Gas-Phase Acylation Reactions. Substrate and Positional Selectivity of Free Acetylium Ions toward Methylbenzenes

Article abstract of DOI:10.1021/ja00529a039

Free acetylium ions, obtained in the diluted gas state from the γ radiolysis of CH3F-CO mixtures, have been allowed to react with methylbenzenes, in the pressure range 380-760 Torr, and in the presence of a gaseous base (NH3).The gaseous cation has been confirmed to be unreactive toward benzene and toluene, whereas it acetylates the xylenes and the other selected polymethylated benzenes.The relative rates of acetylation have been determined in competition experiments, using mesitylene as the reference substrate.The mechanism of acetylation and subsequent isomerization is discussed, and the substrate and positional selectivity of the free CH3CO⁺ ion are evaluated, together with its intrinsic steric requirements.Comparison of the gas-phase results with those of related condensed-phase reactions, involving CH3CH⁺ salts as one of the reactive species, reveals no basic mechanistic differences.Some observed reactivity and selectivity discrepancies, in particular those concerning acetylation of toluene, o- and m-xylene, and hemimellitene, are outlined and their possible causes considered.