NOTES
1610
(H2, C-COHIO, and bicyclohexyl) are enhanced and exchange of hydrogen between eC6Dl2 and the solid is suppressed. The behavior of G(MCP) in Figure 1 is similar to that of G( CaHs) in y irradiation of isopropylbenzene on the same solid5b and on a silica ge1.6 However, the maximum yield in Figure 1 is Gm(MCP) = 0.81 a t F = 0.028 whereas Gm(CaHa)= 1.45l5at F = 0.0018 on the same solid and Gm(CsHa) = 2.16 a t F = 0.0022 on the silica gel. Apparently, cyclohexane has a lower efficiency for positive-hole capture than isopropylbenzene, and the cyclohexane cation yields MCP with a probability lower than that with which the isopropylbenzene cation yields C6H6. In MCP at room temperature, presence of an aluminum halide and a trace of olefin promoter (added16 or produced by y irradiation1') generates carbonium ions which initiate a chain isomerization of MCP. Propagation involves carbonium-ion isomerization followed by hydride transfer. In y irradiation of cyclohexane on silica-alumina, the small value18 of G, (MCP) and the decrease, rather than a continued increase, in G(MCP) at P > 0.028 indicate the absence of a chain isomerization, which is probably precluded by the short ion-pair lifetimes. Comparison with the results of Sagert and Dyne" for irradiation of c-CeD12 on a 760 m2/g silica gel is of interest. Methylcyclopentane was not found; however, hydrogen-atom exchange occurred at low surface coverage with a yield indicative of a high efficiency for positivehole capture on the very high surface solid. The yield of molecules exchanged increases with surface coverage and approaches G % 7 a t complete surface coverage. Such a result suggests that short chains are involved in the exchange reaction. Thus, exchange rate of the cyclohexane carbonium ion must exceed its isomerization rate, a8 found previously for carbonium ions of C&S alkane^;^ rate of the thermoneutral hydride transfer involved in the exchange chain would be expected to exceed the rate of hydride transfer to a carbonium ion of MCP.lg Addition of cyclohexane to irradiated silica-alumina gives MCP as the only measurable liquid product and little or no gas noncondensable at -196' (
