Reactivity Ladder
The reactivity ladder acyl halide > anhydride > ester > amide is not arbitrary. Two factors combine to create the ranking:
- Leaving group ability of the attached group after nucleophile attack.
- Resonance donation of the attached group’s lone pair into the carbonyl in the starting material.
These two effects point in the same direction: groups that are good leaving groups (weak bases like Cl⁻) are also poor resonance donors; groups that are bad leaving groups (strong bases like NR₂⁻) are also strong resonance donors. This coincidence amplifies the reactivity differences across the ladder.
Factor 1: Leaving Group Ability
After nucleophile attack, the tetrahedral intermediate must eject its attached group for NAS to complete. The leaving group ability depends on the conjugate acid pKa:
| Derivative | Leaving group | Conj. acid pKa | LG ability |
|---|---|---|---|
| Acyl halide | Cl⁻ | −7 | Excellent |
| Anhydride | RCOO⁻ | 4-5 | Good |
| Ester | RO⁻ | 16-18 | Poor |
| Amide | R₂N⁻ | 38 | Terrible |
Weaker bases (higher conjugate acid acidity, more stable anions) are better LGs. Cl⁻ leaves effortlessly; R₂N⁻ essentially never leaves.
Factor 2: Resonance Donation
The attached group donates lone pair electrons into the carbonyl via resonance. The more donation, the more electron density on the carbonyl C, and the LESS electrophilic it is:
- Cl in acyl halide: Cl’s 3p lone pair overlaps poorly with C’s 2p. Weak donation. Carbonyl retains full δ⁺ character.
- OR in ester: O’s 2p lone pair donates moderately. Some reduction of δ⁺.
- NR₂ in amide: N’s 2p lone pair donates strongly (N is less electronegative than O). Substantial reduction of δ⁺.
Combine both factors:
- Acyl halide: Cl is a good LG AND a weak donor → carbonyl stays electrophilic AND tetrahedral intermediate collapses quickly. Very reactive.
- Anhydride: carboxylate is a decent LG AND a weak donor (the second C=O pulls electrons away). Quite reactive.
- Ester: alkoxide is a poor LG AND a moderate donor. Moderate reactivity.
- Amide: amide N is a terrible LG AND a strong donor. Very unreactive.
Implications for Interconversion
Going DOWN the ladder (more reactive → less reactive) is thermodynamically favorable:
- Acyl halide + alcohol → ester (go from Cl to OR; Cl- is more stable than OR-).
- Ester + amine → amide (go from OR to NR2; OR- is more stable than NR2-).
Going UP the ladder requires activation:
- Amide → ester requires harsh hydrolysis first (to give COOH), then Fischer esterification.
- Ester → anhydride is possible with DCC or similar activating reagents.
- Ester or acid → acyl halide requires SOCl₂ or PCl₃.
Never try to run the reverse of the ladder directly - you need to activate the starting material (usually via the carboxylic acid + SOCl₂ route).
A Unifying Rule
The rule for predicting whether an NAS interconversion will work:
If the new leaving group (the LG displaced by the nucleophile) is a WEAKER base than the incoming nucleophile, the reaction will work.
- Acyl halide + alcohol: Cl⁻ (weak base, pKa of HCl −7) leaves; RO⁻ comes in. Since RO⁻ is a stronger base than Cl⁻, Cl⁻ is the better LG and the reaction proceeds.
- Ester + hydroxide: OR⁻ leaves; OH⁻ comes in. OH⁻ and OR⁻ have similar basicity; the reaction is thermodynamically neutral unless other factors drive it (like the carboxylate stabilization in saponification).
- Amide + alcohol: NR₂⁻ would have to leave; RO⁻ would come in. NR₂⁻ is much more basic than RO⁻, so NR₂⁻ is the WORSE LG. The reaction does not work spontaneously.