MICELLAR CATALYSIS IN THE NITROSATION OF BENZOYLACETONE IN ALL ACID-MEDIUM

The nitrosation reaction of benzoylacetone (1-phenyl-1,3-butadione) by nitrous acid in an aqueous acid medium is studied in the presence of anionic and cationic micelles. The reaction involves enolization of th e ketone, followed by electrophilic nitrosation of the enol. The prese nce of micelles strongly modifies the keto-enol equilibrium of beta-di carbonyl compounds, like benzoylacetone, in such a way that the enol p ercentage increases with the surfactant concentration The increasing e nol amount with surfactant concentration is quantitatively evaluated b y studying the effect of surfactant addition on the UV-vis spectra of benzoylacetone; the results are used to explain the kinetic data. The addition of hydrogen dodecyl sulfate (HDS) to the reaction mixture inc reases the nitrosation reaction rate continuously until levels off at high HDS concentrations. In the presence of sodium dodecyl sulfate (SD S), the observed rate constant increases at low surfactant concentrati ons, passes through a maximum, and decreases at still higher SDS conce ntrations. The observed kinetic effects in the presence of the cationi c surfactants tetradecyltrimethylammonium salts of Cl-, Br-, NO3-, and SCN- anions, in general TTAX (with X being Cl-, Br-, NO3-, or SCN-), depend on the nature of the micellar counterion; for example, the addi tion of TTANO(3) decreases the reaction rate for the entire concentrat ion range investigated, while the presence of TTASCN calatyzes the rea ction. The varying micellar effects are quantitatively interpreted on the basis of reaction mechanisms, with consideration given to the poss ible nitrosating agent in every case.