13.11: Carboxylic Acids to Esters: Acid-Catalyzed (Fischer) Esterification Overview

The Fischer esterification reaction was developed by the German chemist Emil Fischer in 1895. It is a condensation reaction between carboxylic acids and alcohols in an acidic medium to give esters and water.

Hydroxy-functionalized carboxylic acids undergo intramolecular Fischer esterification to form lactones. The cyclic five- and six-membered lactones are formed spontaneously.

The reaction rate of Fischer esterification is greatly dependent on steric factors. Primary alcohols react fastest with carboxylic acids to form esters, while tertiary alcohols undergo Fischer esterification at a slower rate, forming alkene by-products.

Fischer esterification is an inherently slow reaction with a low equilibrium constant value and never attains completion. During the reaction, an equilibrium is always established between the reactant and the product. Consequently, traces of unreacted acid are always present along with the product ester. Using excess alcohol as a solvent directs the equilibrium towards the product, according to Le Chatelier's principle. Alternatively, removing the water from the reaction mixture using a Dean–Stark trap can also drive the reaction to completion.

Treating carboxylic acids with alcohol under acidified conditions yields esters as the product. Such acid-catalyzed condensation reaction are called Fischer esterification– named after the German chemist Emil Fischer.

Catalysts like HCl and H₂SO₄ protonate the carbonyl group of the carboxylic acid to encourage nucleophilic attack by the alcohol.

Hydroxy acids, that is, acids bearing the –OH group at the γ or δ position, undergo intramolecular esterification to form five- and six-membered ring, lactones.

Notice that both the acid and the alcohol are in equilibrium with the product. Thus, applying Le-Chatelier's principle, the use of excess alcohol as a solvent or the continuous removal of water drives the equilibrium towards the right, increasing the yield of the ester.

The reaction is also influenced by steric factors, and the presence of bulky groups on the acid or alcohol retards the reaction rate. Thus, primary alcohols–with the least steric hindrance–are preferred over phenol or bulky tertiary alcohols which undergo elimination.