Experimental Determination of Internal Energy Barriers in the Gas-Phase Aromatic Alkylation by Dimethylchloronium Ions

Article abstract of DOI:10.1021/ja00209a004

The temperature 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 pressure between 50 and 760 Torr in the range 40-140 deg C.The Arrhenius plot of the empirical k_M/k_X ratio measured at 760 Torr is linear over the entire temperature range investigated, and its slope corresponds to a difference of 2.2 +/- 0.2 kcal mol^-1 between the activation energy for the CH3ClCH3⁺ methylation of the p-xylene and mesitylene.A pressure-dependence study of the same competition reactions carried out at 100 deg C points to 300 Torr as the pressure limit, below which the correspondence between the phenomenological Arrhenius-plot slope and the actual activation-barrier difference is not any longer warranted.This conclusion is further corroborated by a comparison of the present results with those derived for the same reactions from reactant ion monitoring (RIM) "high-pressure" mass spectrometry at 0.5-1.2 Torr.The large discrepancy observed is interpreted as evidence that above 300 Torr the activation mechanism of the CH3ClCH3⁺ methylation of arenes, a typical ion-molecule 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 aromatic alkylations.