Hydrolysis of p-nitrophenyl alkanoates in aqueous organic solvent mixtures. The dispersal of aggregates and the uncoiling of n-alkyl chains

Abstract

Rate constants have been measured for the hydrolysis of six p-nitrophenyl alkanoates (C2, C4, C6, C8, C10, and C12) in basic aqueous media containing up to 80% (v/v) of the cosolvents: ethylene glycol (EG), 2-methoxyethanol (2ME), 1,2-dimethoxyethane (DME, glyme), bis(2-methoxyethyl) ether (diglyme), or DMSO. Addition of these cosolvents reduces and ultimately removes hydrophobic aggregation and coiling of the longer chain alkanoates, with the ether-type solvents (2ME, DME, and diglyme) being more efficient than EG or DMSO. Once the rate-lowering effects of aggregation and coiling have been removed by addition of a cosolvent, the rates of hydrolysis of the C4–C12 esters are all virtually the same, in a given medium. Thus, the apparent "steric size" of the C4–C12 (and even C16) acyl chains does not differ significantly in basic p-nitrophenyl ester hydrolysis. Addition of DMSO brings about rate increases that are greater in a carbonate buffer (pH 10.48) than those in 0.05 M NaOH solution, and these increases are larger than those observed for ethyl acetate in 0.05 M NaOH solution. None of the rate increases rise as quickly as the enhanced basicity of aqueous DMSO media.