AROMATIC ALKYLATION BY GASEOUS ME3C- KINETIC ROLE OF DEPROTONATION OFINTERMEDIATE ARENIUM IONS( IONS )

The alkylation of m-xylene by Me(3)C(+) ions has been studied in the g as phase with the aim of assessing the kinetic role of proton transfer from intermediate arenium ions. Hydrogen kinetic isotope effects (KIE s) emerging from the reaction of m-xylene-4-d and of m-xylene/m-xylene -d(10) mixtures have been exploited as mechanistic probes. The Me(3)C substitution at the 4-position displays a base-strength-dependent KIE related to rate-determining deprotonation of arenium intermediates. Th e Me(3)C(+) reaction at the 5-position is instead characterized by a b ase-independent KIE due to 1,2-hydrogen shift to form a highly stable isomeric arenium ion. When the latter species originates from m-xylene -4-d, its neutralization involves an intramolecular competition of pro ton vs deuteron abstraction, manifesting a net primary KIE. Factors af fecting its magnitude and base-strength dependence are discussed.