JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. zyxwvu 5, 341-348 (1992) EFFECT OF HEXADECYLTRIMETHYLAMMONIUM BROMIDE MICELLES ON THE RATE OF OXIMOLYSIS OF ESTERS DIN0 ZANETTE zyxwvu Departamento de Quimica, Universidade Federal de Santa Catarina, Florianopolis, SC, Brazil AND HERNAN CHAIMOVICH* Departamento de Bioquimica, Instituto de Quimica, Universidade de SZo Paulo, Caixa Postal 20780, CEP 01498, zyx SZo Paulo, SP, Brazil Two amphiphilic oximes, zyxwvutsr 10-phenyl-10-hydroxyiminodecanoic acid (oxime 11) and 4-(9-carboxynonanyl)-l-(9-carhoxy- 1-hydroxyimino nonany1)benzene (oxime 111) were synthesized. The pK. values of oximes zyxw I1 and I11 and acetophenoxime (oxime I) and the rates of oximolysis of p-nitrophenyl acetate (NPA) and p-nitrophenyl octanoate (NPO) were determined in the presence and absence of micellar hexadecyltrimethylammonium bromide (CTAB). The rates of oximolysis increased by up to 3 x 104-fold in the presence of CTAB. Quantitative analysis of micellar effects, using an ion-exchange pseudo-phase model, allowed the determination of the second-order rate constants for the reactions of oximes 1-111 with NPA and NPO in the micellar pseudo-phase. The calculated rate constants in the micellar pseudo-phase were lower than those in water, demonstrating that the rate enhancements were due to substrate concentration in the micelles. Comparison of the rate constants in micelles and water suggests that the sites of reaction of oximes 1-111 with NPO and NPA are similar to those in hulk aqueous solution. Micellar incorporation of the hydrophobic oximes I1 and I11 does not lead to a major change in the nucleophilicity of the oximate anion. INTRODUCTION Quantitative analysis of micellar effects on the rates of chemical reactions has been used to show that the rate constants of bimolecular reactions in the aggregate rarely differ from those in the bulk aqueous phase by more than tenfold. ' This observation also applies to reactions in other amphiphile aggregates such as microemulsions2a9b and vesicles. 2c Most of the data has been obtained with ionic or water-soluble nucleophiles attacking micelle-incorporated substrates. ',* Nucleo- philic reactivity in aqueous solution is limited by partial desolvation of the reactants. Since nucleophilic reac- tivity in micelles and in water are similar, the (micellar) reaction sites are considered to be water-like. It would be interesting to study reactions of nucleophiles designed, in principle, to penetrate the hydrophobic core of a micelle at different positions in order to investigate their reactivity relative to water. * Author €or correspondence. 0894-3230/92/060341-08$09.00 zyxwvut 0 1992 by John Wiley & Sons, Ltd. 1 (1) In this paper we describe the effect of micelles on the rate oximolysis [reaction (l)] of p-nitrophenyl acetate (NPA) and octanoate (NPO) by water-soluble (oxime I), Scheme 1 and hydrophobic oximes (oximes I1 and 111). We found that the calculated rate constants for the oximolysis reactions are very similar. Therefore, for these compounds, the conformational freedom of the micelle-incorporated nucleophiles and the hydration of the attacking group impose a water-like reaction site even between oxime 111 and NPO. Received 6 November I991 Revised 7 February 1992