COMMUNICATIONS TO THE EDITOR
2752
and
Our results agree quite well with the earlier values and seem to us to remove the possibility that the intensity anomaly for the charge-transfer band in pyridine could be due to experimental error. Finally, we should like to report that we have reproduced the changes reported by Reid and Mulliken
as more pyridine is added. We cannot add significantly to their discussion of these puzzling phenomena, except to note our support for their idea that the complex reacts to form 11- as pyridine is first added and then the Ia- reacts to form I- as more pyridine is added. We should stress that the reaction to form 13- is an equilibrium reaction and that the system is in stable equilibrium. Acknowledgments. We are grateful to Public Health Service Research Grants No. GM-10168 and GM14648 for financial support of this work. Discussions and help with the computing from Dr. Larry Julien together with support from the University of Iowa Computing Center are gratefully acknowledged.
COMMUNICATIONS TO THE EDITOR
The Retention of Optical Configuration during Energetic Chlorine Atom Exchange i n Gaseous Alkyl Halides
Sir: The stereochemistry of the substitution at asymmetric carbon atoms of energetic halogen atoms, activated by nuclear recoil, has been previously studied only in condensed phase systems, in which radicalradical cage combination reactions can play an important role.’V2 We have now studied this substitution reaction CP*
+ R C 1 4 R C P + C1
(1)
a t asymmetric positions in gas-phase experiments and have observed that the exchange proceeds with almost complete retention of optical configuration with either DL- or meso-2,3-dichlorobutane. I n the condensed phase, in contrast, both radioactive isomers of 2,3-dichlorobutane are observed in amounts varying with temperature and phase. The total yield of all organic radioactive products is normally much higher in condensed phases than in gas-phase experiments with haloalkanes consistent with the importance of cage combination effect^.^ Table I contains a summary of the observed results for the reactions of C13s formed by the (7,y) reaction on the natural C13’ of the parent 2,3-dichlorobutane. The organic The Journal of Physical Chemistry
products eluting from the gas chromatographic column through the 2,3-dichlorobutane peaks represent approximately 3% of the total C P formed (in experiments without moderators) and the yield of C13a-2,3-dichlorobutanes is about one-tenth of this total-an over-all yield of 0.3% for the substitution of C13*for C1 in these molecules. The other observed organic products include those expected from the replacement of CH3 by C138,H by C138,etcS3 The observed stereospecificity of the substitution reaction is unaffected by the presence of large excesses of argon or xenon. We have also studied the same reactions with energetic C13@ from a nuclear reaction with entirely different characteristics, Ar40(y ,p) C13g. These experiments, carried out in the presence of large quantities of argon, are in complete agreement with those of Table I, as shown in Table 11. No absolute yield measurements have yet been carried out in our system for reactions with Ar40(y,p)C139. The agreement between the results obtained with (1) C. M.Wai, C. T. Ting, and F. S. Rowland, J . Am. Chem. Soc., 86,2525 (1964).
(2) F.5.Rowland, C. M. Wai, C. T. Ting, and G. Miller, “Chemical Effects of Nuclear Transformations,” Vol. 1, International Atomic Energy Agency, Vienna, 1965,p 333. (3) J. E. Willard, “Chemical Effects of Nuclear Transformations,” Vol. 1, International Atomic Energy Agency, Vienna, 1965, p 221.
COMMUNICATIONS TO THE EDITOR
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Table I : Relative Yields of
and meso-ClS8-2,3-Dichlorobutanes from Reactions of (n,y)ClS8 with Gaseous 2,3-Dichlorobutanes DL-
Target materials --(pressures in om)1,3Buta2,3-DCB diene Others
meso 1.5 meso 1.5 meso 1.5 DL DL
DL
1.4 1.4 1.4
1.0 1.0 1.0 1.0 1.0 1.0
-Relative
DL
1 .oo 0.05i 0.02 0 . 0 3 =IC 0.01 1.00 1.00 0 . 0 3 It 0.01 1.00 0.06i 0.02 0.07i0.02 1.00 0.06~k0.02 1.00
... Ar 30 Xe 31
... Ar 30 Xe28
Table I1 : Relative Yields of
yields of ClaW,3-DCB-
mew
DL-
and
meso-C1~-2,3-Dichlorobutanes from Reactions of Arm( y,p)Cl*O with Gaseous 2,3-Dichlorobutanes Target materials --(pressures in om)----
nism on a time scale consistent with transit times for these energetic atoms in molecular collisions-approximately 10-13 sec. The experimental samples were irradiated in essentially standard fashion, either with neutrons in a nuclear or with high-energy y radiation from an electron accelerator.12 Analysis of the labeled products has been performed by radio gas chr~matograpy.’-~ The most important experimental problem in these systems arises from difficulties in eliminating radical-radical reactions leading to both of the C1-labeled 2,3-dichlorobutanes as products. l13-Butadiene, for which the activation energy for radical attack is quite low,l3J4essentially eliminates this radical reaction and permits the observation of the residual direct, stereospecific substitution reaction. The use of compounds with two asymmetric carbon atoms, with DL and meso forms, is dictated by the combined requirements of gas chromatography and half-lives in the less than 1-hr range: C138,illz = 37.3 t l / , = 56 min. min;
1,3-
2,3-DCB
meso 1.5 DL
1.5
Butadiene
Argon
1.0 1.0
30 30
-Relative yields of Cl*@-2,3-DCBmeso DL
1 .oo 0 . 0 3 & 0.01
0.02i 0.01 1 .oo
two different nuclear reactions and two different isotopes suggests that the reacting species in each system may well be the same, and can be assumed to be neutral atomic chlorine with excess kinetic energy.4 The postulate that the reacting species is not ionic chlorine is bolstered by the observation that similar results are obtained with argon and xenon, whose ionization potentials bracket that of C1.6 While the maximum recoil energy of the C138atom recoiling from the (n,r) reaction is 527 ev, cancellation of momenta from excess energy emitted as several y rays with the proper angular correlation and timing could result in much less recoil, and a small fraction of events might fail to lead to bond rupture of the original C-C13’ bond. Experimental observations show that the increase of the concentration of l13-butadiene from 1 to 76 cm results in a decrease in observed C13*-2,3dichlorobutane by a factor of 15, corresponding to an upper limit on the failure to bond rupture of 0.02’%.6 The substitution reactions of energetic tritium atoms also proceed with essentially complete retention of optical configuration a t asymmetric carbon atoms, both in condensed7-9 and gaseous phases.’0#l1 The substitution reaction by an energetic chlorine atom presumably proceeds by an analogous direct mecha-
(4) The much higher energy ClS9would be expected to be able t o attain charge equilibrium, and hence to be neutral in alkyl halide media, barring any complications from long-delayed nuclear decay from nuclear isomeric states. See R. Wolfgang, Prom. Reaction Kinetics, 3 , 97 (1965). (5) Ionization Potentials (in ev): Ar, 15.76; C1, 13.01: Xe. 12.13: CHsCI, 11.46; other halides, usually less than 11.4. (6) A. A. Gordus and C. Hsiung, J. Chem. Phys., 36, 954 (1962), have estimated the upper limit of failure t o bond rupture as 0.1% for 18 alkyl bromide and iodides. No measurements have been reported for C1 isotopes. (7) F. S. Rowland, C. N. Turton, and R. Wolfgang, J . Am. Chem. Soc., 78, 2354 (1956). (8) H. Keller and F. S. Rowland, J . Phys. Chem., 62, 1373 (1958). (9) J. G. Kay, R. P. Malsan, and F. S. Rowland, J. Am. Chem. Soc., 81, 5050 (1959). (10) M. Henchman and R. Wolfgang, ibid., 8 3 , 2991 (1961). (11) Y. N. Tang and F. S. Rowland, unpublished experiments with 2,3-dichlorobutanes. (12) N. Colebourne, J. F. J. Todd, and R. Wolfgang, “Chemical Effects of Nuclear Transformations,” Vol. 1, International Atomic Energy Agency, Vienna, 1965, p 149. We wish to thank Professor Wolfgang, who privately pointed out that the yields from the reaction of Ar40(y,p)Cl’Qwere satisfactorily high enough for hot atom work. (13) E,,t 2.5 kcal/mole. E. W. R. Steacie, “Atomic and Free Radical Reactions,” Reinhold Publishing Corp., New York, N. Y., 1954. (14) Scavenger combinations which were not successful in suppressing radical-radical reactions included: 02-CzH1; 02-isobutene; 01 alone. (15) This work was supported by A.E.C. Contract No. At-(ll-1)-34, Agreement No. 126.
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CHIENM. W A I ‘ ~ F. S.ROWLAND
DEPARTMENT OF CHEMISTRY UNIVERSITY OF CALIFORNIA IRVINE, CALIFORNIA RECEIVED MARCH 6, 1967
Volume 71, Number 8 July 1967
