19.9 Hydrolysis of Esters

Esters are organic compounds derived from carboxylic acids and alcohols. They can undergo hydrolysis — a reaction with water that breaks the ester bond — yielding a carboxylic acid and an alcohol. This reaction can be catalyzed by either dilute acid or dilute alkali, usually with heat.

Acid-Catalyzed Hydrolysis

In the presence of a mineral acid (e.g., $H_{2}S{O}_4$ or $HCl$), ester hydrolysis is a reversible reaction:

$R_{1}COO{R}_2+H_{2}O\underset{H^{+}}{\rightleftharpoons }{R}_{1}COOH+R_{2}OH$

The general steps of the mechanism are:

1. Protonation of the carbonyl oxygen by the acid catalyst, making the carbonyl carbon more electrophilic.
2. Nucleophilic attack of water on the activated carbonyl carbon.
3. Proton transfers and loss of the alcohol group to regenerate the carboxylic acid.

Because the reaction is reversible, an excess of water is used to drive the equilibrium towards hydrolysis products.

Basic Hydrolysis (Saponification)

Basic hydrolysis of esters is specifically known as saponification. This term is derived from the Latin word sapon meaning soap, because this process leads to the formation of alkali salts of carboxylic acids, which are commonly known as soaps.

The general reaction for basic hydrolysis is:

$R_{1}COO{R}_2+\text{NaOH}\stackrel{\Delta }{\to }{R}_1\text{COONa}+R_2\text{OH}$

Where:

• $R_{1}COO{R}_2$ represents an ester
• $\text{NaOH}$ is a strong base
• $R_1\text{COONa}$ is the alkali salt of a carboxylic acid (a soap)
• $R_2\text{OH}$ is an alcohol

Unlike acid-catalyzed hydrolysis, base-catalyzed hydrolysis is irreversible because the carboxylic acid produced reacts immediately with the base to form a resonance-stabilized carboxylate anion ($RCO{O}^{-}$), which cannot react back with the alcohol.

Comparison: Acid vs. Base Hydrolysis