Aromatic substitutions by [3H3]methyl decay ions. A comparative study of the gas- and liquid-phase attack on benzene and toluene

Abstract

A nuclear technique based on the spontaneous decay of tritiated precursors that allows the generation of free carbonium ions of exactly the same nature in different environments, has been exploited in a comparative study of aromatic alkylation by CT₃⁺ ions, both in the gas phase at various pressures and in the liquid phase. The differences between the reactivity pattern of the methyl cation in the two environments can be essentially reduced to the much greater efficiency of collisional stabilization in the condensed phase, allowing a larger fraction of the excited arenium ions, from the highly exoenergetic (ΔH° < 335 kJ mol^–1) attack of the CT₃⁺ ions on benzene and toluene, to survive dissociation and/or isomerization. The mechanism of the major competitive processes promoted by CT₃⁺ attack on arenes, i.e. methylation, tritiodemethylation, and methyldemethylation are discussed, and the substrate and positional selectivity of the CT₃⁺ ions from the decay of CT₄ is compared with that of more conventional alkylating reagents.