Bromination of benzoylacetone in cationic micelles. A strong inhibition effect

Abstract

The bromination reaction of benzoylacetone in aqueous acid medium has been studied in the presence of cationic micelles of tetradecyltrimethylammonium bromide. The reaction involves enolization of the ketone, followed by electrophilic bromination of the enol. The presence of cationic micelles strongly modifies both the Br₂/Br₃^- equilibrium towards the formation of tribromide ion and the keto-enol equilibrium of benzoylacetone towards increasing enol concentration. As the enol is the reactive species in electrophilic substitutions, in the presence of cationic micelles it would be expected that bromination by Br₃^- would occur in the micellar phase, in accordance with previous studies on bromination of alkenes. In contrast, the kinetic results are quite satisfactorily explained by assuming bromination of the enol by molecular bromine, in water as in the micellar pseudophase. The optimized reactivity in water is higher than that in the micellar phase, a less polar medium than bulk water, and bromination is medium dependent because the solvent assists the departure of bromide ion.