Overview
A carboxylic acid derivative is a compound with an acyl group (R-CO-) bonded to a leaving group instead of the OH of the parent carboxylic acid. The four common derivatives are:
- Acyl halide (acid halide): R-CO-X (usually Cl).
- Anhydride: R-CO-O-CO-R’.
- Ester: R-CO-OR’.
- Amide: R-CO-NR’R”.
What They Share
All four have:
- A carbonyl (C=O) with sp² carbon.
- A leaving group directly attached to the carbonyl carbon.
- The capacity to undergo nucleophilic acyl substitution (NAS): nucleophile attacks the carbonyl C, tetrahedral intermediate forms, leaving group departs, C=O reforms.
What Distinguishes Them
They differ in:
- The leaving group: chloride (acyl halide), carboxylate (anhydride), alkoxide (ester), amide nitrogen (amide).
- Reactivity: determined by how good the leaving group is AND how much the LG donates electron density into the carbonyl by resonance.
- Hydrolysis conditions: mild (acyl halides and anhydrides) to harsh (amides require strong acid/base + heat).
- Biological role: esters (fats, phospholipids, aspirin); amides (proteins, urea, nylon).
The Reactivity Order
Acyl halide > Anhydride > Ester > Amide
Acyl halides react fastest because chloride is an excellent leaving group AND does not donate much electron density into the carbonyl. Amides react slowest because amide nitrogen is a terrible leaving group AND donates its lone pair strongly into the carbonyl (resonance), reducing electrophilicity.
Naming the Derivatives
| Class | Suffix / prefix | Example |
|---|---|---|
| Acyl halide | -oyl halide | acetyl chloride (CH₃COCl) |
| Anhydride | … anhydride | acetic anhydride ((CH₃CO)₂O) |
| Ester | alkyl -oate | methyl acetate (CH₃COOCH₃) |
| Amide | -amide | acetamide (CH₃CONH₂) |
Why These Four Are Grouped Together
The unifying idea: all four can be interconverted via nucleophilic acyl substitution. You can convert an acyl halide to an anhydride, ester, or amide (going “down” the ladder). You can convert an amide to an ester with harsh conditions (going up). Understanding the reactivity order lets you predict which interconversions are favorable and which require activation.