Enolate chemistry — new C–C bond
Base-catalyzed aldol addition of acetaldehyde
How the Base-catalyzed aldol addition of acetaldehyde works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: cat. NaOH or KOH. Conditions: H₂O / EtOH, 0–25 °C. Bonds formed: C–C σ bond (α-C to C=O carbon); bonds broken: α C–H; C=O π. Stereochemistry: New stereocentre — racemic. Rate and selectivity: Heating promotes dehydration to the enal (aldol condensation).
At a glance
Step 1: Enolate formation
Hydroxide removes a mildly acidic proton from the carbon alpha to the carbonyl (acidic because the resulting negative charge is stabilized by the adjacent C=O). This forms a resonance-stabilized enolate.
Step 2: The enolate attacks a second aldehyde
The enolate's nucleophilic alpha-carbon attacks the electrophilic carbonyl carbon of a second acetaldehyde molecule. A new C–C bond forms while the C=O π pair shifts entirely onto oxygen.
Step 3: Protonation gives the aldol
Protonation of the resulting alkoxide gives the aldol product — a β-hydroxy aldehyde carrying a brand-new carbon–carbon bond, the key step behind building larger carbon skeletons.
Abbreviations and terms
- EtOH
- Ethanol. Polar protic solvent.
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
- KOH
- Potassium hydroxide. Strong aqueous/alcoholic base.
- pKa
- Acid-dissociation constant (–log). Lower pKa = stronger acid; used to judge which proton a base removes.
How it compares with related reactions
| Base-catalyzed aldol addition of acetaldehyde | Aldol condensation | Claisen condensation of ethyl acetate | |
|---|---|---|---|
| Reagents | cat. NaOH or KOH | NaOH or KOH (cat.) | Ethyl acetate (2 equiv), NaOEt (≥ 1 equiv); then H₃O⁺ |
| Conditions | H₂O / EtOH, 0–25 °C | H₂O/EtOH, heat 50–100 °C | EtOH, reflux; same alkoxide as ester OR group |
| Stereochemistry | New stereocentre — racemic | Mostly E-enal (thermodynamic) | None (achiral product) |
| Rate / selectivity | Heating promotes dehydration to the enal (aldol condensation) | E1cB; loss of HO⁻ from the enolate is rate-determining | Equilibrium pulled forward by deprotonating the β-keto ester (pKa ≈ 11) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Base-catalyzed aldol addition of acetaldehyde?
cat. NaOH or KOH. Conditions: H₂O / EtOH, 0–25 °C.
Which bonds are formed and which are broken?
Formed: C–C σ bond (α-C to C=O carbon). Broken: α C–H; C=O π.
What is the stereochemical outcome?
New stereocentre — racemic
What controls the rate or the selectivity?
Heating promotes dehydration to the enal (aldol condensation)
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go