Gas-phase reaction of tert-butyl ions with arenes. Remarkable selectivity of a gaseous, charged electrophile

tert-Butyl ions from the y radiolysis of neopentane were allowed to react in the gas phase with benzene, toluene, and their mixtures, contained at low concentrations in the gas. The reaction was studied at neopentane pressures ranging from 8.7 to 720 Torr, in the presence of a thermal radical scavenger (OJ. The effects of a gaseous base (EtOH) and of an inert gas (Ar) on the reaction were also investigated. The tert-butyl ions react with toluene in the gas phase, yielding p- and m-tert-butyltoluenes, whose ratio markedly increases with the pressure of neopentane and the concentration of the base. At the higher neopentane and base pressures, the isomeric composition of the products appears to be kinetically controlled and corresponds to a para : 'iZ meta ratio in excess of 35. This value is fully comparable to the highest ratios measured in the study of the liquid-phase tert-butylation and reflects the remarkable positional selectivity of the gaseous, unsolvated tert-butyl cation. Furthermore, the gaseous reagent displays a significant substrate selectivity, as indicated by a kT/kB ratio in excess of 55 measured in the competition experiments. The mechanism of the gas-phase alkylation is discussed and compared with those of different gaseous electrophiles, characterized by a considerably lower selectivity. It is concluded that a low positional and substrate selectivity does not necessarily represent a typical mechanistic feature of the gas-phase aromatic substitutions by charged reagents and that the low selectivity observed in previous studies is likely to arise from the extreme electrophilic character of the reagents so far investigated