Aromatic substitution in the gas phase. Alkylation of xylenes and toluene by isopropyl(1+) ions.

Isopropyl cations, obtained in the dilute gas state from the y radiolysis of propane, have been allowed to react with individual xylene isomers, both neat and in competition with toluene, in the pressure range 20-720 Torr, and in the presence of variable concentrations of a gaseous base (NH3). The mechanism of the substitution, and of subsequent isomerization and dealkylation processes are discussed, and the substrate and positional selectivity, together with the intrinsic steric requirements of the free, unsolvated i-C3H7+ ion, are evaluated. Comparison of the gas-phase alkylation by i-C3H7+ and f-CdHg+ with related reactions occurring in solution reveals no basic mechanistic differences, the gas-phase reactivity of free cations representing the limit more or less closely approached in solution by poorly solvated alkylating agents, of incipient cationic nature.