850
WILLIAMA. PRYOR AND UMBERTO TONELLATO
The Use of Tritiated H2S as a Radical Scavenger in the Y
Radiolysis of Organic Liquids1
by William A. Pryor and Umberto Tonellato2 Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana
70808
(Received July 19, 1968)
We here report the use of tritiated hydrogen sulfide as a scavenger in the y radiation of benzene, cyclohexane, and toluene at 27’. The hydrocarbon solvents were recovered and purified, and their activity was determined. In separate experiments, the isotope effect IC&T for the reaction of phenyl and cyclohexyl radicals with thiols has been measured. If it is assumed that labeling occurs primarily by radical reactions, then the yield of the tritiated hydrocarbon, G(RH-t), and the isotope effect can be combined to give a value for the yield of the parent-minus-hydrogen radical from these three solvents. (See eq 1 and 3.) The values so obtained agree quite well with literature values using iodine as a scavenger. The method has also been evaluated for producing tritium-labeled materials; it gives higher G values for the production of tritiated toluene than does the Wilzbach method and perhaps is somewhat more convenient.
The destruction of organic solvents by radiolysis occurs largely by free-radical and excited-molecule reaction^.^^^ Several scavenger methods4r6 have been developed for quantitatively identifying the radicals which occur in these reactions, since these data can be used in attempts to elucidate the mechanisms of radiodestruction. Some time ago, in connection with another project, we examined tritiated hydrogen sulfide as a scavenger in the y radiolysis of organic liquids. Our studies indicate that HzS-t is a $useful scavenger which gives data comparable with that obtained using other, more well-known scavengers such as iodine. During the course of our work, Ausloos and Lias reported the use of HzS as a radical scavenger in the gas-phase radiolysis of propane,6 Reisz, White, and Kon reported the use of HZS-t in the solid-phase radiolysis of proteins,’ and Ausloos, Scala, and Lias* reported some results using HzS in the liquid p h a ~ e . ~ , ’ ~ We had originally hoped that HzS-t would prove to be a suitable reagent for a modified Wilzbach’l method of synthesizing labeled organic materials. I n the Wilzbach technique, tritiated hydrogen gas is mixed with an organic material, and self-radiolysis leads to tritiation of the organic substance. Since Wilzbach’s orginal discovery, various “accelerated” methods12 have been suggested. We initially felt that y irradiation of organic materials in the presence of HZS-t might have some advantages. However, since HzS-t usually incorporates only one tritium per primary radical, the G values for incorporation of tritium into the substrate are not much larger than those obtained in the Wilzbach procedure. Nevertheless, our method may offer some convenience since HzS-t can readily be prepared from H2O-t and is easier to handle than is tritiated hydrogen gas. It is also possible that the labeled compounds prepared by this method may have lower levels of contamination from radioimpurities or that the The Journal o j Physical Chemistry
chemical nature of these impurities could be more easily predicted. The occurrence of radioimpurities is the greatest drawback of the Wilzbach method.l1S12 It has recently been shown13 that HC1 undergoes a chain reaction in which labeled atoms are incorporated with a G value for exchange of about lo4; this system might be superior for Wilzbach-type labeling work and it should be studied from this viewpoint. Clearly, however, it could not be used as an index of radiodestruction.
Experimental Section The H2S-t was prepared by hydrolyzing Al2S3 with tritiated water in the absence of air,14 was purified by two vacuum distillations, and was transferred to a calibrated bulb equipped with a manometer. A known volume of the HzS was then transferred to an ampoule containing 1-2 ml of the substrate held at liquid nitro(1) Supported in part by the U. 9. Air Force Offlce of Scientific Research, AFOSR(SRC)-OAR, under Grant No. 540-66, 1965-1967; and by the U. 5. Public Health Service, National Institutes of Health, Grant GM-11908-03, 1967-196s. (2) A Postdoctoral Fellow supported by the AFOSR, 1965-1967. (3) For a general review, see J. W. T. Spinks and R. J. Woods, “Introduction to Radiation Chemistry,” John Wiley & Sons, Inc., New York, N. Y., 1964, Chapter 6. (4) (a) F. S. Dainton, Pure A p p l . Chem., 10, 395 (1965); (b) R. A. Holroyd in “Aspects of Hydrocarbon Radiolysis,” T. Gatimann and J. Hoigne. Ed., Academic Press, New York, h-. Y . , 1968, PP 1-32; (c) R. H . Schuler and R . W. Fessenden in “Radiation Research,” North Holland Publishing Co., Amsterdam, 1967, pp 99-112; (d) P. Ausloos. Ann. Rev. P h y s . Chem., 17, 205 (1966). (6) A method which employs a somewhat different approach is the “radical sampling technique” in which isotopically labeled CzH4 or C H d is used, See R. A. Holroyd and G. W. Klein, Int. J . Appl. Radiat. Isotopes, 13, 493 (1962); J . Amer. Chem. SOC.,84, 4000 (1962); J . Phys. Chem., 69, 194 (1965). (6) P. Ausloos and S. G. Lias, J . Chem. Phys., 44, 521 (1966). (7) (a) P. Riesz, F. H. White, and €1. Kon, J . Amer. Chem. SOC.,88, 872 (1966); (b) F. H. White, P. Riesz, and H. Kon, Radiat. Res., 32, 744 (1967). (8) P. Ausloos, A. A. Scala, and 8. C. Lias, J . Amer. Chem. SOC.,89, 3677 (1967).
THE USE OF TRITIATED HzS AS G(RH-t) =
A
851
RADICALSCAVENGER
(H or T atoms from HIS-t incorporated into recovered RH-1) (100 eV absorbed)
(total activity recovered in RH, pCi) -~ 0.5(H2S specific activity, pCi/mol)
gen temperature. The ampoule was constructed so the organic liquid occupied about 95% of its volume. The specific activity of the H2S was measured directly by absorption into a concentrated KOH solution which was counted in a Packard Model 3365 liquid scintillation counter. Corrections for quenching were made using a series of standard solutions purchased from Packard and the automatic external standardization mode of the counter. The irradiation source was a 11,000-Ci cobalt60 pool reactor. The ampoules were rigidly held in a large evacuated glass cylinder which was placed in a diving bell. The temperature in the bell during irradiation was 27 *2’, and the dose rate was 1010 rad/ min (Fricke dosimetry), After irradiation any gaseous products were removed, and the solvent was washed with KOH solutions and water and was then dried. A reverse isotope dilution technique using repeated distillations with a n efficient column and large dilution factors was used to purify the substrate to constant specific activity. Clearly, this type of study could also give yields of products other than the recovered substrate; we did not attempt this type of analysis because we did not have access to a gas chromatograph-flow counter system.
Results Hydrogen sulfide is an excellent candidate as a radical scavenger.6-8r10 It reacts with radicals with a very large rate constant,16 but the HS. which is produced does not abstract hydrogens from any but the most activated CH bonds.6--8f16Thus when tritiated H2S is used not more than one tritium label should be introduced into the products for each primary radical formed for most organic substrates. If the isotope effectfor the reaction of radicals with H2Sis determined, the yield of radioactive products can be related to the total yield of primary radicals. Table I gives the data. We have expressed the amount of exchange in terms of G(RH-t), the yield, in molecules per 100 eV absorbed, of recovered labeled substrate. This G value is corrected for the specific activity of the H2S-t used but not for any possible isotope effectlS (see eq 1). The activity of the HZS-t was kept low so that any possible destruction caused by self-radiolysis would be small. (See run 7, Table I.) Runs 1-6 show that increasing the HZS-t concentration in cyclohexane from 1.1 to 5.6% does not affect G(RH-t). Therefore, these concentrations of HzS are sufficient to scavenge all the free cyclohexyl radicals.*O Run 8 shows the effect of a
6.023 X loza (100 eV absorbed)
larger absorbed dose. Although the accuracy of the data may not be sufficient, it would appear that G(RH-t) may decrease a t higher doses. (9) A number of related references can be cited, some of which have been reviewed by 157. A. Pryor in “Annual Reports on the Mechanisms of Reactions of Sulfur Compounds,” Vol. 2, N. Kharasch, B. S. Thyagarajan, and A. I. Khodair, Ed., Intra-Science Research Foundation, Santa Monica, Calif., 1967. Some of the more pertinent references are given: (a) Meissner and Henglein used DzS to scavenge hydrogen atoms produced in the radiolysis of liquid hexane (G. Meissner and A. Henglein, Ber. Bunsenges. Phys. Chem., 69, 264 (1965)); (b) these workers also studied the radiolysis of solid, liquid, and gaseous HsS (G. Meissner and A. Henglein, Z . Naturforsch., 20b, 1005 (1965)) ; (c) McNaughton has reported the radiolysis of HiS in the liquid phase (G. S. McNaughton, Trans. Faraday Soc., 62, 1812 (1966)) ; (d) Myron and Johnsen reported the radiolysis of liquid ethanethiol (J. J. J. Myron and R. H. Johnsen, J . Phys. Chem., 70, 2951 (1966)); (e) it has long been known that HzS adds t o olefins under radiolytic conditions; see ref 10 and for a recent reference see K. Sugiomoto, W. Ando, and S. Oae, Bull. Chem. SOC.Jap., 38, 221 (1965); (f) Lunde and Hentz have studied the radiolysis of thiophene (G. Lunde and R. R. Hentz, J . Phys. Chem., 71, 863 (1967)); (9) Bergdolt and Schulte-Frohlinde have reported the yields of hydrogen in the radiation of solutions of thiophenol-dl in cyclohexane (A. Bergdolt and D. Schulte-Frohlinde, Z. Naturforsch., 22b, 270 (1967)); (h) Karmann, Meissner, and Henglein have reported the pulse radiolysis of aqueous solutions of HzS (W. Karmann, G. Meissner, and A. Henglein, Z. Naturforsch., 22b, 273 (1967)): (i) Kroh and Hankiewicz have reported the radiation of benzene and toluene in the presence of KzO-t (J. Kroh and E . Hankiewice, Chem. P h y s . Lett., 1, 542 (1968)) and concluded that labeling occurs primarily via excited molecule reactions. (10) W. A. Pryor, “Free Radicals,” McGraw-Hill Book Co., Inc.? New York, N. Y . , 1966. (11) (a) K. E. Wilzbach, J . Amer. Chem. Soc., 79, 1013 (1957); (b) K. E. Wilzbach, Advan. Tracer Methodol., 1 , 4, 28 (1963); (c) P. Reisz and K. E. Wilrbach, J . Phys. Chem., 62, 6 (1958). (12) This has been reviewed: (a) J. R . Jones, Lab. Fract., 14, 433 (1965); (b) A. P. Wolf, Proceedings of the Second International Conference on Methods of Preparation and Storage of Labeled Compounds, Euratom Publishing Co., 1966; (c) H. L. Bradlow, D. K. Fukushima, and T. F. Gallagher, Atomlight, 2 (1959); (d) M. L. Whisman and B. H. Eccleston, Nucleonics, 20, 98 (1962); (e) M. Weneel and P. E. Schulee, “Tritium LMarkierung Darstellung, Messung und Anwendung nach Wilebach Markierter Verbindungen in Medinein, Chemie, Landwirtschaft Industrie,” Walter de Gruyter and Go., Berlin, 1962. (13) (a) J. W. Fletcher and G. R. Freeman, Can. J . Chem., 44, 2645 (1966) ; (b) this result could have been predicted from the studies of H. L. Benson and J. E. Willard, J. Amer. Chem. Soc., 83, 4672 (1961), 88, 6689 (1966), and those of R. H. Wiley, W. Miller, C. H. Jarboe, J. R. Harrell, and D. J. Parish, Rad& Res., 13, 479 (1960); see also P. J. Homer and A. J. Swallow, J. Phys. Chem., 65, 963 (1961). (14) This technique is similar to t h a t used t o prepare DzS by A. Kruis and K. Clusius, 2. Phys. Chem. (Leipzig), B38, 158 (1938). (15) The following data can be cited: (a) the rate constant for the reaction of pentyl radicals with pentanethiol a t 25O is 7 x 104 M-1 sec-1 (R. D. Burkhart, J. Amer. Chem. Soc., 90, 273 (1968)); the rate constant for the reaction of CFa. with Has is 4.5 X lo8 exp(-3900/RT) M-1 sec-1 (J. C. Amphlett and E. Whittle, Trans. Faraday Soc., 63,2695 (1967)) ; the activation energy for this reaction has also been reported to be 3.9 kcal/mol by N. L. Arthur and T. N. Bell, Can. J. Chem., 44, 1445 (1966); (c) the rate constant for the reaction of methyl radicals with HzS is 2.6 X lo* exp(-2600/RT) M-1 sec-1 (N. Imai and 0. Toyama, BUZZ. Chem. Soc. Jap., 33, 662 (1960)); (d) the rate constant for the reaction of OH. with HzS in ~ ~ the rate constant aqueous solution is 1.1 X 1O’O M-1 ~ e c - 1 ; (e) for the reaction H H B + HZ HS . is loDM-1 sec -1 in aqueous solution (G. Meissner and A. Henglein, Ber. Bunsenges. Phys. Chem., 69, 3 (1965)).
+
+
Volume 75,Number 4 April 1960
WILLIAMA. PRYOR AND UMBERTOTONELLATO
852
Table X : y Radiation of Organic Liquids in the Presence of H2S-1
Run
1 2 3 4 5 6 7 8 9
10
Activity in
wt,
Mol %
HzS-1,
Substrate
g
of HIS4
pCi
Cyclohexane Cyclohexane Cyclohexane Cyclohexane Cyclohexane Cyclohexane Cyclohexane Cyclohexane Toluene Benzene
1.408 0.949 1.511 1.377 0.889 0,721 1.506 1.206 1.072 1.553
1.07
41.1 48.4 95.0 120.2 83.0 116.6 235.5 211.3 50.3 22.1
1.74 2.27 2.40 3.17 5.61 4.30 4.82 2.12 0.93
-Tritium I n nonvolatile products
10-ro X absorbed dose, eV
1.16
0.180
1.00 1.11
... ...
1.32 0.94 0.76
0.288
...
2.32 0.84 1.21
In purified
substrate
0.159 0.132 0.213 0.239 0.127
...
0.104
