Hydroxide as General Base in the Saponification of Ethyl Acetate

Abstract

The second-order rate constant for the saponification of ethyl acetate at 30.0 degrees C in H(2)O/D(2)O mixtures of deuterium atom fraction n (a proton inventory experiment) obeys the relation k(2)(n) = 0.122 s(-1) M(-1) (1 - n + 1.2n) (1 - n + 0.48n)/(1 - n + 1.4n) (1 - n + 0.68n)(3). This result is interpreted as a process where formation of the tetrahedral intermediate is the rate-determining step and the transition-state complex is formed via nucleophilic interaction of a water molecule with general-base assistance from hydroxide ion, opposite to the direct nucleophilic collision commonly accepted. This mechanistic picture agrees with previous heavy-atom kinetic isotope effect data of Marlier on the alkaline hydrolysis of methyl formate.

Mechanistic investigations of the hydrolysis of amides, oxoesters and thioesters via kinetic isotope effects and positional isotope exchange

“…The conclusion that water is the actual nucleophile is also strongly supported by a proton inventory study that found that at least two hydrogen atoms are involved in the transition state. This is consistent with the general base mechanism, but not the direct attack by hydroxide [38].…”

The enthalpy and entropy of activation for ethyl acetate saponification

“…The isotope effect on the oxygen nucleophile was interpreted as a molecular event, where the attacking nucleophile in aqueous alkali is water, with general base assistance from hydroxide. The results from a proton inventory experiment for the saponification of ethyl acetate [7] agree with the mechanism proposed by Marlier. A nuclear magnetic resonance study was performed to the rates of hydroxide-promoted hydrolysis regarding formamide within an aqueous media of varying mole fraction D 2 O to shed light on whether this mechanism involves a nucleophilic attack of OH − on the C O or OH − , acting as a general base for removing protons from an attacking water.…”

Determination of the kinetics of biodiesel saponification in alcoholic hydroxide solutions

“…1a). However, recent studies [7,9] found that the reactions did not occur via simple bimolecular collisions, but rather required a molecule of water to form the tetrahedral intermediate. This indicates that a water molecule stabilises the transition-state complex through hydrogen bonding (Fig.…”

“…For instance, the Arrhenius activation energies of 41.40-63.18 kJ mol À1 [3,5,6,11] for alkyl esters saponification in aqueous/protic medium were too small to account for the desolvation energy of 423.4 kJ mol À1 [9] required for the desolvation of OH À in the aqueous medium. The values of the activation energies for alky esters saponification suggest that the direct collision of desolvated OH À ions is implausible.…”