Enolate Chemistry
High-Yield Summary
- Keto-enol tautomerization: carbonyl compounds exist in rapid equilibrium between the keto form (C=O, more stable/abundant) and the enol form (C=C-OH) — constitutional isomers differing by one proton and one double bond position.
- Enol = neutral C=C-OH tautomer (transient intermediate). Enolate = its conjugate base, formed by base removing the α-hydrogen — a highly reactive nucleophile that drives most carbonyl C-C bond formation.
- Michael addition (conjugate addition): enolate's nucleophilic α-carbon attacks the β-carbon of an α,β-unsaturated carbonyl compound (conjugation makes that β-carbon electrophilic) — a 1,4-addition.
- Kinetic enolate: deprotonation at the less-substituted α-position, fastest, favored by low temperature + bulky strong base (LDA). Thermodynamic enolate: more-substituted α-position, more stable double bond, favored by higher temperature + weaker base + reversible conditions.
- Enamines are the nitrogen analog of enols. Synthetically useful enamines form directly from ketone/aldehyde + secondary amine (no isolable imine, since the secondary amine has no N-H to lose) — used as base-sensitive equivalents of enolates.
Kinetic vs. Thermodynamic Enolate
| Enolate | Conditions that favor it |
|---|---|
| Kinetic (less-substituted α-position, forms fastest) | Low temperature, bulky strong base (LDA) |
| Thermodynamic (more-substituted α-position, more stable C=C) | Higher temperature, weaker base, reversible/equilibrating conditions |
Michael Addition (Conjugate Addition)
- 1An enolate forms (base removes an α-hydrogen from a carbonyl compound).
- 2A separate molecule presents an α,β-unsaturated carbonyl — conjugation pulls electron density away from the β-carbon, making it electrophilic.
- 3The enolate's nucleophilic α-carbon attacks that β-carbon.
- 4The double bond's electrons shift to accommodate the new bond, delivering the enolate to the β-position and forming a new C-C bond (1,4-addition).
Key Terms
- Tautomers
- Constitutional isomers that rapidly interconvert, differing only in the position of one proton and one double bond (e.g. keto vs. enol form).
- Enol vs. enolate
- Enol = neutral C=C-OH tautomer. Enolate = its conjugate base (deprotonated at the α-carbon); enolate is the far more reactive nucleophile.
- α,β-unsaturated carbonyl
- A carbonyl group directly conjugated to a C=C double bond; the conjugation makes the β-carbon electrophilic, the target of Michael addition.
- Enamine
- Nitrogen analog of an enol (C=C-NR₂); synthetically important enamines form directly from a ketone/aldehyde + secondary amine, used as base-sensitive enolate equivalents.
Common MCAT Trap
- The keto form dominates at equilibrium, but the enol/enolate is usually the species that actually reacts — don't assume the minor tautomer is unimportant.
- In a Michael addition, the nucleophile attacks the β-carbon, not the carbonyl carbon itself — this is what distinguishes conjugate (1,4) addition from direct (1,2) carbonyl addition.
- Kinetic vs. thermodynamic enolate is decided by which α-hydrogen is removed (less- vs. more-substituted position), not by the base's identity alone — conditions (temperature, reversibility) matter just as much as the base.
Quick Recall
What structurally distinguishes an enol from its keto tautomer?
In a Michael addition, which carbon does the enolate attack, and why is it electrophilic?
What conditions favor the kinetic enolate over the thermodynamic enolate?
Why do synthetically useful enamines form without passing through an isolable imine?