Salt Effects on Solvolysis Reactions of p-Nitrophenyl Alkanoates Catalyzed by 4-(Dialkylamino)pyridine- Functionalized Polymer in Buffered Water and Aqueous Methanol Solutions

Specific salting-in effects that lead to striking substrate selectivity were observed for the hydrolysis of p -nitrophenyl alkanoates 2 ( n = 2−16) catalyzed by 4-(dialkylamino)pyridine-functionalized polymer 1 in aqueous Tris buffer solution at pH 8.0 and 30 °C. Macromolecule 1 was found to exhibit clear substrate preference for 2 ( n = 6) in 0.05 M aqueous Tris buffer solution, as contrasted with the corresponding reaction in 0.05 M aqueous phosphate or borate buffer solutions where the substrate selectivity is absent. The formation of a reactive catalyst−substrate complex, 1 · 2, appears to be promoted by the presence of tris(hydroxymethyl)methylammonium ion, an efficient salting-in agent, from the Tris buffer system. The salting-in effect on formation of 1 · 2 complex is presumed responsible for the substrate specificity. The salting-out effects of sodium chloride on the solvolysis of 2 catalyzed by 1 were also investigated in 1:1 (v/v) methanol−water solution at pH 8.0 and 30 °C. The rate of 1 -catalyzed solvolysis of 2 ( n = 10−16) was found to vary inversely with NaCl concentration (0−1.0 M). The magnitude of the salting-out effects is dependent on the alkyl chain length in 2 and the concentrations of 1 and NaCl. At 7.5 × 10 - 5 unit mol L - 1 1 and 0−1.0 M NaCl the order of reactivity for 2 ( n = 10−16) was n = 10 > 12 > 14 > 16. However, at 5.0 × 10 - 6 unit mol L - 1 1, a revised reactivity order, 2, n = 14 > 12 > 16, was obtained at [NaCl] < 0.15 M. A significant decrease in the substrate preference for 1 -catalyzed solvolysis of 2 ( n = 10−16) was observed at higher NaCl concentrations. We suggest that the reduced catalytic efficiency and selectivity expressed by 1 in the presence of sodium chloride should be attributed to changes in the morphology and composition of aggregates containing 1 and 2 in aqueous methanol solution that lead to decreased dependence of aggregate formation on the hydrophobicity of the substrate.