PETERRIESZAND KENNETH E. WILZBACH
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LABELING OF SOME Cs HYDROCARBONS BY EXPOSURE TO TRITIUM' BY PETERRIESZAND KENNETHE. WILZBACH Contribution f r o m the Cheniistry Division, Argonne NationaJ Laboratory, Lemont, Illinois Received June 84, 1967
G values for the incorporation of tritium into n-hexane, cyclohexane and benzene and into products of their decomposition have been determined for exposure of the hydrocarbon vapors to two pressures of tritium gas a t 25'. The values are such that ionizing radiation rather than recoil tritons must be the important factor in the labeling process. G values were also determined for products formed in chemical amounts from n-hexane. The correspondence of these products to the tritiated products suggests the possible use of tritium as a radiation source to facilitate ident,ification of products.
Introduction Most of the work on tritium labeling2 by exposure of organic compounds to tritium gas has been directed toward obtaining the labeled parent compound; the investigation of the tritiated by-products has been largely neglected. Information concerning these by-products is of practical value in the selection of techniques for purification of the parent compound. The relationship of the by-products to the parent compound and to radiolysis products can provide information on the mechanism of the process and the possible use of tritium as a source to facilitate identification of radiolysis products. The labeling of some Ce hydrocarbons, n-hexanes, cyclohexane and benzene, by exposure to tritium gas was investigated to obtain such information. G values for tritium incorporation in pa,rent compound and other tritiated products were determined for reactions of the hydrocarbon vapors with tritium gas, at two different pressures, at 25". For n-hexane the tritiated products were related to those formed in chemical quantities.
Experimental Procedures Reagent grade hydrocarbons, brought to a purity of 99.9 mole % by fractional freezing, and stored over CaH2, were used. The tritium gas used in the experiments was essentially isotopically pure, but contained about 50% helium-3 from @-decay; its tritium content was determined from ion current measurements3 on a quantitatively diluted sample. The transfer of tritium to the irradiation vessel was carried out in a conventional glass vacuum system mounted in a well-ventilated hood. Irradiations were carried out in spherical bulbs equipped with a capillary side-arm and a break-seal. I n a typical experiment the hydrocarbon, stored over calcium hydride, was carefully degassed and allowed to fill the irradiation vessel to a m e a p e d pressure somewhat less than its vapor The irradiation vessel was immersed in pressure at 25 liquid nitrogen and a measured volume of the tritium gas was introduced by toeplerization. The bulb was then sealed off at the capillary side arm and placed in a thermostat at 25.0" for an appropriate period. At the end of the irradiation, the vessel was attached to the vacuum line, immersed in liquid nitrogen and opened. The non-condensable gas was transferred to a storage vessel through a trap cooled to -195", and any condensable material carried with it was returned to the irradiation vessel. The reaction product was then analyzed, either directly, or after separation into components of different volatility. A separation into volatile, main and polymeric components was accomplished by adding suitable carriers (e.g., n-pentane, the parent hydrocarbon, and n-octane) and allowing the mixture to distil from an ice-bath into traps maintained
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(1) Work performed under the auspices of the U. S. Atomic Energy Commission; presented a t the 132nd Meeting of the American Chemical Society, September 8-13, 1957. (2) K. E. Wilzbach, J . A m . Chem. Soc., 79, 1013 (1957). (3) K . E. Wilzbach, A. R. Van Dyken and Louis Kaplrtn, Anal. Chem., 26, 880 (1954).
at -95 and -195". The polymeric fraction, retained in the irradiation vessel, waa dissolved in the parent hydrocarbon for analysis. Total amounts of tritium in the irradiation products were determined by ion current measurementsa on gas samples obtained by the zinc fusion procedure.4 The non-polymeric irradiation products were also examined on a vapor fractometer (Perkin-Elmer Corporation Model 154) modified by addition of a small ionization chamber within the heated enclosure and in series with the thermal conductivity cell. Chemical and radiochemical peaks were registered simultaneously on synchronized recording potentiometers. From such charts the radioactivity of a given component was determined from the total tritium content of the mixture and the ratio of the area of its activity peak to the total area of all radioactive peaks. The reliability of this method depends upon the elution of all radioactive components and the introduction of a sample identical with that used for tritium assay. (.The latter condition is difficult to achieve in samples which contain volatile components.) Alternatively, the activity of a given component was obtained by comparing the area of its activity peak with that of a component present in chemical amounts, the ratio of areas under chemical and activity peaks having been established by calibration with a radiochemically pure compound.
Results and Discussion Quantities of reactants used in this study were such that absorption of the radiation energy was virtually complete since Dorfman6 has shown that half of the energy of the tritium @-particle is absorbed when the product of the gas density and the radius of the spherical container is 0.08 mg./cm.2. G values for tritium incorporation (the number of T atoms introduced per 100 e.v. absorbed) were calculated on the basis that 7.6 X 10'' e.v. are available6 per hour for one curie of tritium. The amounts of tritium incorporated when the vapors of n-hexane, cyclohexane and benzene were exposed at 25" to two different pressures of tritium gas were determined. The "G" values for incorporation of tritium in the parent compound, in "polymer," and in all products are shown in Table I. The results show that, for a given hydrocarbon, "G" values for tritium incorporation increase with, and are roughly proportional to, the pressure (or mole ratio) of tritium. Such a result cannot be explained on the basis of labeling by recoil tritons, but is compatible with 1a.beling by reactions induced by absorption of radiation energy. From a practical point of view, the advantage of using higher pressures (or mole ratios) of tritium to increase labeling of the parent compound is offset to some extent by the concomitant increase in the. proportion of tritium in other products. (4) K. E. Wilzbach, L. Kaplsn and W. G. Brown, Science, 118, 522 (1953). (5) L. M. Dorfman, Phus. Rev., 96, 393 (1954). (6) G. H. Jenks, J. A. Ghormley and F. H. Sweeton, ibid., 76, 701 (1949).
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LABELING OF C6 HYDROCARBONS BY EXPOSURE TO TRITIUM
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R E L A T I V E RETENTION VOLUME. Fig. 1.-Chromatogram of chemical products from n-hexane. 41
Compd.
Benzene Cyclohexane Hexane
TABLE I GI t VALUESFOR TRITIUM INCORPORATION AT 25’
Preasure. mm. hydrocarbon Tz
73 98 80 86 102 89
22.4 105 17.5 107 4.5 17.8
Reaction vessel, 00.
107 9.9 107 9.9 524 107
At comparable tritium pressures, in the neighborhood of 20 mm., the “G” value for incorporation of tritium in benzene is more than ten times that for hexane or cyclohexane. Further, the fraction of total tritium found in the parent compound is greatest for benzene, and greater for cyclohexane than for hexane. This is clearly shown in Table 11, which presents the distribution of tritium among non-polymeric by-products for these irradiations. Products arising from rupture of C-H rather than C-C bonds account for 95% of the volatile tritium in the case of benzene, and about 60y0 in the case of cyclohexane, the C7 and Cs fragments being methylcyclohexane and ethylcyclohexane. The effect is similar to that encountered in the radiolysis of benzene and cyclohexane. The radiation stability of benzene is well known; the only products found in radiolysis of the vapor are acetylene, ethylene and polymer.7 Dewhurst has commented8 on the preference for rupture of C-H rather than C-C bonds in the irradiation of liquid cyclohexane. The relationship between unlabeled and labeled (7) J. P. Manion and 19. Burton, THISJOURNAL,66, 560 (1952) (8) H.A. Dewhurst, J . Chem. Phys., 24, 1254 (1956).
Hr.
236 19 113
18 162 91
Total energy absorbed, e.v. X 1020
CLment
13.5 0.47 5.1 0.44 9.0 4.3
0.11 .35 .0052 .022 .0019 ,0088
Gpo~vmer
0.05 .52 ,010 .092 .0036 ,0092
Gtor.1
0.165 .89 .021 .17 .009 .048
TABLE I1 FRACTIONS O F “VOLATILE” TRITIUM IN Compd. Benzene Cyclohexane n-Hexane
VARIOUS PRODUCTS Fraction in various products Cn Ca CI C, Ce C7 Ca Cs 0.05 0.95’” .10 0.16 0.13 0.04 .49* 0.04 0.04 .07 0.11 0.19 0.11 .27’ 0.09 0.11 0 . 0 5
lOOyo of Ca activity in benzene. 96% of Ce activity in cyclohexane. 89% of Ceactivity in straight chain compounds.
products from n-hexane was investigated in one experiment (Table I, tritium pressure equals 4.5 mm.) where a larger amaunt of hydrocarbon was irradiated. The extent of conversion of n-hexane to products in this experiment was about 3.570. The chemical and radiochemical quantities of products found in a sample analyzed by vapor fractometry at 52” are shown in Figs. 1 and 2, respectively. The results, expressed in terms of the number of unlabeled molecules produced and the number of tritium atoms incorporated per 100e.v. absorbed, Gc and GT,respectively, are shown in Table 111. The values of Gc for volatile components represent lower limits because of possible loss from the sample, but
PETER RIESZAND KENNETH E. WILZBACH
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RELATIVE RETENTION VOLUME.
Fig. 2.-Chromatogram
of tritiated products from n-hexane.
TABLE 111 same carbon skeleton is found for every product PRODUCTB FORMED ON EXPOSURE OF *HEXANETO TRITIUM detected in chemical amounts. Conversely, chemProduct0
Qc
UT
x
104
QC/QT
4.2 HrCHd 0. 13b Ethane 4.1 320 .67* Propane 5.8 1160 lob Isobutane 2000 0.5 .77b *Butane 1510 5.1 .126 Isopentane 1.3 920 n-Pentane .18 1.5 1200 2-Methylpentane 4800 .24 0.5 n-Hexane 18.6 (105,000)" 2-Methylhexane .064 1.0 640 3-Methylhexane 1.0 .064 640 n-Hep tane 1.1 3-Methylheptane .47 2.0 2350 3-Ethyl hexane n-Octane 0.11 .08 Branched CS .35 2300 Branched CO .04 .04 Branched Cp n-Nonane .07 Branched CIO .07 Branched CU .05 Presence of some unsaturated compounds with the same carbon skeleton is not excluded. b Values represent lower limit. Molecules of hexane per molecule containing a tritium atom.
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values for the ratio of Gc to G;., a measure of the specific activity, are not subject to this limitation. The assignment of carbon skeletons shown in this table is based on comparison of retention volumes on a didecyl phthalate column with those of known compounds; resolution of some unsaturated hydrocarbons from saturated ones with the same skeleton was not possible with this column. The results establish that a tritiated product with the
ical amounts of products are found for every major tritiated component (with the possible exception of n-heptane). The detection of chemical amounts of products corresponding t o minor tritiated components is limited by the fact that sensitivity of detection of radiochemical peaks is 106 times that of chemical peaks. The values of Gc/GT establish that several hundred to several thousand molecules of a given component are produced for each one that contains a tritium atom. Values of Gc/GT are equal for the two methylhexanes and much lower than for methylpentane. This result suggests that both methylhexanes are formed by the same process, presumably the combination of C1and Csmoieties, and that the methylpentane is formed by a different mechanism, presumably the rearrangement of the parent compound. Similarly, the occurrence of chemical and radiochemical peaks where 3methylheptane and 3-ethylhexane would be eluted (indistinguishably) suggests attachment of a Cz fragment a t the 2- and 3-positions of the hexane chain. The appearance of radiochemical, but not chemical, peaks corresponding to n-heptane and noctane indicates that attachment a t the terminal carbon of the hexane chain is a less favored process. As the tritiated products from benzene and cyclohexane are similar to those of radiolysis, so are the chemical and tritiated products from n-hexane similar in nature to those which already have been reported by Dewhurst*.g for the irradiation of the liquid with electrons and y-rays. In addition, the "G" value for gas (Hz CH4)production is in good
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(9) H.A. Dewhurst and E. H. Winslow, J . Chdm. Phys., 86, 969 (1957).
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Jan., 1958
EFFECTS OF PHASE ON RADIO-INDUCED CHEMICAL REACTIONS
agreement with that found on irradiation of nhexane vapor u~by a-particles. Conclusions Two fundamentally different processes for labeling by exposure t o tritium gas should be considered: first, reactions of recoiling triton particles as in the method of Rowland and Wolfgang”; and secondly, reactions between molecules which have been activated by absorption of radiation energy. Recoil tritons, produced by P-decay of Tz molecules, can be eliminated as a major source of labeled products. Since 5700 e.v. are absorbed for each triton produced, it is impossible to obtain a G 7 , value for T incorporation greater than 0.02 from recoil tritons. I n addition, the dependence of the observed “G” values on tritium pressure is incompatible with labeling by recoil tritons. The reactions largely responsible for labeling, then, must occur either between ionized or excited organic molecules and tritium or between tritium atoms or ions and unexcited organic molecules. The results of the present work, particularly those from the experiment with hexane, in which both chemical and radiochemical products were determined, indicate that the former is the favored process. The one t o one correspondence between the carbon skeletons of major labeled and unlabeled products from n-hexane implies that the same process or combination of processes is responsible for their formation, and it is difficult to account for the quan(10) V. P. Henri. C. R. Maxwell, W. C. White and D. C. Peterson. THISJOURNAL, 66, 158 (1053). (11) F. 9. Rowland and R. Wolfgang, Nudeonicu. 14, No. 8, 58 (1950).
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tity of the chemical products on the basis of H atom reactions. The proportions of hexane and tritium were such that only 0.2% of the energy was absorbed by tritium, and the relative values for the ionization potentials and first excited states of the two molecules are such that transfer of charge and of excitation energy from hexane t o tritium molecules is not likely. Asiuming that the “G” value of 1812for the production of H atoms from Ht represents an upper limit for production of T atoms, the number of unlabeled molecules produced is more than 100 times the number of T atoms avaiIable. The results suggest strongly, therefore, that the production of excited and ionized organic molecules and their subsequent reactions determine the nature and distribution of the carbon skeletons of labeled and unlabled products. This conclusion is supported by the striking correspondence between products obtained in previous radiolysis studies of benzene and n-hexane with the products reported here. Since the presence of tritium does not appear to change the nature of the products, the use of tritium as a radiation source may provide a means of identifying products which are undetectable by other methods. This may facilitate the study of compounds which show great radiation stability or permit the use of low dosages t o avoid secondary reactions involving initial products. Acknowledgment.-It is a pleasure to acknowledge valuable discussions with Dr. Louis Ilaplan of this Laboratory. (12) H. E y i n g , J . 0. Hirschfelder a n d H. S. Taylor, J . Chem. P h y s . 4, 570 (1030).
EFFECTS OF PHASE ON REACTIONS INDUCED BY RADIATION IN ORGANIC SYSTEMS BY T. 0. JONES, R. H. LUEBBE,JR.,J. R. WILSONAND J. E. WILLARD Contribution from the Department of Chemistry of the University of Wisconsin, Madison, Wisconsin Received June 84, 1067
This paper summarizes some of the data in the literature on the effects of phase on chemical reactions induced in organic systems by light, ionizing radiation and nuclear transformations. It also resents new data which show that: (1) the ratio of HI to 1 2 roduced in the radiolysis of pure isopropyl iodide with 8 0 6 0 ?-rays is about tenfold higher in the solid phase a t -190’ &an in the liquid a t room temperature; (2) the yield of iodine from the radiolysis of isopropyl iodide in either the liquid or solid is dependent on the past hietory of irradiation of the sample in the other phase; (3) when the spectrum of ethyl iodide glass a t 190’ is examined after exposure of the solid to 2537 A. light, little or no change is ohserved as a result of the irradiation, but when the glass is then melted and immediately refrozen, an absorption peak appears a t 3700 A.; a similar effect is caused by the self-irradiation of tritiated ethyl iodide glass; (4)the ratio of HI to Tp produced by the photolysis of ethyl iodide is approximately ten-fold higher in the glass at -190” than in the liquid a t room temperature.
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Introduction The life history of an atom or molecule activated by radiation may depend critically on whether it is in a gas, a liquid or a solid. In the gas the mean free path and average time between collisions are relatively large. There are no caging effects. Two radicals formed by the dissociation of a molecule have negligible chance of undergoing primary recombination, and they can be prevented from combining with other radicals in the system by the
addition of low concentrations of radical scavengers. In the liquid or solid, however, radicals formed from the same molecule may undergo primary recombination and radicals formed from molecules in adjacent portions of a “spur” may likewise combine with each other before undergoing sufficient diffusion to react with low concentrations of additives. Electrons ejected from molecules in the gas phase by ionizing radiation have no chance of recombining with the parent ion. In c condensed
