Acetylene radiolysis

The main products of acetylene radiolysis are benzene and the polymer cuprene. Less significant products include vinylacetylene, diacetylene, cyclooctatetraene, phenylacetylene and butadiene . Despite the fact that the radiolysis of acetylene has been studied a number of times ° 3-466 gf many years it... 

The buildup reactions dominate the radiation chemistry of acetylenes in acetylene radiolysis benzene and the oligomer cuprene are the main products (Cserep 1981). 

Both this and previous studies demonstrate the existence of rather long chains of consecutive ion-molecule reactions in methane, ethylene, and acetylene, and thus they provide direct evidence for ionic mechanisms of condensation or polymerization in these gases. Polymers have been found in relatively high yields among the radiolysis products of these... 

Application to Ethylene Radiolysis. The predominant ions in the mass spectrum of ethylene (1) are ethylene, vinyl, and acetylene ions, which together account for over 85% of the total ionization. A total of 38% of all ions are C2H4+, and since kF(ethylene) = 25.9 e.v./ion pair, the parent ion should be produced with a yield of at least 1.5 ions/100 e.v. absorbed in ethylene. Similar calculations for the probable yields of the other major ions lead to estimates of 0.96 vinyl ions/100 e.v. and 0.94 acetylene ions/100 e.v. Successive dissociations are relatively unimportant in ethylene. 

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. 

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). 

Photoexcitation of an acetylene molecule results in either dimerization or dissociation of the molecule (C2 + H2 or C2H + H). In the mass spectrometer, the major positive ions are C2H2+ (75%) and C2H+ (15%). However, at STP no gaseous products are seen under radiolysis. There are only two major products, benzene and a polymer, cuprene with an empirical formula QJH. The detailed mechanisms are still debatable. However, the following remarks may be made ... 

Hug GL, Bonifacic M, Asmus K-D, Armstrong DA (2000a) Fast decarboxylation of aliphatic amino adds induced by 4-carboxybenzophenone triplets in aqueous solutions. A nanosecond laser flash photolysis study. J Phys Chem B 104 6674-6682 Hug GL, Carmichael I, Fessenden RW (2000b) Direct EPR observation of the aminomethyl radical during the radiolysis of glycine. J Chem Soc Perkin Trans 2 907-908 Hunter EPL, DesrosiersMF, Simic MG (1989) The effect of oxygen, antioxidants and superoxide radical on tyrosine phenoxyl radical dimerization. Free Rad Biol Med 6 581-585 Ito O (1992) Flash photolysis study for reversible addition reactions of thiyl radicals with olefins and acetylenes. Trends Phys Chem 3 245-266... 

The mechanism of benzene and higher polymer formation remains uncertain with further work on isotopic mixtures needed to help determine the processes which occur. It is interesting to note that, in the P- and X-ray radiolysis of C2H2-C2D2 mixtures, Mains et observe all benzenes do to in the products and conclude that a C-H rupture must occur in the radiolysis. Dorfman and Wahl have shown that in the helium-sensitized radiolysis of acetylene, where only ionized states of acetylene are formed, there is no formation of benzene. The strong pressure-dependence of the benzene formation in the direct photolysis still provides the strongest evidence for a molecular mechanism such as given in the reaction scheme. 

The radiolysis of ethane has been studied almost exclusively in the gas phase. The products of reaction are mainly hydrogen, n-butane, ethylene, propane and methane with smaller quantities of acetylene, isobutane and isopentane - °°. When the radiolysis is conducted with NO added as a radical scavenger, the hydrogen and n-butane yields are reduced and propene and butene are observed as products > The radiolysis of ethane with iodine vapor has shown that the radicals H, C2H5, and CH3 along with smaller quantities of C3H7, C4H9 and CH2 are present . 

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. 

The following reaction scheme includes both free-radical and excited-molecule reactions as well as ionic reactions all of these reactions along with other unknown processes may be occurring in the radiolysis of acetylene. 

The radiolysis of benzene has been the subject of many studies in both the liquid phase and the gas phase. In all cases the radiolysis leads mainly to polymer formation, but hydrogen and acetylene as well as other minor products are formed in much lower yields. 

Considerably less work has been done on the kinetic aspects of the radiolysis of fluorine compounds than on other halogen compounds. With °Co y-radiation, perfluoromethane produces perfluoroethane , and perfluoroethane produces (refs. 351, 352) a mixture of perfluoromethane, -propane, -butane, -cyclopropane, and -acetylene. Similar radiolysis studies on perfluorocyclobutane , perfluoro-cyclohexane and other perfluoro-compounds show that the dominant process is one of rupture of the carbon-carbon bond with the formation of a variety of perfluoro-compounds as products. 

The effect of Linear Energy Transfer (let) on the radiolysis of methyl iodide has been studied by Sturm and Schwarz The increase of the yields of ethane, methane, hydrogen, ethylene and acetylene with let supports the view of Gillis et that these products are formed by diffusion-controlled reactions of radicals... 

Figure 2 also gives the G-values of the various products in the radiolysis of mixtures of cyclopropane-propene. The maximum in the acetylene yield is at a cyclopropane molar fraction of 0.75 and in the ethylene yield at cyclopropane molar fraction of 0.25 indicating probably that they are produced in competing reactions of the same intermediate. The curve of the yields of isobutane, 4-methyl-1-pentene and 1,5-hexadiene indicate bimolecular reactions since none of them is formed in the pure components. Foldiak and Horvath" suggested that the formation of these hydrocarbons may be explained by combination reactions between the reactive intermediates existing in high steady state concentrations in the system, as in equations 48 and 49 ... 

Products of y-radiolysis of pyridazine are acetylene, nitrogen, hydrogen, and a polymer. ... 

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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