Radiolysis of cyclohexane

Excited molecules produced by the radiation may also transfer their energy to other substrates. The yield of hydrogen in the radiolysis of cyclohexane-benzene solutions is considerably lower than would be expected from the electron fraction of each component and the known hydrogen yields for the pure compounds125. 

Zhang, N., Wu, J.L. 1991. Scavenger effects in the radiolysis of cyclohexane 4-methyl-4-phenyl-2-pentanone system and cyclohexane tributyl phosphate system. Radiat. Phys. Chem. 38 383-388. 

Trifimac AD, Sauer MC, Jonah CF. (1985) Pulse-radiolysis of cyclohexane The identity of the high-mobiKty positive ion. Chem Phys Lett 113 316-319. 

The radiolysis of cyclohexane is perhaps the most completely understood of all hydrocarbons owing not only to the many studies" but also to the rather simple mechanism. Details of the radiolysis, however, are receiving more complete investigation. 

The gas-phase radiolysis of cyclohexane shows an increase in products resulting from breaking of the cyclohexane ring . The major products of radiolysis in inert gas matrices are the same as for the liquid-phase radiolysis ° . 

Irradiation of saturated aliphatic compounds typically results in imsaturation, polymerization, and isomerization. The radiolysis of cyclohexane illustrates all three of these processes. If the radicals are very energetic, cyclohexene can be formed by the abstraction of hydrogen from a cyclohexyl radical either by a hydrogen atom or by another cyclohexyl radical. If the radicals become thermalized, recombination of radicals can occur to give bicyclohexyl. A less frequent process is rearrangement, followed by hydrogen atom capture to yield methylcyclopentane. 

Radiolysis of Cyclohexane at Very High Dose Rates... 

Using one of these sources we have determined product yields from the radiolysis of cyclohexane and the effects on these yields of adding nitrous oxide, benzene, and sulfur hexafluoride. 

The product yields from the radiolysis of cyclohexane containing benzene, nitrous oxide, and sulfur hexafluoride, with electron pulses at a dose rate of 6 X 1027 e.v./gram sec. and a dose of 7 Mrads, are shown in Figures 4, 5, 6, and 7. Also included in these figures are data of other workers (12, 17) at ca. 1016 e.v./gram sec. with 60Co y-rays. Comparison between the results at the two dose rates shows that the effect of the additives is qualitatively the same. However, there are some differences in the amounts by which the yields change. 

Pure Cyclohexane. The major products from the radiolysis of cyclohexane are hydrogen, cyclohexene, and bicyclohexyl. Dewhurst (4) proposed that these could be formed by the production of hydrogen atoms and cyclohexyl radicals, followed by abstraction by hydrogen atoms and recombination and disproportionation of the cyclohexyl radicals. Dyne and Jenkinson (5) showed that part of the hydrogen yield is unimolecu-larly formed. It was suggested that excited cyclohexane molecules give... 

An increase in the ratio of cyclohexene to bicyclohexyl, but no decrease in hydrogen yields, has been observed in the radiolysis of cyclohexane with high LET radiation (I). As our results imply that a reduction in the hydrogen yield should accompany an increase in this ratio, the constant value of the hydrogen yield is presumably caused by other reactions occurring in the tracks that produce hydrogen. 

In the 1470-A. photolysis of cyclohexane-nitrous oxide solutions, nitrous oxide reacts with excited cyclohexane molecules to form nitrogen and oxygen atoms. The reaction of N20 with photoexcited 2,2,4-trimethylpentane molecules is much less efficient than with cyclohexane. In the radiolysis of these solutions, G(N2) is the same for different alkanes at low 5 mM) N20 concentrations. At higher concentrations, G(N2) from the radiolysis of cyclohexane is greater than G(N2) from the radiolysis of 2,2,4-trimethylpentane solutions. The N2 yields from 2,2,4-trimethylpentane are in excellent agreement with the theoretical yields of electrons expected to be scavenged by N20. The yield of N2 in the radiolysis of cyclohexane which is in excess of that formed from electrons is attributed to energy transfer from excited cyclohexane molecules to nitrous oxide. 

Photoionization of the hydrocarbon followed by dissociative electron attachment (Reaction 1) should be considered since the ionization potential of a molecule is less in the liquid phase than it is in the gas phase. For hydrocarbons the ionization potential is 1 to 1.5 e.v. less in the liquid phase (24). The photon energy at 1470 A. is about 1.4 e.v. below the gas-phase ionization potentials of cyclohexane and 2,2,4-trimethylpentane (14). Some ionization may therefore occur, but the efficiency of this process is expected to be low. Photoionization is eliminated as a source of N2 for the following reasons. (1) If photoionization occurred and the electron reacted with nitrous oxide, then O" would be formed. It has been shown in the radiolysis of cyclohexane-nitrous oxide solutions that subsequent reactions of O result in the formation of cyclohexene and dicyclohexyl (I, 16, 17) and very little cyclohexanol (16, Table III). In the photolysis nitrous oxide reduces the yield of cyclohexene and does not affect the yield of dicyclohexyl. This indicates that O is not formed in the photolysis, and consequently N2 does not result from electron capture. (2) A further argument against photoionization is that cyclohexane and 2,2,4-trimethylpentane have comparable gas-phase ionization potentials but exhibit quite different behavior with respect to N2 formation. 

On the other hand, it has been argued by Burton et al. (2, 3, 4) that the lifetime of singlet excited cyclohexane molecules is too short ( 10-13 sec.) to be observed. This conclusion is based largely on the behavior of scintillators containing fluorescent solutes dissolved in cyclohexane. It is important at this point to emphasize that at solute concentrations close to millimolar (which are usually employed in scintillator studies) excited cyclohexane molecules cannot be detected by nitrous oxide. Higher concentrations of both nitrous oxide and benzene (25) are required to observe energy transfer. This study confirms that in the radiolysis of cyclohexane, solutes at millimolar concentrations interact mainly with the electron and not with excited molecules. 

Cyclohexane (ionization energy, 9.88 e.v.) has been photolyzed at 1236 A. (10.0 e.v.) and 1048-1067 A. (11.6-11.8 e.v.). All major products have been determined in the absence of free radical scavengers and in the presence of added NO and 02. In addition, cyclo-C6Di2-H2S mixtures were irradiated to determine the free radicals formed in the decomposition of the superexcited cyclohexane molecule. Accurate quantum yield determinations have been made, both by use of saturation current measurements during photolysis and by chemical actinometry. It is seen that an increase in photon energy results in an increase in the relative importance of processes producing H atoms or alkyl radicals while the yields of products attributed to "molecular elimination processes, such as the formation of molecular hydrogen, diminish. Similar trends are seen in the solid-phase photolysis. The relative importances of the various primary processes are derived. The application of this information to the interpretation of the radiolysis of cyclohexane is discussed. 

Radiolysis of cyclohexane in the presence of 20 mmoFL of N O resulted in the reduction of the H yield by about one third [Scholes and Simic, 1964]. Later, Asmus et al. [1970] estimated the contribution of ionic mechanisms to the radiolytic production of hydrogen from hydrocarbons, to be about one third. These observations suggest that the recombination of electrons with the cations contributes to the formation of H in reactions such as (R-5) and (R-7). In the presence of N O, the reactions (R-5) and (R-7) could be modified resulting in reduced yield of hydrogen because of the occurrence of (R-16) and the following types of reactions [Foldiak, 1981] ... 

Thus in the presence of halogenated compounds, the radiolytically produced electron may preferentially react with the halogenated molecules. Also, the free radical moiety produced from the halogenated molecule, e.g., the free radicals formed in reaction (R-20), may participate in the reactions leading to the formation of several new products. For example, radiolysis of cyclohexane in the presence of carbon tetrachloride was reported to lead to a chain reaction [Henglein et al., 1963 Stone and Dyne, 1964]. 

Table 11J2. Variation of hydrogen and cyclohexene yields as a function of dose, on radiolysis of cyclohexane [Ho and Freeman, 1964]...

Even more important are reactions of radiolytic products with the precursors of these excited states. Results from recent experiments in which the solute luminescence and magnetic field effect were compared for radiolysis of cyclohexane or isooctane with 0.5-2.2 MeV electrons, 1-5 MeV protons, and 2-20 MeV a-particles suggested that the decrease in the solute luminescence and the magnetic field effect was due to both the increasing importance of crossrecombination and the "intervention of radicals or other transient species with the precursors" with the fluorescent states [63]. The effects of spin relaxation and ion-radical reactions in dense spurs were identified as likely causes for reduced magnetic field effects, fluorescence yields, and probabilities 0 in spurs from 17-40 keV x-rays as compared to the spurs from fast electrons [80]. 

Bums, W.G., Hokoyd, R. A., and Klein, G.V. 1966. Radical yields in the radiolysis of cyclohexane with different kinds of radiation. J. Phys. Chem. 70 910. 

TABLE 5. Yields of products, in G-value units of (100eV) in the radiolysis of cyclohexane in the gas and liquid phase... 

TABLE 6. Fragmentation pattern of the ions in cyclohexane obtained mass spectrometrically (70 eV electrons) and from gamma radiolysis of cyclohexane at 55 torr ... 

FIGURE 2. The yield of excess electrons scavenged by methyl bromide as a function of methyl bromide concentration in the gamma radiolysis of cyclohexane (lower scale). The yield of survival as a function of time for the charged species after instantaneous irradiation in cyclohexane (upper scale) the upper curve has been obtained with equation 31, the lower one with a more extensive treatment (see text)... 

FIGURE 6. The yield of products in the radiolysis of cyclohexane as a function of LET" ... 

TABLE 12. Yields ofproducts in the gamma radiolysis of cyclohexane ... 

The pentacarbonyls of Cr, Mo, and W have been produced by pulse radiolysis of cyclohexane solutions of the corresponding hexacarbonyls by the following reactions ... 

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