Optimization of micellar catalysis of nucleophilic substitution reactions in buffered solutions of cetyltrimethylammonium halide surfactants, part 2:buffers in the pH range 7-8
N. Ouarti et al., Optimization of micellar catalysis of nucleophilic substitution reactions in buffered solutions of cetyltrimethylammonium halide surfactants, part 2:buffers in the pH range 7-8, J PHYS ORG, 14(11), 2001, pp. 823-831
Binding of the phosphate, tris-(hydroxymethyl)-methylamine, aminomethylprop
anediol, and glycinemethylester buffers by cetyltrimethylammonium chloride
(CTAC1) in aqueous solutions has been probed by investigating: (1) the depe
ndence of the buffer pH (starting pH 7.9) on [CTAC1], and (2) the micellar
effect on the kinetics of dephosphorylation of p-nitrophenyldiphenylphospha
te (PNDPP) by the anion of isonitrosoacetylacetone (INAA) in CTAC1 solution
s in the presence of the same buffers. The pH-[CTAC1] profiles showed a mar
ked dependence on the buffer employed and the coion, Y-, of its acidic comp
onent, RNH3+Y- The sizeable pH decrease observed with phosphate buffer (0.4
3 pH units for [Buffer] = 10-2 M at [CTACI] = 2 X 10(-2) M) indicates that
both buffer components, namely H2PO4- and HPO42-, exchange with the surfact
ant counterion, Cl-. This ion exchange occurs at the expense of the nucleop
hile (anion of INAA)-Cl- counterpart. Indeed, the micellar acceleration of
the phosphate-buffered reaction is the smallest, k(max)/k(w) = 410 (k(max)
and k(w) are the maximum pseudo first-order rate constants in buffered mice
llar solutions and bulk water, respectively). Although CTAC1 micelles do no
t seem to incorporate the neutral component of amino buffers, the pH-[CTAC1
] profiles were found to depend on the nature in Y- (F-, Cl- or AcO-). The
micellar accelerations (k(max)/k(w) approximate to 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-o
rder rate constant of the micelle-mediated reaction, smaller (ca one-third)
than that in bulk aqueous solution, is discussed in terms of the propertie
s (ionic strength and microscopic polarity) of interfacial water. Copyright
(C) 2001 John Wiley & Sons, Ltd.