Experimental-determination of Internal Energy Barriers In the Gas-phase Aromatic Alkylation By Dimethylchloronium Ions

The temp. and pressure dependence of the substrate selectivity of the alkylation of mesitylene (M) and p-xylene (X) by radiolytically formed CH3ClCH3+ ions have been investigated in CH3Cl gas at 40-140°/50-760 Torr. The Arrhenius plot of the empirical kM/kX ratio measured at 760 Torr is linear over the entire temp. range investigated, and its slope corresponds to a difference of 2.2 ± 0.2 kcal mol-1 between the activation energies for the CH3ClCH3+ methylation of p-xylene and mesitylene. A pressure-dependence study of the same competition reactions carried out at 100° points to 300 Torr as the pressure limit, below which the correspondence between the phenomenol. Arrhenius-plot slope and the actual activation-barrier difference is no longer warranted. This conclusion is further corroborated by a comparison of the present results with those derived from the same reactions from reactant ion monitoring (RIM) high-pressure mass spectrometry at 0.5-1.2 Torr. The large discrepancy obsd. is interpreted as evidence that above 300 Torr the activation mechanism of the CH3ClCH3+ methylation of arenes, a typical ion-mol. process, is essentially thermal and that, below this limit, coexistence of both thermal and electrostatic activation mechanisms as well as incomplete equilibration of the internal energy of the reactants make Arrhenius plots hardly a measure of the activation barriers involved in the gas-phase arom. alkylations.