Esterification with acetic anhydride: practice and mechanism
The alcohol (or phenol) attacks acetic anhydride at the C=O group and acetate leaves. An acetate ester (–O–CO–CH₃) forms; pyridine captures the acid released.
Example
Acetic anhydride is a reactive acylating agent. The alcohol (or phenol) attacks a carbonyl group and acetate leaves. Unlike the Fischer esterification this is not an equilibrium and also works with phenols.
Mechanism step by step
- 1
The alcohol oxygen attacks one C=O group of acetic anhydride: a tetrahedral intermediate.
- 2
Acetate (a good leaving group) is expelled and the C=O bond re-forms.
- 3
Pyridine takes the proton and binds the acetic acid. The acetate ester forms.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- That would be a methyl ether. Acetic anhydride transfers an acetyl group (CH₃–C=O), not a methyl group.
- That is a formate. With acetic anhydride the ester carbon also carries a methyl group.
- Without a Lewis acid acetic anhydride does not acylate the ring (that would be Friedel–Crafts). The phenol oxygen attacks and the phenyl ester forms.
- The acetyl group goes to the oxygen, not the carbon. The alcohol oxygen is the nucleophile, so an ester forms, not a ketone.
Often confused with: Fischer esterification · Ester from acyl chloride
Frequently asked questions
Why not a Fischer esterification?
The Fischer esterification is an equilibrium and barely works with phenols. Acetic anhydride reacts completely and also with phenols.
An everyday example?
Aspirin: salicylic acid is acetylated at its phenolic OH group with acetic anhydride to give acetylsalicylic acid.