Optimization of micellar catalysis of nucleophilic substitution reactions in buffered solutions of cetyltrimethylammonium halide surfactants, part 2: buffers in the pH range 7–8

Abstract

AbstractBinding of the phosphate, tris‐(hydroxymethyl)‐methylamine, aminomethylpropanediol, and glycinemethylester buffers by cetyltrimethylammonium chloride (CTACl) in aqueous solutions has been probed by investigating: (1) the dependence of the buffer pH (starting pH 7.9) on [CTACl], and (2) the micellar effect on the kinetics of dephosphorylation of p‐nitrophenyldiphenylphosphate (PNDPP) by the anion of isonitrosoacetylacetone (INAA) in CTACl solutions in the presence of the same buffers. The pH–[CTACl] profiles showed a marked dependence on the buffer employed and the coion, Y−, of its acidic component, $\hbox{RNH}_{3}^{+}\hbox{Y}^{-}$. The sizeable pH decrease observed with phosphate buffer (0.43 pH units for [Buffer] = 10−2 M at [CTACl] = 2 × 10−2 M) indicates that both buffer components, namely $\hbox{H}_{2}\hbox{PO}_{4}^{-}$ and $\hbox{HPO}_{4}^{2-}$, exchange with the surfactant counterion, Cl−. This ion exchange occurs at the expense of the nucleophile (anion of INAA)–Cl− counterpart. Indeed, the micellar acceleration of the phosphate‐buffered reaction is the smallest, kmax/kw = 410 (kmax and kw are the maximum pseudo first‐order rate constants in buffered micellar solutions and bulk water, respectively). Although CTACl micelles do not seem to incorporate the neutral component of amino buffers, the pH–[CTACl] profiles were found to depend on the nature in Y− (F−, Cl− or AcO−). The micellar accelerations (kmax/kw ≈ 600), however, were not strongly altered by a change in the buffer coion, except where Y− = F−. In the interfacial region, the partially desolvated fluoride ion behaves as a nucleophile, competing with INAA anion for the dephosphorylation of PNDPP. The rate–[surfactant] profiles were interpreted in terms of the pseudophase ion‐exchange model, as applied to a reaction scheme involving competitive exchanges of the oximate and Y− for the surfactant counterion. The second‐order rate constant of the micelle‐mediated reaction, smaller (ca one‐third) than that in bulk aqueous solution, is discussed in terms of the properties (ionic strength and microscopic polarity) of interfacial water. Copyright © 2001 John Wiley & Sons, Ltd.