The gas-phase methylation of benzene and toluene

The reactions of the methyl cation with benzene and toluene in the gas phase have been examined using the flowing afterglow-selected ion flow-drift tube technique. With benzene four product ions are formed, C₆H₆ ⁺ by electron transfer, C₆H₅ ⁺ by addition and loss of CH₄, C₇H₇ ⁺ by addition and loss of H₂, and an adduct C₇H₉ ⁺. Deuterium and carbon-13 labeling experiments were carried out to provide mechanistic insights. In agreement with earlier work, deuterium labeling (CD₃ ⁺ with C₆H₆ or CH₃ ⁺ with C₆D₆) shows that partial H/D scrambling between the methyl group and the ring occurs during the formation of C₆H₅ ⁺ and C₇H₇ ⁺. However, in contrast to earlier work, no carbon-13 scrambling was observed between the methyl and ring carbons, thus ruling out a ring expansion and contraction mechanism to account for the H/D scrambling. Nor did we find H/D scrambling in the electron transfer product ion, C₆H₆ ⁺. When collision-induced dissociation (CID) was carried out on the adduct ion, extensive H/D and carbon-13 scrambling was found, indicating that at least some ring expansion occurs during its formation. Reaction of C₆H₅ ⁺ with methane at room temperature exclusively forms the adduct ion; in a drift field, this adduct ion fragments by loss of CH₄ and H₂. Mechanisms are proposed which account for our results, and these are supported by ab initio calculations. Similar studies were carried out with toluene as the neutral reagent. Besides the four analogous product ions, we found hydride transfer from the methyl group of toluene to be a major reaction channel and addition with loss of ethylene to be a minor channel.