13.11 : カルボン酸からエステルへ: 酸触媒 (フィッシャー) エステル化の概要

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.