The gas-phase methylation of benzene and toluene

Abstract

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.