Aromatic substitution in the gas phase. Alkylation of arenes by gaseous C4H9+ cations.

Butyl cations, obtained in the dilute gas state from the radiolysis of butane in the pressure range from 70 to 750 torr, have been allowed to react with benzene, toluene, and their mixtures or with trace amounts of o-xylene in the gaseous system. The gas-phase butylation yields invariably sec-butylarenes, remarkably free of isomeric byproducts, namely n- and fert-butylarenes. Other alkylation experiments, where gaseous butyl cations from the reaction of butane with radiolytically formed H3+io ns were used as reagent, confirmed the exclusive formation of sec-butylarenes. The butylation process displays the positional and substrate selectivity and the dependence of orientation on the pressure of the system, typical of other gas-phase ionic substitutions. At high pressures, ortho-para orientation predominates in the sec-butylation of toluene, with a 0rtho:meta:para ratio of 43:30:27 at 715 torr. As the pressure is reduced, a gradual shift in favor of the thermodynamically most stable meta-substituted arenium ion is observed, leading to a 0rtho:meta:para ratio of 31:48:21 at 70 torr.