Ester from acyl chloride: practice and mechanism
The alcohol replaces the Cl on the carbonyl carbon and pyridine traps HCl: acyl chloride → ester, completely and without equilibrium.
Example
The alcohol attacks the highly reactive acyl chloride and chloride leaves. Pyridine traps the HCl formed. Unlike the Fischer esterification, the reaction goes to completion without an equilibrium.
Mechanism step by step
- 1
Ethanol attacks the carbonyl carbon of the acyl chloride.
- 2
The tetrahedral intermediate expels Cl⁻ and C=O re-forms.
- 3
Pyridine takes up the proton and binds HCl as pyridinium chloride: ester.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- The carboxylic acid would form with water (hydrolysis). Here ethanol is the nucleophile, so the ethyl ester forms.
- A ketone would only form with a carbon nucleophile (e.g. a cuprate). Ethanol attacks with its oxygen.
- The C=O group stays: only the Cl is swapped for O–Et. An ether without C=O does not form.
Often confused with: Amide from acyl chloride · Fischer esterification
Frequently asked questions
Why is this better than the Fischer esterification?
There is no equilibrium; the reaction is fast, complete and needs no strong acid, good for sensitive molecules.
What else does pyridine do?
It can act as a nucleophilic catalyst activating the acyl chloride (acylpyridinium ion); DMAP is even better.