Fischer Esterification is an acid-catalyzed method to turn carboxylic acids into esters through nucleophilic acyl substitution.
General Reaction:
Fischer esterification (aka Fischer-Speier esterification and acid-catalyzed esterification) is a great way to take a carboxylic acid and convert it into an ester. All that’s required is a carboxylic acid, a strong acid catalyst, and an alcohol. Let’s go ahead and use acetic acid, H3O+ (same as writing H+), and ethanol to demonstrate the mechanism. To be clear, any acid like H2SO4 or HCl works just fine; we’re just going to use protonated water here.
Mechanism:
Starting material, reagents, and product
Protonation
The first step of this reaction is the protonation of the carbonyl oxygen to form an electrophilic carbon. Either resonance form can be used for the rest of the mechanism.
Nucleophilic attack
The alcohol then comes in and attacks the carbon to form a tetrahedral intermediate.
Proton transfer
Next an intramolecular proton transfer occurs to form a hydronium ion.
Reform the carbonyl
Once that happens, the carbonyl is reformed and water is kicked off.
Deprotonation
All that’s left to do is deprotonate the carbonyl.
Now that you've learned about Fischer esterification, you know the game plan for nucleophilic acyl substitution (NAS). NAS can be used to make tons of molecules like benzoic acid and aspirin!
Summary:
Notice that we ended up with our acid again? That’s why this is considered an acid-catalyzed mechanism. Of course, there’s also base-catalyzed esterification. Be sure to check out my lesson dedicated to carboxylic acid derivatives. Good luck studying!
