Four functional groups share one skeleton: a carbonyl carbon bonded to a heteroatom group that can leave. They differ enormously in reactivity — across a range of about 10¹³ — and the ordering is not arbitrary. It follows from one structural question asked of each: how strongly does the attached group donate into the carbonyl?
Four derivatives, one core structure
Every carboxylic acid derivative is RCO–X, where X is a group that can act as a leaving group: acid chloride (X = Cl), anhydride (X = OCOR), ester (X = OR), and amide (X = NR₂). Nitriles are usually treated alongside them, since they hydrolyze to the same acid.
Each X donates a lone pair into the carbonyl by resonance, exactly as the OH does in a carboxylic acid, and the degree of that donation sets the entire reactivity order.
The reactivity ladder
| Derivative | X | Resonance donation | X as leaving group | Reactivity |
|---|---|---|---|---|
| acid chloride | Cl | very poor | excellent | highest |
| anhydride | OCOR | poor (shared) | good | high |
| ester | OR | moderate | moderate | moderate |
| amide | NR₂ | strong | terrible | lowest |
Two rules produce this order and they reinforce each other.
Better resonance donation means a less electrophilic carbonyl. Nitrogen is the best donor of the four — least electronegative, holding its lone pair most loosely — so an amide's carbonyl is the least electron-poor. Chlorine is a poor donor: its lone pairs are held tightly, and its 3p orbital overlaps badly with carbon's 2p, a size mismatch. So an acid chloride's carbonyl is the most electrophilic.
Better leaving group means a faster collapse. Chloride is the conjugate base of HCl, pKa −7, and leaves readily. An amide nitrogen would have to leave as R₂N⁻, conjugate base of an amine at pKa 38, which it essentially never does. An anhydride's carboxylate (pKa 5) and an ester's alkoxide (pKa 16) sit in between.
Amide resonance is worth its own paragraph
Nitrogen's donation into the carbonyl is strong enough to have measurable structural consequences. The C–N bond in an amide has about 40% double-bond character, which means rotation about it is restricted — a barrier of roughly 18 kcal/mol, enough that the two methyls of dimethylformamide appear as separate NMR signals at room temperature. The whole amide unit is planar, and the nitrogen is not pyramidal as an amine's would be.
This has a consequence far outside organic chemistry. The peptide bond is an amide, and its planarity and restricted rotation are what limit a protein backbone to two rotatable angles per residue. Alpha helices and beta sheets exist because amide resonance takes a third degree of freedom away.
It also explains why amides are not basic. An amine's lone pair is available and it is a decent base (conjugate acid pKa ~10); an amide's lone pair is tied up in the carbonyl and it is essentially non-basic (conjugate acid pKa ~0). Same nitrogen, ten orders of magnitude apart, because of one resonance interaction.
Esters: common, mild, versatile
Esters form from a carboxylic acid and an alcohol under acid catalysis — Fischer esterification, an equilibrium that must be driven by excess alcohol or by removing water — or from a more reactive derivative such as an acid chloride, which is faster and irreversible.
They undergo acyl substitution with a range of nucleophiles. Hydrolysis under acid is reversible and returns the acid and the alcohol; under base it is saponification, and because the product carboxylate is deprotonated and unreactive, the reaction is irreversible and goes to completion. Saponification with a long-chain ester is literally soap-making, which is where the name comes from.
Esters are reduced to primary alcohols by LiAlH₄, but not by NaBH₄ — a selectivity difference worth remembering. Grignard reagents add twice, because the ketone formed after the first addition is more reactive than the ester was, giving a tertiary alcohol with two identical new R groups.
Amides: the least reactive, and biology's backbone
Amides are so unreactive that hydrolysis requires strong acid or base with prolonged heating. That stability is exactly why they are the linkage holding proteins together — a bond that must survive ordinary aqueous conditions for years while remaining selectively breakable by dedicated enzymes.
LiAlH₄ does reduce amides, and distinctively: the product is an amine rather than an alcohol, because the nitrogen stays attached throughout. That makes amide formation followed by reduction a standard two-step route to an amine, and it is taken up again in Module 12.
You have benzoic acid and need N-methylbenzamide.
Direct reaction of the acid with methylamine gives only an ammonium carboxylate salt — the amine is basic and simply takes the proton.
So go up first, then down: SOCl₂ converts the acid to benzoyl chloride, top of the ladder. Then methylamine, with a second equivalent (or pyridine) to mop up the HCl released, gives the amide cleanly.
Two steps, because the ladder only runs downhill and you had to climb back to the top before descending.
What carries forward
The ladder governs every conversion in the next section and is the single most tested idea in Module 10. Amide resonance recurs in Module 12 as the explanation for why amides are not basic, and in any discussion of protein structure. And the recognition that reactivity ordering here is really a leaving-group ranking connects this chapter straight back to Module 2.