Influence of anionic and cationic reverse micelles on nucleophilic aromatic substitution reaction between 1-fluoro-2,4-dinitrobenzene and piperidine

The nucleophilic aromatic substitution (SNAr) reaction between 1-fluoro-2,4 -dinitrobenzene and piperidine (PIP) were studied in two different reverse micellar interfaces: benzene/sodium 1,4bis(2-ethylhexyl),sulfosuccinate (AO T)/water and benzene/benzyl-n-hexaecyl dimethylammonium chloride (BHDC)/wat er reverse micellar media. The kinetic profiles of the reactions were inves tigated as a function of variables such as surfactant and amine concentrati on and the amount of water dispersed in the reverse micelles, W-0 = [H2O]/[ surfactant]. In theAOT system at-W-0 = 0, no micellar effect was observed a nd the reaction takes place almost entirely in the,benzene pseudophase, at every AOT and PIP concentration. At W-0 = 10, a slight increment of the rea ction rate was observed at low [PIP] with AOT concentration, probably due t o the increase of micropolarity of the medium. However, at [PIP] greater th an or equal to 0.07 M the reaction rates are always higher in pure,benzene than in the micellar medium because the catalytic effect of the amine predo minates in the organic solvent. In the BHDC system the reaction is faster i n the micellar medium than in the pure solvent. Increasing the BHDC concent ration accelerates the overall reaction, and the saturation of the micellar interface is never reached. In addition, the reaction is not base-catalyze d in,this micellar medium.:Thus, despite the partition of the reactants in both pseudophases the reactions effectively take place at the interface of the aggregates. The kinetic behavior can:be quantitatively explained taking into account the distribution of the substrate and the nucleophile between the bulk, solvent- and the micelle interface. The results were used to eva luate the amine distribution constant between the micellar pseudophase and organic solvent and the second-order rate coefficient of SNAr reaction the interface. A mechanism to rationalize the kinetic results in both interface s is proposed.