The effect of pressures up to 3 lcbar on the rate of the acid-catalyzed hydrolysis of methyl, ethyl, and t-butyl acetates in dilute aqueous acid and of ethyl acetate in concentrated hydrochloric acid has been measured. The volume of activation for t-butyl acetate is zero within experimental error, showing that the lnechanism is unimolecular. 'Those for methyl and ethyl acetates are near -9 cm3 mole-I in both dilute and concentrated acid. We deduce from this that the mechanism is the same in 9.2-114 hydrochloric acid as in dilute acid, that the transition state is not highly polar, and that if the proton in the reactive protonated ester is on the carbonyl oxygen the11 the attaclcing water molecule adds, and if the proton is on the ether oxygen then the attacking \\rater molecule substitutes.
INTRODUCTIOSI n spite of a large amount of experimental work, the hydrogen-ion-catalyzed hydrolysis of simple aliphatic esters is not well understood. There is little doubt, because, for example, of the relative rates in water and heavy water (1-3), that a pre-equilibrium proton transfer occurs, but the location of the proton in the reactive conjugate acid is not known. There is little doubt, mainly because of substituent effects (4, p. 770), that hydrolysis occurs by an attack of a water nlolecule on the conjugate acid, but it is not known whether the attack is a n addition to a water molecule to form a stable intermediate or whether it is a simple substitution. I t is, however, known that when ethyl benzoate is hydrolyzed by acid catalysis, the carbonyl oxygen of the unhydrolyzed ester exchanges with the oxygen of the solvent water (5, G), but i t is not lcnown whether the hydrolysis and exchange follow similar or unrelated mechanisms. There are differing opinions (e.g. refs. 7-11) about whether the transition states that determilie the rates have high dipole ~nonlents or not.The effect of pressure on the rates of acid-catalyzed hydrolyses can provide useful information about the mechanisms (11-lG), and a preliminary discussioll of the hydrolysis of methyl acetate based on a few results reported by Bogojawlenslty and Talnnlann (17) has already been given (11). In this paper we report measurelnents of the effect of pressures up to 3 kbar on the rates of hydrolysis of methyl and ethyl acetates in dilute hydrochloric acid.Tal'vik and Pal'm (18) have argued that in collcelltrated acids ethyl acetate becomes essentially conlpletely protonated, and that if allowance is made for the protonation, then the rate becomes nearly proportional to EIarnmett's acidity function ho rather than to the concentration of hydrogen ion. By using the Zuclter-EIammett hypothesis (19, 20) they deducecl that the slow step is An attempt to verify this mechanism in concentrated acids has been tilade by measuring the volume of activation in concentrated hydrochloric acid.
