A LITTLE LIGHTBULB MOMENT
Carboxylic acid derivatives, enolates and amines
Anhydrides leave as carboxylates
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Anhydrides split into two acyl pieces.
The idea behind the cue
Anhydrides are reactive acyl donors because nucleophilic attack can expel a carboxylate leaving group. That leaving group is relatively stable, so substitution occurs readily compared with esters and amides.
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Short & simple
Anhydrides split into two acyl pieces.
Acronym
AC — Airborne Cats.
Story
Two chefs hold a shared lasagna tray. A nucleophile bumps one side, and the other side walks off with the tray handle, grumbling as a carboxylate.
Connect the pieces
- A (Airborne) → Anhydride: Anhydrides are reactive acyl donors.
- C (Cats) → Carboxylate: They leave as a relatively stable carboxylate, so substitution happens readily.
- Airborne · Anhydride: Anhydrides are reactive acyl donors.
- Cats · Carboxylate: They leave as a relatively stable carboxylate, so substitution happens readily.
Know the limits
Mixed and cyclic anhydrides can be used in different ways, but both still undergo nucleophilic acyl substitution by carboxylate expulsion.
Further reading · OpenStax Organic Chemistry ↗Check your understanding
What is the usual leaving group in anhydride substitution?
A. Hydride
B. Carboxylate
C. Alkoxide
D. Amide
Reveal explanation
Carboxylate
Anhydrides commonly expel a carboxylate group when attacked by a nucleophile.