Key Mechanism Fischer Esterification

The Fischer esterification mechanism (see the Key Mechanism box) is an acid-catalyzed nucleophilic acyl substitution. The carbonyl group of a carboxylic acid is not sufficiently electrophilic to be attacked by an alcohol. The acid catalyst proto-nates the carbonyl group and activates it toward nucleophilic attack. Loss of a proton gives the hydrate of an esto-. 

Loss of water from the hydrate of the ester occurs by the same mechanism as loss of water from the hydrate of a ketone (Section 18-14). Protonation of either one of the hydroxyl groups allows it to leave as water, forming a resonance-stabilized cation. Loss of a proton from the second hydroxyl group gives the esto-. 

The mechanism of the Fischer esterification would seem long and complicated if you tried to memorize it, but we can understand it by breaking it down into two simpler mechanisms (1) acid-catalyzed addition of the alcohol to the carbonyl and (2) acid-catalyzed dehydration. If you understand these mechanistic components, you can write the Fischer esterification mechanism without having to memorize it. 

Part 1 Acid-catalyzed addition of the alcohol to the carbonyl group. 

EXAMPLE Acid-catalyzed formation of methyl benzoate from methanol and benzoic acid. 

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