CHIENM. WAI AND F. S. ROWLAND
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The Stereochemistry of Energetic Chlorine Atom Exchange in Alkyl Halides1 by Chien M. Wai and F. S. Rowland Department of Chemistry, University of California, Irvine, California 91664
(Received May 9, 1969)
Energetic chlorine atoms produced in nuclear recoil substitute for C1 atoms in either meso- or dl-2,3-dichlorobutane with almost complete ( 93%) retention of configurationat the asymmetric centers in the gas phase. The same result is obtained for hot chlorine atoms from the nuclear reactions 37Cl(n,y)38C1 and 40hr(y1p)a9C1.
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Introduction Energetic chlorine atoms are able to replace C1 atoms in alkyl chlorides, both in the gaseous and condensed I n the present experiments, we have focused our attention on the stereochemistry of the substitution process a t an asymmetric carbon atom in an effort toward better understanding of the mechanism of the energetic substitution process. The major experimental difficulty involved in stereochemical experiments with molecules possessing only one asymmetric center is the problem of complete resolution of a racemic mixture into its pure d- and Z- forms. In our case, an additional restriction arises from the relatively short half-lives (95% purity by low-temperature stereospecific addition of Cl2 to trans- and cis-2-butene1 respectively,12 and then purified by gas chromatography. The actual limitation on purit,y is set, not by the The Journal of Phgsical Chemistry
initial impurity level after preparative gas chromatography, but by the radiation chemical format,ion of the “opposite” stereoisomer from all radiation (neutron, gamma, etc.) present during the thermal neutron irradiation of the samples. The measured macroscopic fraction of the “opposite” isomer was always less than the fraction of chlorine radioactivity found in that isomeric form, indicating that the radioactivity observed in the form ol the inversion product could not arise solely from radiation chemical damage to the radioactive, stereochemically pure parent isomer. However, thermal reaction mechanisms of thermalized radiochlorine atoms-such as the reaction suppressed by l,$butadiene, as discussed helow-could account for all or part of the higher specific activities found for the inversion products. Experimental Techniques. The details of sample preparation, irradiation, and radio gas chromatographic analysis are adequately described elsewhere, and are essentially unchanged from these previous descripA typical radio gas chromatogram, taken on a 15-ft tritolylphosphate column with 3gClradioactivity, is shown in Figure 1. The recorded activities are then corrected for the decay of the particular
(1) This research was supported by AEC Contract No. AT-(ll-1)-34, Agreement No. 126, and constituted part of the thesis submitted by C. M. Wai in partial fulfillment of the requirements for the Ph.D. degree at the University of California, Irvine. (2) J. E. Willard, “Chemical Effects of Nuclear Transformations,” Vol. 2, International Atomic Energy Agency, Vienna, 1965, p 221. (3) F. 8. Rowland, C. M. Wai, C. T. Ting, and G. Miller, “Chemical Effects of Nuclear Transformations,” Vol. 2, International Atomic Energy Agency, Vienna, 1965, p 333. (4) R. Wolfgang, Progr. Reaction Kinetics, 3, 97 (1965). (6) C. M. Wai and F. S. Rowland, J . Phys. Chem., 71,2752 (1967). (6) C. M. Wai and F. S. Rowland, J. Amer. Chem. Soc., 90, 3638 (1968). (7) C. M. Wai and F. S. Rowland, J . Phys. Chem., 72,3049 (1968). (8) L. Spicer and R. Wolfgang, J. Amer. Chem. SOC.,90,2426 (1968). (9) C. M. Wai and F. S. Rowland, ibid., 91, 1053 (1969). (10) C. M. Wai, Ph.D. Thesis, University of California, Irvine, 1967. (11) L.Spicer, Ph.D. Thesis, Yale University, 1968. (12) H. J. Lucas and C. W. Gould, Jr., J . Amer. Chem. Soc., 63,2541 (1941). (13) J. K. Lee, E. K. C . Lee, B. Musgrave, Y.-N. Tang, J. W. Root, arid F. 8. Rowland, A?zaZ.Chem., 34,741 (1962).
STEREOCHEMISTRY OF C1 ATOMEXCHANGE IN ALKYLHALIDES 39CI-MES0 5g4v)
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RETENTION TIME (MIN.) Figure 1. Gas chromatographic records of meso- and dG2,3dichlorobutane peaks from the reactions of energetic a9C1with meso-2,3-dichlorobutane in the gas phase. Upper trace, 3QC1 radioactivity; lower trace, thermal conductivity.
isotope during the time between chromatographic peaks.14 The chief experimental problem is the low total radioactivity of labeled 2,3-DCB in gas-phase experiments with the pure molecules. This difficulty arises from two factors-the low vapor pressure of the target molecule (about 1.8 cm in room temperature irradiations) and the low absolute percentage yield (
