Acyl chlorides
Acyl chlorides
- Acyl chlorides are made from carboxylic acids and are very reactive.
- They react with many nucleophiles, always releasing HCl.
- They follow an addition–elimination mechanism.
Reactions (all give HCl)
| Reactant | Product |
|---|---|
| water | the carboxylic acid |
| an alcohol | an ester |
| phenol | an ester |
| ammonia | an amide |
| a 1°/2° amine | an amide |
Practice
An acyl chloride reacting with ammonia gives:
Ammonia (or a 1°/2° amine) reacts with an acyl chloride to give an amide.
Practice
An acyl chloride reacting with water gives:
Hydrolysis of an acyl chloride regenerates the carboxylic acid (very vigorously).
Addition–elimination
- A nucleophile adds to the slightly positive carbonyl carbon → a tetrahedral intermediate.
- Then the C=O reforms and $\text{Cl}^-$ leaves as HCl (elimination).
- Ease of hydrolysis: $\text{acyl chloride} \gg \text{alkyl chloride} \gg \text{aryl chloride}$ (the aryl C–Cl is strengthened by the ring).
Practice
The reactions of acyl chlorides follow which mechanism?
A nucleophile adds to the carbonyl carbon; the C=O reforms and Cl⁻ leaves as HCl.
Practice
The order of ease of hydrolysis is:
Acyl chlorides hydrolyse violently, alkyl slowly, and aryl chlorides not at all (ring-strengthened C–Cl).
You've got it
Key idea
- acyl chlorides are very reactive; with water → acid, alcohol/phenol → ester, ammonia/amine → amide (all + HCl)
- mechanism = addition–elimination (nucleophile adds to the carbonyl carbon, then HCl is eliminated)
- hydrolysis ease: acyl ≫ alkyl ≫ aryl chloride