Acetylene from radiolysis

It is now clearly demonstrated through the use of free radical traps that all organic liquids will undergo cavitation and generate bond homolysis, if the ambient temperature is sufficiently low (i.e., in order to reduce the solvent system s vapor pressure) (89,90,161,162). The sonolysis of alkanes is quite similar to very high temperature pyrolysis, yielding the products expected (H2, CH4, 1-alkenes, and acetylene) from the well-understood Rice radical chain mechanism (89). Other recent reports compare the sonolysis and pyrolysis of biacetyl (which gives primarily acetone) (163) and the sonolysis and radiolysis of menthone (164). Nonaqueous chemistry can be complex, however, as in the tarry polymerization of several substituted benzenes (165). 

Acetylene Ion. No evidence for the contribution of ion-molecule reactions originating with acetylene ion to product formation has been obtained to date. By analogy with the two preceding sections, we may assume that the third-order complex should dissociate at pressures below about 50 torr. Unfortunately, the nature of the dissociation products would make this process almost unrecognizable. The additional formation of hydrogen and hydrogen atoms would be hidden in the sizable excess of the production of these species in other primary acts while the methyl radical formation would probably be minor compared with that resulting from ethylene ion reactions. The fate of the acetylene ion remains an unanswered question in ethylene radiolysis. 

The effect of electrical fields on the radiolysis of ethane has been examined by Ausloos et and this study has shown that excited molecules contribute a great deal to the products. The experiments were conducted in the presence of nitric oxide, and free-radical reactions were therefore suppressed. The importance of reactions (12)-(14) was clearly demonstrated by the use of various isotopic mixtures. Propane is formed exclusively by the insertion of CH2 into C2H6 and the yield is nearly equal to the yield of molecular methane from reaction (14). Acetylene is formed from a neutral excited ethane, probably via a hot ethylidene radical. Butene and a fraction of the propene arise from ion precursors while n-butane appears to be formed both by ionic reactions and by the combination of ethyl radicals. The decomposition of excited ethane to give methyl radicals, reaction (15), has been shown by Yang and Gant °° to be relatively unimportant. The importance of molecular hydrogen elimination has been shown in several studies ° °. ... 

The radiolysis of propane has been studied extensively in experiments that have included a wide range of techniques. The gas-phase radiolysis in the absence of inhibitors yields the products hydrogen, ethane, propene, 2,3-dimethylbutane, methane, ethylene, isobutane, acetylene, isopentane and n-butane as well as small quantities of butene-1, -pentane, 2-methylpentane and -hexane ° ° . At high conversions the yield of ethylene, propene, 2,3-dimethylbutane and isobutane are all reduced. The reduction in ethylene arises from hydrogen atom addition, while the reduction in the other products may arise from the reaction of propyl ions with propene to remove both C3H6 and the source of isopropyl radicals. 

Benzene, toluene, ethylbenzene, and the three xylenes have been irradiated in the vapor phase with gamma rays. Products and yields have been compared with those in liquid-phase radiolysis. G values for disappearance in the vapor phase range from 6 to 10, more than five times greater than in the liquid phase. The principal product in each case is polymer. All of the identified products are also found in the liquid phase, but relative yields are markedly different. The high yields of acetylene and some other products in the vapor phase suggest that ionic processes are more important here than in the liquid phase. 

It would appear, therefore, that all of the products identified in the vapor-phase radiolyses could be formed from excited molecules. Some products, however, are so much more abundant in vapor-phase radiolysis than in photolysis or liquid-phase radiolysis as to suggest the likelihood of additional precursors. In particular, the formation of acetylene, the isomerization of the xylenes, and the replacement of aromatic hydrogen by methyl groups are difficult to explain solely in terms of reactions of excited molecules. 

On the basis of previous discussion, the only additional source of ethylene in the radiolysis is the decomposition of excited neutral ethyl chloride molecules. If the ion-molecule contribution to this product is subtracted from the total yield reported in Table III, an ethylene-acetylene yield of G = 4.24 may be attributed to excited neutral decomposition. If we now assume that the photolysis experiments provide a direct measure of the neutral excited molecule decomposition to be expected in the radiolysis, this ethylene yield may be used as a basis for normalization to estimate the contributions from this source to the other products using the relative photolysis distributions from Table IV. In this way, the contributions from excited neutral decomposition reported in Table V were derived. 

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