13.14: Carboxylic Acids to Esters: Acid-Catalyzed (Fischer) Esterification Mechanism
Carboxylic acids react with alcohols to yield esters via an acid-catalyzed condensation reaction called Fischer esterification. This is a nucleophilic acyl substitution reaction that proceeds via a tetrahedral intermediate, where a water molecule is eliminated as the leaving group.
The mechanism of this reaction was confirmed by Robert and Urey (1938) through a radioisotope labeling experiment, where esterification of a carboxylic acid was carried out using 18O-labeled alcohol. The resulting ester was found to be labeled with an 18O atom, establishing the fact that the –OH group of the carboxylic acid was replaced by the –OR group from the alcohol.
Modifications of Fischer esterification use either a boron trifluoride ether complex or an organotin complex as the catalyst in an acid-free condition.