Reactivity Principles
High-Yield Summary
- General reactivity order toward nucleophilic acyl substitution: amides (least reactive) < esters ≈ carboxylic acids (moderate) < anhydrides (most reactive).
- Four structural factors drive this order: steric hindrance, induction, conjugation, and ring strain — all act by changing how easily a nucleophile can reach and attack the carbonyl carbon.
- Steric hindrance: bulky groups near the carbonyl carbon physically block nucleophilic approach (a spatial, not electronic, effect) — exploited deliberately via protecting groups in synthesis.
- Induction: electronegative atoms near the carbonyl pull electron density away through sigma bonds, raising electrophilicity. Anhydrides' bridging O + second carbonyl withdraw density (highly reactive); amides' N donates density via resonance instead (less reactive).
- Ring strain: small cyclic derivatives (e.g., β-lactams) carry angular strain that raises energy and drives faster nucleophilic attack than open-chain counterparts.
Induction: Anhydrides vs. Amides
| Derivative | Inductive effect on carbonyl carbon |
|---|---|
| Anhydrides | Bridging O + adjacent 2nd carbonyl pull electron density away → more electrophilic → more reactive |
| Amides | N lone pair donates density into carbonyl via resonance → less electrophilic → less reactive |
Key Terms
- Steric hindrance
- Physical crowding of the carbonyl carbon by bulky nearby groups, blocking nucleophilic approach — a spatial effect.
- Induction
- Electronic pull of electron density through sigma bonds due to electronegativity differences between bonded atoms.
- Conjugation
- Alternating single/double bonds allowing electron delocalization; can either reduce electrophilicity (ground-state spreading) or enhance reactivity (transition-state stabilization).
- Ring strain
- Elevated energy in small rings from bond angles forced away from ideal values; drives faster ring-opening nucleophilic attack (e.g., β-lactams).
- Protecting group
- A bulky substituent deliberately installed near a reactive site to sterically shield it from unwanted nucleophilic attack.
Common MCAT Trap
- Esters are grouped with carboxylic acids as "moderately reactive" — don't rank esters between amides and anhydrides as a distinct middle tier without remembering they're tied with carboxylic acids specifically.
- Conjugation's effect isn't one-directional — it can either dampen reactivity (spreading electron density lowers ground-state electrophilicity) or boost it (stabilizing the transition state, lowering activation energy). Don't assume conjugation always means "less reactive."
- Amide nitrogen doesn't withdraw electron density like anhydride oxygen does — it donates via resonance. This is the opposite direction of anhydride's inductive effect, and is exactly why amides are least reactive despite N and O being in similar positions.
Quick Recall
Rank amides, esters, and anhydrides by reactivity toward nucleophilic acyl substitution.
Why are amides so much less reactive than anhydrides, even though both have an atom adjacent to the carbonyl?
Why do β-lactams react so readily with nucleophiles despite being amides (the least reactive derivative class)?