Acylating agents
Acylation introduces an acyl group into another molecule. Acyl chlorides and acid anhydrides are reactive carboxylic-acid derivatives that can be used as acylating agents.
Acyl chloride
The group of the corresponding carboxylic acid is replaced by chlorine.
Ethanoyl chloride
General structure:
Acid anhydride
An oxygen atom joins two acyl groups.
Ethanoic anhydride
A symmetrical acid anhydride may be represented as .
Naming
| Derivative | Change from carboxylic-acid name | Example |
|---|---|---|
| Acyl chloride | Replace -oic acid with -oyl chloride | ethanoic acid → ethanoyl chloride |
| Acid anhydride | Replace -oic acid with -oic anhydride | ethanoic acid → ethanoic anhydride |
Why they are reactive
Acyl chlorides and acid anhydrides react more readily than carboxylic acids and esters because the groups displaced from the acyl carbon are effective leaving groups. Chloride leaves from an acyl chloride, while a carboxylate-containing group leaves from an acid anhydride.
The two derivatives therefore undergo similar changes in functional group. Their by-products differ: reactions of acyl chlorides commonly lead to hydrogen chloride, whereas corresponding acid-anhydride reactions produce a carboxylic acid or a carboxylate-containing salt.