Isotope effect studies on elimination reactions. VI. The mechanism of the bimolecular elimination reaction of 2-arylethylammonium ions

The mechanism of the elimination reaction of 2-arylethyltrimethylammonium ions with ethoxide ion in ethanol has been examined using tracer and kinetic isotope effect techniques. Absence of exchange with solvent of both 2-phenylethyltrimethylammonium-2,2-d 2 bromide and 2-(p-trifluoromethylphenyl)-ethyltrimethylammonium-2,2-d 2 bromide and the observation of nitrogen isotope effects of 1.3 and 0.9%, respectively, have eliminated a two-step process involving a freely-solvated carbanion intermediate for both salts. The observation of only a slight change in the magnitude of the nitrogen isotope effect when the solvent is changed from ethanol to water has also excluded a zwitterionic intermediate which is specifically hydrogen-bonded to the molecule of ethanol formed by removal of a β-hydrogen by ethoxide ion. Finally, tracer studies using α- and β-dideuterated substrates have eliminated the less probable reaction pathways involving ylide and carbene intermediates. It is concluded that the reaction of 2-arylethyltrimethylammonium salts with ethoxide ion is a concerted E2 process.