Acetate-Catalyzed Bromination and Deuterium Exchange of 2-Butanone (I). The Mechanism for the Bimolecular Displacement Reactions of α-Haloketones (II)

Abstract

<p> The regioselectivities of bromination and deuterium exchange of 2-butanone are shown to be the same, under identical conditions. This work firmly establishes that enolization is the rate-determining step for the former reaction, contrary to some recent reports in the literature.</p> <p> The steric effects and activation parameters in the bimolecular nucleophilic displacement reactions of a series of α-haloketones and alkyl halides are shown to be inconsistent with either a bridging or conjugation mechanism for the observed rate enhancements of haloketone over alkyl halide.</p> <p> The stereoelectronic requirements of this mechanism are tested in a system where the stereochemistry is known (cis- and trans-chlorocyclohexanones). The activation parameters suggest that only in the case where the geometry is correct for maximum conjugation (trans-chlorocyclohexanone) is there an appreciable difference in mechanism (stereoelectronically) from displacement at ordinary saturated carbon.</p>