the ratio of retention to inversion. The authors suggested that these results could only be interpreted by assuming two channels for hot substitution: (1) direct replacement without a change in configuration in accordance with the “impact model” and (2) a backside attack with inversion giving rise to a highly excited product molecule, stabilized by the liquid cage walls. However, in a recent study, Wu and Achelo found no general conformation effect for various diastereomeric dichloroalkanes. 1978 American Chemical Society
