MICELLAR EFFECTS ON THE NITROSATION BY PIPERIDINES BY 2-BROMOETHYL NITRITE AND 1-PHENYLETHYL NITRITE IN BASIC-MEDIA

The nitrosation of piperidine (P), 2-methylpiperidine (2-MP), and 4-methylpiperidine (4-MP) by 2-bromoethyl nitrite (BEN) and 1-phenylethyl nitrite (PEN) is kinetically studied in basic media and in the presence of tetradecyltrimethylammonium bromide (TTABr) and sodium dodecyl sulfate (SDS) micelles. In the case of cationic micelles of TTABr and with BEN as the nitrosating agent, the results are explained assuming that only the alkyl nitrite is distributed between the aqueous and the micellar pseudophases, whereas the reaction takes place in water. When the nitrosating agent is 1-phenylethyl nitrite, reaction in the micellar pseudophase is also observed. This result is evidence for the kinetic relevance of stoichiometrically negligible amounts of hydrophilic reagents in the micelles in the case of reactions involving highly hydrophobic substrates. The previous estimation, by indirect methods, of the binding constant of the unprotonated amine to the micelles allows a quantitative estimation of the rate constants in the micellar pseudophase, which could be compared with those in water. With anionic micelles of SDS, the results are analyzed by using the simple pseudophase ion-exchange (PPIE) model taking into account the partitioning of alkyl nitrites between the micellar and aqueous pseudophases and, simultaneously, the change in the basic ionization equilibria of the amine, because of the competition between the ammonium ion of the amine and Na+, for binding to the micelle. The electrostatic interaction of protonated amine with the micellar surface reduces the quantity of total unprotonated amine, the reactive form. Rate constants for the reaction in the micellar pseudophase and in water, obtained from the kinetic analysis, are reported and discussed. Binding constants of the unprotonated amine to the SDS micelles, as well as the ion exchange constants between the counterions of the micelle, Na+, and the ammonium ions of the corresponding amine, were previously determined from absorbance measurements.