Aggregation of p-Nitrophenyl Alkanoates in Aqueous Solution. Limitations on Their Use as Substrates in Enzyme Model Studies

Abstract

p-Nitrophenyl alkanoates are aggregated at extremely low concentrations in aqueous solution. Second-order rate constants for imidazole or hydroxide catalyzed hydrolysis are independent of ester concentration up to a critical concentration of ester, and then decrease with increasing ester concentration. The critical concentrations at 25 °C are: p-nitrophenyl hexanoate, 1 × 10−4 M; octanoate, 9.6 × 10−6 M; decanoate, 1.2 × 10−6 M. For the dodecanoate ester, the critical concentration was estimated as ca. 10−7 M. Addition of 10% methanol causes only a modest increase in the critical concentration. The critical concentration probably represents the solubility limit of the ester. The rate constants for the hydrolysis of the longer chain esters evaluated here are significantly larger than those in the literature, suggesting that the earlier work was done using conditions where the esters were aggregated.