Esters can be synthesized by an acid-catalyzed nucleophilic acyl substitution between alcohol and carboxylic acid; this process is called the Fischer esterification reaction. Because the alcohol oxygen is a poor nucleophile, the carbonyl carbon is made more electrophilic by protonation of the carbonyl oxygen. The steps of the synthesis are all reversible. The reaction is generally driven to completion by using an excess of the liquid alcohol as a solvent and by distilling off the product.
The curved arrows show the movement of electrons in this step of the mechanism.