Reaction of Cyclohexane with Mercury-6(3P1) Atoms

An investigation has been made of the mercury-6(3P1)-photosensitized decomposition of cyclohexane at 29.30±0.01°C, under static conditions. The products of the reaction are hydrogen, bicyclohexyl, and cyclohexene, in order of decreasing importance. The mean quantum yields of cyclohexane disappearance, hydrogen formation, and cyclohexene formation were found to increase slightly with increasing substrate pressure; at 90-mm cyclohexane pressure, the values are 0.83, 0.43, and 0.03, respectively. Extrapolation to initial conditions of data on the variation of mean quantum yield with time at constant substrate pressure gave 0.55 for the initial quantum yield of hydrogen formation, and 0.17 for cyclohexene formation, independent of substrate pressure. From the foregoing initial quantum yield data, taken in conjunction with the stoichiometry of the reaction, a value of 0.93 was obtained for the initial quantum yield of cyclohexane consumption. Absence of hexane, dodecanes, n-hexylcyclohexane, and low molecular weights products was taken as evidence of the stability of the cyclohexane ring during reaction. The results of the investigation can be adequately accounted for by the following simple mechanism:(1) cyclo C6H12+Hg 6(3P1)→cyclo C6H11+H+Hg 6(1S0),(2) H+cyclo C6H12→H2+cyclo C6H11,(3) 2 cyclo C6H11→cyclo C6H10+cyclo C6H12,(4) 2 cyclo C6H11→bicyclo C12H22,(5) H+cyclo C6H10→cyclo C6H11. Steps (1) to (4) describe the reaction under initial conditions. From the initial quantum yield data the ratio k4/k3, the ratio of the rates of recombination to disproportionation for cyclohexyl radicals, was evaluated at 2.2. In short, recombination is approximately twice as fast as disproportionation for cyclohexyl radicals.