Optimization of micellar catalysis of nucleophilic substitutions in buffered cetyltrimethylammonium salt solutions. 1. Buffers for the 9-10 pH range

The rate-surfactant concentration profiles for the reaction of p-nitrophenyldiphenyl phosphate, PNDPP, with butanedione monooximate (pK = 9.3), Ox(-), in aqueous solutions of cetyltrimethylammonium salts, CTAX, are measured for X- = Br-, Cl-, and Ac-, in the presence of various buffers working in the 9-10 pH range. All these profiles are typical of micelle-assisted bimolecular reactions involving interfacial ion-exchanges. Their analysis in terms of pseudophase-ion-exchange model fails, whatever the counterion and the buffer. The micellar effects, k(max)/k(w) (k(max), the largest rate constant in the rate-[CTA(+)] profile, and k(w), the rate constant at the same [Ox(-)] in water), depend on the counterion and on the buffer. The counterion effect on these ratios follows the unexpected sequence Br- much less than Cl- approximate to AcO-. The buffer dependence of the micellar effect, in CTACl micelles, in the order carbonate < berate < glycine less than or equal to ethanolamine = AMP (2-amino-2-methyl-1,3-propanediol), going from 125 to 820 when the buffer concentration is 10(-2) M, is interpreted in terms of increasing micellar concentrations of the oximate, [Ox(-)](m). Nevertheless, the [Ox(-)](m) dependence on the buffer does not parallel the micellar binding of these buffers observed in pH measurements. A new scheme involving competition between three exchange equilibria, K-Cl(B), K-Cl(Ox), and K-B(Ox), is used to interpret the data, emphasizing that the base-nucleophile exchange, and not only that between the surfactant counterion and the nucleophile, controls the rates in the low [CTA(+)] range. These results are discussed in the light of previous interpretations of the buffer role in micellar catalysis.