Structure and reactivity in the hydrolyses of aliphatic carboxylic acid esters and chlorides

Abstract

For hexanoic acid and its seven isomers, relative rates have been determined for acid catalysed esterification with methanol, and compared with those for saponification of the methyl esters. A good correlation between logarithms of relative rates for the two reactions is obtained, and it is suggested that the eight isomers provide a test set of compounds in which steric effects alone act on reactivity at the acyl carbon. A full set of steric parameters ($E_{\rm s}^\prime$ values) are presented. Rates of solvolyses of the acid chlorides of the isomers have been determined conductometrically in 3:1 wt:wt acetonitrile water. Logarithms of relative rates show a poor correlation with $E_{\rm s}^\prime$, and, taking into account the solvent dependence of the rates, the pattern excludes both rate‐limiting formation of a tetrahedral intermediate and rate‐limiting dissociation of chloride to form acylium ions. The remaining possibilities, a concerted process (AND N‡) and rapid reversible formation of a hydrate followed by rate‐limiting dissociation of chloride (AN + D N‡) are considered. Copyright © 2007 John Wiley & Sons, Ltd.

Concerted SN2 mechanism for the hydrolysis of acid chlorides: comparisons of reactivities calculated by the density functional theory with experimental data

“…It is to be noted, however, that acetyl chloride is not protonated in solvolyses under normal reaction conditions 48, 49. It has also been suggested that dissociation of the chloride ion may follow the hydration of acetyl chloride,23, 32, 33 though neither the fast10, 11 nor the slow14 prior formation of the hydrate was supported by experimental data 42…”

“…The hydrolyses of acid chlorides were also supposed23, 32, 33 to proceed through the hydrated substrates, though these intermediates have neither been synthesized nor detected in the reaction mixtures. Acetyl chloride hydrate could be formed if the nucleophilic attack of water on the carbonyl group would be accompanied by a proton transfer towards the oxygen atom of the CO group.…”

“…Nevertheless, it was also suggested that not the mechanisms but only the structure of the TSs changes with the substituents and solvents 29, 30. The hydrolysis of acid chlorides was also supposed to occur at the hydrated form of the substrates 31–33. Hydrates of acid chlorides could be the intermediates of the oxygen exchange reaction,34 too.…”

Solvolyses of Benzoyl Chlorides in Weakly Nucleophilic Media

“…Estimates based on kinetic data at 0 °C show that acetyl chloride reacts about 5 orders of magnitude faster than predicted from Equation 3, and a prior hydration mechanism could not be excluded [24,37]. Recent extensive DFT calculations included a wide range of acid chlorides and optimized structures of assemblies including molecules of water and acetone.…”

Steric effects and mechanism in the formation of hemi‐acetals from aliphatic aldehydes

“…Logarithms of relative rates for the acid-catalysed esterifications provide the traditional steric substituent parameters, E s , and we have tabulated those for the C 5 H 11 isomers in our earlier paper. [1] It is clear that relative rates for the ester saponifications might equally be used to compare steric demand within this series.…”