13.13: 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.