Micellar Catalysis in the Nitrosation of Benzoylacetone in an Acid Medium

Abstract

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 the ketone, followed by electrophilic nitrosation of the enol. The presence of micelles strongly modifies the keto−enol equilibrium of β-dicarbonyl compounds, like benzoylacetone, in such a way that the enol percentage increases with the surfactant concentration. The increasing enol amount with surfactant concentration is quantitatively evaluated by 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 increases the nitrosation reaction rate continuously until levels off at high HDS concentrations. In the presence of sodium dodecyl sulfate (SDS), the observed rate constant increases at low surfactant concentrations, passes through a maximum, and decreases at still higher SDS concentrations. The observed kinetic effects in the presence of the cationic 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 addition of TTANO3 decreases the reaction rate for the entire concentration range investigated, while the presence of TTASCN calatyzes the reaction. The varying micellar effects are quantitatively interpreted on the basis of reaction mechanisms, with consideration given to the possible nitrosating agent in every case.

Effects of β-Cyclodextrin on the Keto−Enol Equilibrium of Benzoylacetone and on Enol Reactivity

“…It is hown that the order Br - > Cl - > SCN - is only applicable when the nucleophile is deactivated by hydration, whereas the “natural” order of nucleophilicity SCN - > Br - > Cl - is observed in solvents such as acetonitrile, acetone, etc or acetonitrile−water mixtures and also for the nitrosation in the micellar phase …”

“…In previous works, we have shown that the study of the influence of micellar solutions on the absorption spectrum of β-dicarbonyl compounds, like benzoylacetone, can provide a new method for determining keto−enol equilibrium constants of β-dicarbonyl compounds in water. , As a consequence of the perturbation of the keto−enol equilibrium of benzoylacetone (BZA) brought on by the presence of micelles, the nitrosation reaction of BZA in aqueous acid micellar solutions is greatly modified . In the present study, we investigate the effects of β-cyclodextrin (β-CD) on the keto−enol equilibrium of BZA and dibenzoylmethane (DBM) and on the nitrosation reaction of the enol of BZA, DBM, and acetylacetone (AcAc) in aqueous acid medium.…”

Cationic Micelles Induced Nitrosation of 1,1,1-Trifluoro-3-(2-thenoyl)acetone in Mild Acid Medium: Reactivity of the Enolate

“…The nitrosation of monoketones as well as β-diketones, β-ketoesters, and other related structures to give usually the corresponding oximes is nowadays a well-known process. − The reaction proceeds via the enol tautomer, but either the enolization or the reaction of the enol can be rate-limiting, depending on the relative rates of enolization and nitrosation of the enol and also on the enol content. For the more acidic enols, a reaction pathway via the enolate anion is also possible and in some cases has been detected. , …”

Solvent effects versus concentration effects in determining rates of base-catalyzed keto-enol tautomerization

“…By contrast, when the two reagents bind to micelles, k o increases with the surfactant concentration and reaches a constant value when the surfactant counterions are not inert. [20][21][22] With inert counterions, and on the basis of the original assumption that a substrate in one micelle does not react with a reactant in another and that equilibrium is maintained between aqueous and micellar pseudo-phases, rate maxima are observed as a consequence of the dilution of the reagents in the micellar interface at high micelle concentration. 23,24 This is the situation of the buffer-catalyzed tautomerization in TTABr micellar solutions or the H + catalyzed tautomerization in SDS micelles.…”