Interconversion between the Cation Radicals of Toluene and Cycloheptatriene: An Evaluation of the Difference Between the Gas Phase and Solution

The cation radicals of toluene and 1,3,5-cycloheptatriene may be prepared in solution by various means, and the literature in this area offers no evidence for the rearrangement that interconverts these materials in the gas phase. Thermodynamic calculation including estimates for solvation terms suggests a pKa of-12 for these cation radicals. This suggests rapid proton loss, which is consistent with existing measurements. Published theoretical calculations suggest an energy of activation of about 30 kcal mol^-1for rearrangement in the gas phase, and this is consistent with lifetime measurements long known in the photodissociation experiments. The information above leads to the conclusion that proton loss from the cation radicals in solution is preemptive of the arrangement. This is contrasted with the McLafferty rearrangement of ketones, which occurs both in the gas and in solution. In this case, proton loss is not a route to stabilization in the cation radicals for unrearranged aliphatic ketones.