Benzene vapor-phase 7-radiolysis

Most photodecarbonylation reactions of cyclic ketones, especially in the vapor phase, have been postulated to proceed from various vibrational levels of excited singlet states.321 However, the elimination reaction leading to unsaturated aldehydes has now been shown to occur largely via excited triplet states. In solution, where the lowest vibrational levels of the excited states are rapidly reached, to-alkenals are the major products observed in both photolysis and radiolysis of cyclopentanone and cyclohexanone. The reaction is quenched by oxygen and dienes,322-324 as well as by the alkenal produced in the reaction.325 The reaction is also sensitized by benzene triplets.322,323 With cyclopentanone, quenching by 1M piperylene occurs some 20 times as fast... 

Vapor-Phase y-Radiolysis of Benzene, Toluene, Ethylbenzene, and the Xylenes... 

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. 

Tr he stability of aromatic hydrocarbons to radiation has been cited so frequently in the literature that it has come to be accepted (5, 6) as a characteristic of aromaticity, a consequence of electron delocalization. These conclusions are based on the results of irradiations in the liquid phase. That they may not apply to the isolated aromatic molecule, however, is suggested by the few reported studies of the radiolysis of aromatic hydrocarbons in the vapor phase. Such studies have thus far been limited to two compounds—benzene (8, 10, 11) and isopropylbenzene (cumene) (9), and differences in the character of the radiation or the conditions of irradiation preclude a simple assessment of the effect of phase on these systems. 

To provide further information on this point, we have investigated the y-radiolysis of benzene, toluene, ethylbenzene, and the xylenes in the vapor phase and have determined yields of the gaseous products, 4 polymer, and some products of intermediate volatility. These results are compared with those of parallel irradiations of liquid toluene and o-xylene and with published (2, 12) data for the other hydrocarbons in the liquid phase. 

Our results on the y-radiolysis of benzene, toluene, ethylbenzene, and the xylenes in the vapor phase, and of toluene and o-xylene in the... 

Comparison of the results of vapor- and liquid-phase radiolysis shows that the yields of all products from each hydrocarbon are markedly greater in the vapor phase. As in the liquid phase, polymer is the predominant product, accounting for 83-96% of the hydrocarbon consumed. The 100-e.v. yields (G values) of polymer increase with alkyl substitution from 6 for benzene to 8.7 for ethylbenzene the yield in each case is about five to six times that observed in the liquid phase. 

Table I. y—Radiolysis of Benzene, Toluene, and Ethylbenzene in Vapor and Liquid Phase...

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