675
NOTES T
Acknowledgment. We gratefully acknowledge partial support from Public Health Service Research Grant GRiI-08893-07 from the National Institute of General Medical Sciences, Public Health Service.
- FOR-
H ; RETENTION
IN 2,3-DICHLOROBUTANE
Retention of Configuration during Recoil Tritium Reactions at Asymmetric Carbon Positions in 2,3-Dichl~robutane~ ,4+++-+44*iU,4,4L+.+j-t.*&4* ++++.&+#.+,++++ I
I
I..
I
+ I
Y ++++*b. I
I
I
by Yi-No0 Tang, C. T. Ting, and F. S. Rowland Department of Chemistry, University of California, Irvine, California 92664 (Received July 16, 1969)
Figure 1. Radio gas chromatograms of tritiated 2,3-dichlorobutane-t molecules from reactions of recoil tritium with 2,3-dichlorobutane in 1,a-butadiene plus 02-scavenged gas phase experiments. Upper diagram (displaced upward by 1500 cpm); T* meso-DCB; Lower diagram, T* (EZ-DCB; insets, same chromatograms wit>hcounts summed over 10-min intervals.
+
One of the most important, and common, reactions of energetic tritium atoms from nuclear recoil is the substitution for hydrogen in a C-H bond, as in eq 1.2-6 Earlier experiments have demonstrated that this substitution at an asymmetric carbon atom is accompanied by retention of optical configuration in both the crystalline and gaseous phase^.^-^ However, the experimental procedures have generally involved (a) numerous chemical reactions and separations and (b) sufficiently high specific radioactivity to permit measurement following all of these steps, together with the heavy radiation damage that accompanies such high specific activity. Consequently, measurements have been performed on very few systems, and confirmatory experiments with other systems are thus desirable. T*+RH+RT+H
(1)
The most significant of the previous experiments have involved the reactions of recoil tritium atoms with an optically active molecule, addition of both d- and Zcarriers, optical resolution of the dl mixture, and degradation of each to determine the intramolecular ~ molecules distribution of tritium a c t i ~ i t y . ~ -Target containing two or more asymmetric carbon atoms substitute the much simpler chemistry involved in the separation of diastereomers for the resolution of optical isomers. Since the meso and dl forms of 2,3-dichlorobutane (DCB) can be readily separated and analyzed by radio gas chromatography,’O the experiments can be carried out with lower radiation damage (and specific activity) in the sample. We have carried out a series of experiments in all three phases designed to measure the retention-inversion characteristics of the recoil tritium substitution reaction with these molecules. The experiments have not involved actual determination of the intramolecular location of the tritium in 2,3-DCB.
+
Experimental Section The dl and meso forms of 2,3-DCB were prepared by low-temperature stereospecific addition of Clz to transand cis-Zbutene, respectively, and were then purified further by preparative glpc. The measured macroscopic meso content of dl samples, after irradiation, was approximately 0.5% for condensed phase samples, and less than 0.1% for gas samples. Neutron irradiations were performed on liquid phase samples of DCR in capillaries containing LiF powder and on gaseous samples with 3He as the tritium s0urce.~-5 Enough argon was added to the gas-phase samples to minimize losses of activity by recoil into the walls. The irradiated samples were analyzed in the usual manner by radio gas chromatography, without any special difficulties. The meso- and dl-2,3-DCB peaks separate very satisfactorily on a 35-ft tri tolyl phosphate column, as shown in Figure 1. (1) This research was supported by A. E. C. Contracts No. AT-(11-1)407 and AT-(11-1)-34, Agreement No. 126. (2) R. Wolfgang, Progr. Reaction Kinetics, 3,97 (1965). (3) R. Wolfgang, Ann. Rev, Phys. Chem., 16, 15 (1965). (4) F. Schmidt-Bleek and F. S. Rowland, Angew. Chem.. Int. Ed., 3, 769 (1964). (5) “Chemical Effects of Nuclear Transformations,” Vol. 1 and 2, International Atomic Energy Agency, Vienna, 1965. (6) F. 6. Rowland, C. N. Turton, and R. Wolfgang, J . Amer. Chem. Soc., 78,2354 (1956). (7) H. Keller and F. S. Rowland, J . Phys. Chem., 62, 1373 (1958). (8) J. G . Kay, R. P. Malsan, and F. S. Rowland, J.Amer. Chem. SOC., 81,6050 (1959). (9) M. Henchman and R. Wolfgang, ibid., 83, 2991 (1961). (10) C. M.Wai and F. S. Rowland, J . Phys. Chem., 71, 2762 (1967). (11) J. K. Lee, E. K. C. Lee, B. Musgrave, Y.-N. Tang, J. W. Root, and F. 8. Rowland, Anal. Chem., 34,741 (1962). Volume 74, Number 3 February 6, 1970
NOTES
676
Results Under all conditions of irradiation as summarized in Table I, the radioactivity of the tritiated parent molecule was at least 20 times larger than that of the isomer corresponding to inversion at one carbon atom. I n each experiment, however, a measurable yield of the tritiated “inversion” compound was obtained. These small yields show up more clearly when the observed counts are accumulated into 10-min interval plots than in the 1-min plots used for the parent molecules. Many other labeled compounds are also observed, usually not in positions which interfere with the determination of the dl- and meso-DCB activities. An extra, unidentified radioactivity peak was found between the meso and dl peaks in some 02-scavenged samples; this peak was completely suppressed by the inclusion of l,&butadiene as an additional scavenger. In the absence of information about 12-DCB solid solubilities, the intracrystal scavenging situation is unknown for the solid samples. Small correction factors, proportional to the relative masses, have been applied to the meso data for the macroscopic mass conversion of dl to meso during liquid phase irradiations.
Table I
(A) Ratio of meso- and dl-2,3-Dichlorobutane-t from Recoil Tritium Reactions with 2,3-Dichlorobutanes Relative yield Phase
Gas Gas Gas Gas Liquid Liquid Solid
Sample composition, cm Hg di-DCB Scavenger Ar
1.4 1.4 1.5 1.5
Oz(1.8) oz(3.7) 02(4.4) Og(8.0) None
*He
13.2 11.3 57.9 55.4
1.6 1.6 3.4 7.2
,..
LiF LiF
...
I2 I2
,..
Of
meso-DCB-t
(dl-DCB-t = 100)
LiF
1.4 f0.4 -2 1.2 f0.4 1.3 f0.4 3 . 2 f0.8 2 . 3 f 0.6 4.5 f 1.0
(B) Butadiene/Oz Scavenger Phase
DCB
Gas
dl(1.4)
Gas
meso(l.4)
Scavengers
sHe
Relative Yields dl-DCB-t msso-DCB-t
Oz(4.0) 1,3-Bu(2.6) 1 . 9 100 Od3.6) 1,3-Bu(3.1) 2 . 0 4 . 6 f 0 . 5
1.2 f0.3 100
Coyfigurations Following T-for-HSubstitution at a n Asymmetric spx Carbon Atom. The data of Table I confirm completely the general conclusion from the previous experiments-substitution of T-for-H occurs very preferentially with retention of configuration at the asymmetric spa carbon atom. Inversion of configuration during substitution would require appreciable motion by the other substituent atoms and groups, and has been postulated to be highly disfavored beThe Journal of Physical Chemistry
cause insufficient time is available during the substitution process for completion of the requisite atomic motions.2-s Retention of configuration, on the other hand, need not involve extensive adjustment of position by the other substituents and could readily occur sec. The present exon a time scale of periments are completely consistent with this hypothes i ~ ; ~in- this, ~ as in the earlier experiments, all of the other substituents are at least as heavy as a methyl group and the question of retention vs. inversion with light substituents (i.e., H or D atoms) has not been studied. Formation of the “Opposite” Isomer in Gas Phase. All of the experiments demonstrate the presence of the “opposite” radioactive isomer in small yield. Precise determination of the per cent retention/inversion a t the asymmetric carbon atom would require a determination of the intramolecular distribution of tritium between the CH3- and -CHCI- positions of each product molecule. Comparisons with other inter- and intramolecular competitions suggests that the per-bond substitution of T-for-H will be quite similar for both bond types (perhaps from 70 to 100% as much -CTClas CH2T-, compared per b ~ n d ) , ‘ ~and ~ ’ ~thus that 18-25% of the tritium activity of the parent stereoisomer is actually in the asymmetric C-H position. Even if all of the tritium activity of the meso isomer from the reaction of T* with dl-DCB is in the asymmetric position, the gas-phase preference for retention of configuration is 293% with this parent molecule. The corresponding lower limit for the meso isomer is approximately 2 80%. The presence of the labeled opposite isomer is clearly demonstrated by the data of Figure 1 and Table I, keeping open the question of the possible existence of some bona fide hot substitution-with-inversion reactions. However, similar gas-phase experiments involving the stereochemistry of the substitution of 38Cl or asCl for C1 in 2,3-DCB have shownl0 (a) low, but detectable, yields of the! opposite isomer (opposite/ parent -0.02) in butadiene-02-scavenged systems and (b) substantial yields of the opposite isomer (ratios from 0.50-1.0) unless butadiene is present. Since the gas-phase yield of the opposite isomer in the tritium experiments is very small (absolute yield -O.l%), and some small uncertainties exist concerning the detailed scavenging mechanisms under (a) and (b), we do not consider that the small peaks observed in these experiments are positive proof of the presence of a substitution-with-inversion mechanism. The desirability, and great difficulty, of intramolecular determination of the tritium location in the opposite isomer makes further experiments with the 2,3-DCB molecules less advantageous than experiments with similar dl, meso (12) T. Smail and F. S . Rowland, J. Phys. Chem., 72,1845 (1968). (13) J. W. Root and F. S. Rowland, J . Amer. Chem. Soc., 84, 3027 (1962).
NOTES pairs containing hydrogen only in the asymmetric positions, e.g., CHFClCHFCl. Inversion Products in Condensed Phases. The yields of meso-2,3-DCB-t from the reactions of T * with the dl parent are definitely higher in the condensed phases than in the gas phase, even though still small. The most probable explanation for these increased yields is that some dl-2,3-DCB-t molecules, ini tially formed by T-for-H substitution, are sufficiently excited to decompose by C-C1 bond rupture. The residual C ~ H T TC1 radical can then undergo either (a) immediate recombination with the C1 atom, retaining the original stereochemistry; or (b) racemization to a mixture of radicals of both “dl” and “meso” form, and then recombination with the C1 atom, providing additional yields of both labeled isomeric forms. These recombination processes can be facilitated in the condensed phase by the surrounding solvent or crystal cage, capable of retaining atom and radical in close proximity to one another. Studies of 38Clreactions with 2,3-DCB in the condensed phases show large yields of both labeled isomers, in both liquid and solid. The presence of both labeled forms in nonequilibrium amounts in the 38Cl experiments has been attributed in close competition between the time scales for racemization of newlyformed CH3CHC1CHCHa radicals and combination of these radicals with others in the cage. With W 1 in the liquid phase a t room temperature, about 30% of the reformed molecules have the “opposite” configuration from that of the parent.14 Such cage recombinations, however, play only a small percentage role in the tritium experiments. Assuming similar competitive rates for racemization and recombination, the contribution of cage reactions to the total parent yield is no more than 8% in recoil tritium reactions with 2,3-dichlorobut ane. An alternate hypothesis seems much less likely, but cannot be eliminated by present data. If a substitution-with-inversion mechanism is postulated to require the simultaneous deposition of very large amounts of excitation energy,I5 the fraction of such molecules surviving in the gas phase could be substantially smaller than for labeled molecules formed by substitutionwith-retention, in agreement with the observations of Table I. Such a postulate is not wildly unreasonable when inversion would be accompanied by motion of CH,, C1, and CHClCH3 groups. I n any event, the total yield corresponding to stabilized “inversion” product, as measured in condensed phases, is very much smaller than that of the “retention”product, and the heavily predominant mechanism of T-for-H substitution must still be identified as the retention mode. (14) F. 8. Rowlmd, C. M. Wai, C. T. Ting, and G. Miller, “Chemical Effects on Nuclear Transformations,” Vol. I, IAEA, Vienna, 1965, p 333.
(15) See E. K. C. Lee and F. S. Rowland, J , Amer. Chem. Sac,, 85, 897 (1963).
677
A Multilayer Isotherm with Sensible Spreading Pressure Limits
by C. M. Greenlief and G. D. Halsey Department of Chemistry, University of Washington, Seattle, Washington 98106 (Received July $8, 19f30)
Isotherm equations that cover the entire range of adsorption have exhibited one of two types of thermodynamic difficulties: either the lack of a Henry’s law region at low coverage, or in the case of the BET equation a spreading pressure that approaches infinity as the pressure tends toward the saturation vapor pressure, Po.’ One of us has proposed an equation that has the virtue of having the correct limiting behavior at both low and high coverage.2 It is the purpose of this note to explore this equation further. The isotherm is a linear combination of a virial expansion at low coverage and a term at high coverage due to the decay of surface energy with the third power of the distance. Specifically
P = bn exp[(c/b)n]
+ POexp[-a(~/v,)-~]
(1)
where c is negative and a and b are positive. n is the amount of gas adsorbed, v is the volume of gas adsorbed, and urn is the volume of a monolayer of gas; a is a measure of the adsorption potential. For low coverages, eq 1 becomes P = b n + c n 2 + ..,
(2)
These constants can be expressed in terms of gas-solid (b = KT/BAs and C = k T C A A B / virial ~oefficients,~ PAS). The expansion is valid only if c is negative, but this is to be expected in the temperature range below the critical temperature of the adsorbate, in the region where a multilayer isotherm becomes appropriate.4 At high coverages eq 1approaches the limit In (P/Po) = -aa/03
(3)
where e = v/vrn, the surface coverage in monolayers. Typical isotherms of eq 1 are shown in Figure 1. The isotherm as proposed has no empirical parameters, and its only arbitrary feature is the choice of a simple exponential blending function. I n Figure 2, experimental data of Prenzlow and Halsey5 for the adsorption of argon on one layer of xenon preadsorbed on graphitized carbon black are fitted to eq 1. The constants have been adjusted to the Cassel, J , Phys, Chem.,48, 195 (1944), (2) G. D. Halsey, “The Solid-Gas Interface,” E. A. Flood, Ed., Marcel Dekker, New York, N. Y., 1967. (3) W. A. Steele, Advan. CoZZoidInterfac.Sei., 1 , 3 (1967). (4) G . D. Halsey, J . Chem. Phys., 36,1688 (1962). (5) C. F. Prenzlow and G . D. Halsey, J . Phys. Chem., 61, 1158 (1957). (1)
Volume 74, Number d February 6, 1970
