Bromination of benzoylacetone in cationic micelles. A strong inhibition effect

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 elec trophilic bromination of the enol. The presence of cationic micelles strong ly modifies both the Br-2/Br-3(-) equilibrium towards the formation of trib romide ion and the keto-enol equilibrium of benzoylacetone towards increasi ng enol concentration. As the enol is the reactive species in electrophilic substitutions, in the presence of cationic micelles it would be expected t hat bromination by Br-3(-) 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 e nol by molecular bromine, in water as in the micellar pseudophase. The opti mized 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.