Q16P - Organic Chemistry (Mcmurry)

Question

Why is the saponification of an ester irreversible? In other words, why doesn’t treatment of a carboxylic acid with an alkoxide ion yield an ester?

Step-by-Step Solution

Answer

The given step in equilibrium is irreversible as the carboxylic acid is a much stronger acid than alcohol so, equilibrium lies towards right.

 

Step 1: Saponification Process

Hydrolysis of an ester in presence of an aqueous hydroxide is known as saponification. This process is used in making of soap. The mechanism of this process involves reaction of OH^- ion on the carbonyl carbon as shown:

In next step, the alkoxide ion, reacts with the acid to give the carboxylate salt and the alcohol.

Step 2: Why the Reaction is Irreversible

The equilibrium in the given reaction,

lies far to the right (according to Le Chatelier’s principle) because the carboxylic acid is a much stronger acid than methanol (or any other alcohol). Thus, the carboxylic acid is removed from the equilibrium and its salt is formed and the hydrolysis goes to completion. Hence, saponification is effectively irreversible.