Ester Hydrolysis in Base Saponification

Unlike its acid-catalyzed counterpart, ester hydrolysis in aqueous base is irreversible. 

This is because carboxylic acids are converted to their corresponding carboxylate anions under these conditions, and these anions are incapable of acyl transfer to alcohols. 

To isolate the carboxylic acid, a separate acidification step following hydrolysis is necessary. Acidification converts the carboxylate salt to the free acid. 

Ester hydrolysis in base is called saponification, which means soap making. Over 2000 years ago, the Phoenicians made soap by heating animal fat with wood ashes. Animal fat is rich in glycerol triesters, and wood ashes are a source of potassium carbonate. Basic cleavage of the fats produced a mixture of long-chain carboxylic acids as their potassium salts. 

Potassium and sodinm salts of long-chain carboxylic acids form micelles that dissolve grease (Section 19.5) and have cleansing properties. The carboxylic acids obtained by saponification of fats are called fatty acids. 

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