931
RADIOLYSIS OF CYCLOHEXANE SOLUTIONS
Electron Scavenging in the Radiolysis of Cyclohexane Solutions of Alkyl Halides 1 by John M. Warman, K.-D. Asmus, and Robert H. Schuler Radiation Research Laboratories, Mellon Institute, Carnegie-Mellon University, Pittsburgh, Pennsylvania (Received September $ 0 , 1968)
15813
The formation of alkyl radicals in irradiated solutions of CHSC1, CH3Br, and CzH6Br in cyclohexane has been investigated at concentrations from lov4 to 0.5 M . Experiments with added ethylene and cyclopropane show that the radicals do not result from reactions of either hydrogen atoms or positive ions. From this it appears that radical formation results exclusively from electron capture. The observed concentration dependence can be very well described by the expression
The low concentration limit (Gti) is identified with the free-ion yield and is found to be -0.1. It is believed that the high concentration limit (Gfi f Ggi), which is found to be 3.9 f 0.1, can be identified with the total yield of electrons available for reaction with solutes. The above expression has been used to treat competitive electron capture by two alkyl halides. Deviations from ideality observed in the CH&l-C&,Br system are interpreted in terms of electron transfer from a CHaCl anion to the bromide. It is shown that such secondary ionic reactions can be described quantitatively.
In recent years there has been considerable interest in the role of electron-scavenging processes in the radiolysis of hydrocarbon solutions. A fair number of studies on this subject have now been carried out, and the current status of knowledge has been summarized in an article which appears elsewhere.2 The first attempt to examine the importance of electron capture in the radiolysis of hydrocarbon solutions was made in 1954 by Williams and HamilF when they studied the radical yields produced from solutions of alkyl halides. They found that appreciable yields of radicals were produced from the solute (G's > 1) a t concentrations M and attributed these yields for the most above part to dissociative electron-capture processes. It has, however, always been difficult to rule out other possible mechanisms which might also result in formation of radicals from alkyl halides. Interest in details of the chemical effects of the electron-capture process has been renewed by the finding of Scholes and Simic4 that large yields of nitrogen are formed in the radiolysis of solutions of nitrous oxide in cyclohexane. Because this solute has been used as a specific electron scavenger in aqueous systems6 with considerable success by numerous workers and is known to give nitrogen as a result of electron-capture processes in the radiolysis of gaseous mixtures,6 the qualitative conclusion of Scholes and Simic that nitrogen results from electron capture in the nitrous oxide-cyclohexane solutions seems very likely to be valid. However, the nitrogen yields observed are very high and indicate that some additional effects which involve secondary processes are also present. The primary aim of the present research was to reinvestigate the radical yields from alkyl halide so-
lutions in order to examine as critically as possible the extent to which electron capture contributes to them and to determine the Concentration dependence of electron scavenging over as wide a concentration range as possible. The latter is of particular interest in view of recent theoretical treatments of the subject. A knowledge of the concentration dependence is also necessary for the quantitative treatment of competitive scavenging in systems containing more than one solute. Attention has been focused here on the alkyl bromides and chlorides. It is known that in the case of methyl iodide solutions considerable methane is formed.' Because of this and other complications due to hydrogen atom and positive-ion attack on the iodides, these solutes will be treated ~eparately.~ p8
Experimental Section Cyclohexane (Phillips research grade) was chromatographed through a silica gel column to remove polar and olefinic impurities. Matheson methyl chloride, methyl bromide, and ethylene were distilled (at -80') and then stored on a vacuum line. Ethyl (1) Supported in part by the U. 9. Atomic Energy Commission. (2) J. M. Warman, K.-D. Asmus, and R. H. Schuler, Advances in Chemistry Series, No. 82, American Chemical Society, Washington, D. C., 196S,p 25. (3) R. R. Williams and W. H. Hamill. Radial. Res., 1, 158 (1954). (4) G. Scholes and M. Simic. Nature, 202, 895 (1964). (5) F. 5. Dainton and D. B. Peterson, ibid., 186, 878 (1960): Proc. Roy. Sot.. A267, 443 (1962). (6) G. R. A. Johnson and J. M. Warman, Nature, 203, 7 3 (1964); Trans. Faraday SOC.,61, 1709 (1965). (7) R. H.Schuler, J . Phys. Chem., 61, 1472 (1957). (8) P. R. Geissler and J. E. Willard, J . Amer. Chem. SOC.,84, 4627 (1962). (9) R. H.Schuler and J. L. McCrumb, to be published. Volume 75, Number 4
April 1969
J. M. WARMAN,K.-D. ASMUS,AND R. H. SCHULER
932 bromide was purified gas chromatographically and then outgassed and stored on the vacuum line. Radioiodine ( I3lI2)was prepared as described previously.1o A measured pressure-volume of the solute vapor was added to 5 ml of an outgassed solution of cyclohexane containing radioiodine of known specific activity. Vapor volumes above the sample were -0.5 cc so that essentially all of the solute was in the liquid phase. Initial experiments indicated that for solute concentrations less than 1M the yields from samples prepared in the conventional way were possibly too low because of the failure to remove COz during the outgassing procedure. Therefore for solute concentrations M cyclohexane was dried over sodium and below pumped a number of times on the vacuum line at room temperature to remove CO2. Radioiodine of known specific activity and the desired amount of alkyl halide were then added on the vacuum line without further exposure of the sample to air. The iodine concentration in this case was determined optically after the irradiation. Preparation of the samples in this way considerably increased the yields at concentrations of -lo-* M but had little or no effect a t concentrations above M . In general, iodine concentrations were -0.4 mM or higher except for experiments a t the lowest solute concentrations where the iodine concentration was always lower than that of the solute. Comparison experiments a t different iodine concentrations showed that the iodine affected the observed radical yields only when present a t a concentration higher than those of the other solutes. The samples were irradiated with 6oCoy rays at an absorbed dose rate of 5 X lo1* eV cc-l hr-l in cyclohexane. Total doses were 4 X 10'' eV cc-I except where low iodine concentrations required more limited irradiations. At the doses used only l0-20% of the iodine reacted during the irradiation. Alkyl radicals produced from the alkyl halides during irradiation were measured as the alkyl iodide produced by the reaction
R e+ 13112 --+
R131I
+ 1-
From the results on methyl-radical scavenging in 2,2,4trimethylpentane' (where 50% of the methyl radicals react with iodine at 6 X 10-7 M ) scavenging should be quantitative (>99%) a t the iodine concentration employed. Several experiments intercomparing results a t 10-4 and 10-3 M iodine confirmed this expectation. Alkyl iodides were separated gas chromatographically, and their yields were measured as described previously.1o The yields of CH3 and CzHs radicals from cyclohexane in the absence of alkyl halides were found to be
