14.18 : 酯到羧酸：皂化

Esters can undergo either acid-catalyzed hydrolysis or base-promoted hydrolysis through typical nucleophilic acyl substitutions.

The base-promoted hydrolysis of esters also called saponification, involves treatment with an aqueous base, followed by acid to give carboxylic acids.

Since a stoichiometric amount of base participates in the reaction, it acts as a reactant and not as a catalyst.

The mechanism begins with the nucleophilic attack by the hydroxide ion at the ester carbonyl carbon, forming a tetrahedral intermediate.

The second step is the reconstructing of the carbonyl group with the departure of an alkoxide ion.

Subsequently, deprotonation yields a carboxylate ion and alcohol. This step is the driving force that pushes the equilibrium towards the product.

Finally, acidification of the carboxylate ion gives free acid.

The mechanism is supported by the isotope-labeling studies using an ester labeled with the ¹⁸O isotope. After hydrolysis, all the ¹⁸O label appears in the alcohol, indicating acyl-oxygen bond cleavage.

Consequently, one of the oxygens in the carboxylate ion comes from the nucleophile.