Gas-phase acylation reactions. Substrate and positional selectivity of free acetylium ions toward methylbenzenes

Free Ac+ ions, obtained in the dild. gaseous state from the γ-radiolysis of MeF-CO mixts., reacted with methylbenzenes at 380-760 Torr in the presence of a gaseous base (NH3). The gaseous cation is unreactive toward C6H6 and PhMe but acetylates the xylenes and other selected polymethylbenzenes. The relative rates of acetylation were detd. in competition expts., using mesitylene as the ref. substrate. The mechanism of acetylation and subsequent isomerization is discussed, and the substrate and positional selectivity of the free Ac+ ion are evaluated, together with its intrinsic steric requirements. Comparison of the gas-phase results with those of related condensed-phase reactions involving Ac+ salts as 1 of the reactive species reveals no basic mechanistic differences. Some obsd. reactivity and selectivity discrepancies, in particular those concerning acetylation of PhMe, o- and m-xylene, and hemimellitene are outlined, and their possible causes considered