Competitive concerted and stepwise addition of free arylium ions to propane in the gas phase

Abstract

Labelled tolylium ions from the decay of ring-multitritiated toluene have been allowed to react with propane in the gas phase, at pressures ranging from 20 to 744 Torr, yielding isomeric propyltoluenes as the major addition products. The relative composition of the n-versus iso-propyltoluenes (σ_p :σ_s 3.2–4.7) is found to depend appreciably upon the total pressure of the system and the presence of added bases (NH₃ or CH₃OH). Pressure and base effects on the isomeric distribution of both n- and iso-propyltoluenes are also investigated. The results are consistent with a reaction pattern involving preliminary formation of an electrostatic adduct, wherein fast hydride-ion transfer from a secondary C–H bond of C₃H₈ to the arylium ion takes place. The same mechanism does not seem operative when the first interaction occurs between the tolylium ions and a primary C–H bond of the substrate. The behaviour of isomeric tolylium ions towards propane is discussed and compared with related studies involving unsubstituted phenylium ions. A mechanistic model is proposed for both reactions, which accounts for the apparent discrepancy between the indiscriminate affinity of arylium ions for any kind of substrate and their ‘abnormally’ high site selectivities.