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 n2-16 catalyzed by 4-dialkylaminopyridine-functionalized polymer 1 in aqueous Tris buffer solution at pH 8.0 and 30 deg C. Macromolecule 1 was found to exhibit clear substrate preference for 2 n6 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 trishydroxymethyl 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 11 vv methanol-water solution at pH 8.0 and 30 deg C. The rate of 1-catalyzed solvolysis of 2 n10-16 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 x 10exp -5 unit molL 1 and 0-1.0 M NaCl the order of reactivity for 2 n10-16 was n10 12 14 16. However, at 5.0 x 10exp -6 unit molL 1 a revised reactivity order 2 n14 12 16, was obtained at NaCl 0.15 M. A significant decrease in the substrate preference for 1-catalyzed solvolysis of 2 n10-16 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