Kinetic consequences of molecular aggregation in the hydrolysis of p-nitrophenyl carboxylates

Abstract

Alkaline hydrolysis of p-nitrophenyl carboxylates was investigated in mixed aqueous–organic solvents. Esters with a long alkyl chain tend to aggregate by intermolecular hydrophobic interaction above the critical concentration. Among acetate, hexanoate, decanoate, dodecanoate, and hexadecanoate, the last showed the lowest critical concentration, while acetate and hexanoate did not demonstrate any detectable aggregation even in 1.0%(v/v) aqueous dioxan. The reactivity of monomeric esters decreased with increasing alkyl-chain length, due to the selfcoiling behaviour of the alkyl chain which acts to mask the ester bond from hydroxide attack. The critical concentration for dodecanoate decreased in the reaction medium order: 10.9%(v/v) dioxan > 10.9%(v/v) acetonitrile [graphic omitted] 1.0%(v/v) dioxan > 10.9%(v/v) ethanol. The rate increase observed for hydrolysis of monomeric dodecanoate by increasing the organic content of a solvent was attributed to elongation or decoiling of the monomeric ester. As for the effect of hydroxide ion concentration on aggregation behaviour, the monomer fractions of dodecanoate and of hexadecanoate above their critical concentrations were found to decrease with increasing [OH–] due to the structure-making ability of hydroxide ion. Urea acted to raise somewhat the critical concentrations of the esters and to decrease their aggregation numbers as exemplified by the alkaline hydrolysis of dodecanoate. The saltingout effects of inorganic salts gave a reduction in the fraction of monomeric ester and hence in the overall hydrolysis rate.