Ethyl Cyclohexanone-2-carboxylate in Aqueous Micellar Solutions. 2. Enol Nitrosation in Anionic and Cationic Micelles

Abstract

Micellar-mediated rates of both the ester hydrolysis and enol nitrosation reactions of ethyl cyclohexanone-2-carboxylate, ECHC, were measured in aqueous acid media. In equal acid concentration, the enol nitrosation reaction rate is more than 10 times faster than the ester hydrolysis reaction rate; this fact makes possible the separate (independent) study of both reactions undergone by ECHC. Anionic micelles of sodium dodecyl sulfate, SDS, inhibit the ester hydrolysis; by contrast, the nitrosation reaction goes through maxima as the [SDS] increases above the critical micelle concentration. A similar pattern of behavior is observed in the study of the effect of anionic micelles of hydrogen dodecyl sulfate, HDS, on the rates of the two reactions: the first-order rate constant (ko) of the ester hydrolysis goes through minima as [HDS] increases, even though the rates are actually affected very little by HDS addition; in sharp contrast, the overall rate constant of the nitrosation reaction is enhanced by HDS addition and levels off at high surfactant concentration. The values of the rate constant at high surfactant concentration is more than 6 times that determined in the absence of HDS. Despite the different micellar effects observed in the two reactions occurring in the same substrate, experimental data fit the pseudophase ion exchange (PPIE) model and predict similar values for the second-order rate constant of each reaction in the different types of micelles of a given counterion. The implications of these findings for the reactivity in micelles are discussed. Cationic micelles inhibit the nitrosation reaction throughout the surfactant concentration range; although the reaction at the micellar phase is not negligible, the physicochemical properties of this reaction region account for the lower reactivity in this medium. Experimental data are quantitatively explained by means of the pseudophase model, and kinetic rate constants or equilibrium constants appearing in the proposed reaction scheme are reported.