Carboxylic acids react with alcohols to yield esters via anacid-catalyzed condensation reaction called Fischer esterification. This is a nucleophilic acyl substitution reaction that proceeds viaa tetrahedral intermediate, where awater molecule is eliminated as the leaving group.

The mechanism of this reaction was confirmed by Robert and Urey (1938) through a radioisotope labeling experiment, whereesterification of a carboxylic acid was carried out using ¹⁸O-labeled alcohol. The resulting ester was found to be labeled with an¹⁸O atom, establishing the fact that the –OH group of the carboxylic acid was replaced bythe –OR group from the alcohol.

Modifications of Fischer esterificationuse either a boron trifluoride ether complex or an organotin complex as the catalyst in an acid-free condition.