14.19:
Esters to Carboxylic Acids: Acid-Catalyzed Hydrolysis
Acid-catalyzed hydrolysis is another important reaction of esters.
In this reaction, an ester is hydrolyzed in the presence of aqueous acid to form carboxylic acid and alcohol.
This is a reversible reaction and precisely the reverse of a Fischer esterification reaction.
The mechanism begins with the protonation of the carbonyl oxygen by the acid catalyst, rendering the carbonyl carbon more electrophilic.
This is followed by a nucleophilic attack by water at the carbonyl carbon, forming a tetrahedral intermediate.
Next, deprotonation of the tetrahedral intermediate gives a neutral tetrahedral addition intermediate.
Subsequently, protonation of the alkoxy oxygen converts the alkoxy group into a better leaving group.
In the next step, the carbonyl group is re-formed with the departure of alcohol as a leaving group.
Finally, deprotonation yields a carboxylic acid and regenerates the acid catalyst.
Overall, an acid catalyst increases the electrophilicity of the carbonyl carbon and decreases the basicity of the leaving group.
14.19:
Esters to Carboxylic Acids: Acid-Catalyzed Hydrolysis
Hydrolysis of esters under acidic conditions proceeds through a nucleophilic acyl substitution. In the presence of excess water, the reaction proceeds in a reversible manner, forming carboxylic acids and alcohols.
During hydrolysis, the ester is first activated towards nucleophilic attack through the protonation of the carboxyl oxygen atom by the acid catalyst. The protonation makes the ester carbonyl carbon more electrophilic. In the next step, water acts as a nucleophile and adds to the carbonyl carbon, followed by the loss of a proton to give the tetrahedral addition intermediate. Further, the protonation of the alkoxy group converts it into a better leaving group. Subsequently, the carbonyl group is reconstructed with the expulsion of alcohol as a leaving group. Finally, deprotonation yields a carboxylic acid as the final product and regenerates the acid catalyst.
An acid catalyst protonates the carbonyl oxygen, thereby increasing the formation rate of the tetrahedral intermediate. Moreover, the catalyst decreases the basicity of the leaving group. Notably, the reaction is exactly the reverse of a Fischer esterification reaction.