Characterization of Different Reverse Micelles Interfaces using The Reaction of 4-fluoro-3-nitrobenzoate with Piperidine.

CORREA, N.M.; DURANTINI E. N.; SILBER J.J.

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY

John Wiley & Sons

Lugar: New York, USA; Año: 2005 vol. 18 p. 121 - 127

0894-3230

Abstract The characterization of different reverse micellar interfaces, benzene/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/water and benzene/benzyl-n-hexadecyldimethyl ammonium chloride (BHDC)/water, were studied using an aromatic nucleophilic substitution reaction, SNAr, between acid 4-fluoro-3-nitrobenzoic (FNBA) and piperidine (PIP). The reaction was also studied in homogeneous media, water and benzene, varying the ionic strength in the aqueous solution. The kinetic profiles were investigated as a function of variables such as surfactant, amine concentration and the amount of water dispersed in the reverse micelles, W0 = [water]/[surfactant]. In the pure solvents, water and benzene, there is no genuine base-catalyzed reaction. In water the reaction is faster than in benzene, due to the better stabilization of the intermediate Z. Also, the reaction rate in water is accelerated by the addition of salt. In both micellar systems FNBA is totally incorporated to the interface of the aggregate, because, in the working conditions of excess of amine it is ionized to carboxylate, The reactions kinetics in these media can be quantitatively explained taking into account the distribution of the nucleophile between the bulk solvent and the micelle interfaces. In the AOT reverse micelles at W0 = 10, the anionic substrate seems to be located in the waterside of the interface while in BHDC it resides at the oil side of the interface. The results were used to evaluate the intrinsic second-order rate coefficient of the SNAr reaction at the interface. From these data properties such as, micropolarity and ionic strength of this environment can be deduced. Thus, as expected, the micropolarity of the reverse micelles systems are higher than the pure benzene and increases upon water addition. In the AOT system and due to the location of the probe, the average ionic strength of the AOT reverse micelles could be estimated and a value around 4.5 M was found.