Kinetic study of the cyclohexadienyl radical. Part 3.—Mutual interaction and thermal decomposition

Abstract

The unusual pattern of the reactions shown by the cyclohexadienyl radical below 200° is consistent with the high stabilization energy and complex pattern of spin density in the radical, and with its close relationship to benzene. We have investigated the scope of the reactions of the cyclohexadienyl radical in the gas phase, in the absence of significant concentrations of other radicals and in the presence of an excess of cyclohexadiene-1,4. Metathesis between these species would be revealed by a process of chain isomerization of cyclohexadiene-1,4 to cyclohexadiene-1,3, but this did not occur at a significant rate below 200°. Mutual interaction was the only significant process of consumption of cyclohexadienyl radicals below 100°, and conformed to the pattern : 2 cyclo-C₆H₇·→ C₆H₆+ cyclo-C₆H₈-1,3 (11%)→ C₆H₆+ cyclo-C₆H₈-1,4(20%)→ C₁₂H₁₄ isomers (69%) between 63 and 101°. At 136° cyclohexene was formed in this system, and the relative abundance of both cyclohexene and benzene increased rapidly as the reaction temperature was raised, until at 186° the rate of formation of each exceed the rate of the primary generation of cyclohexadienyl radicals. The results are consistent with the thermal dissociation of the cyclohexadienyl radical : cyclo-C₆H₇·→ C₆H₆+ H·; E_a= 31kcal/mole, followed by the formation of cyclohexene and the regeneration of the cyclohexadienyl radical in a chain sequence. The enthalpy of formation of the cyclohexadienyl radical was estimated as 44 kcal/mole, and the stabilization energy as 25 kcal/mole.