Aldol condensation: practice and mechanism
On heating the aldol loses water: an α,β-unsaturated aldehyde forms (C=C conjugated with C=O).
Example
As in the aldol addition, the β-hydroxy aldehyde forms first. On heating it loses water (E1cB), giving an α,β-unsaturated aldehyde whose C=C is conjugated with the C=O.
Mechanism step by step
- 1
First the aldol addition to the β-hydroxy aldehyde takes place.
- 2
The base forms an enolate at the α carbon again.
- 3
The enolate expels hydroxide (E1cB): α,β-unsaturated aldehyde, driven by conjugation of C=C and C=O.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Background
Addition and condensation
The aldol reaction first gives a β-hydroxy carbonyl (the aldol). On heating, with base or acid, water is lost and an α,β-unsaturated carbonyl compound forms. This second stage is the aldol condensation; under basic conditions the elimination runs E1cB via the enolate.
Why losing water is so easy
The new C=C double bond is conjugated with the C=O group, often also with an aromatic ring. That conjugation stabilises the product and drives the reaction.
Crossed aldol condensation
Two different carbonyl compounds easily give mixtures. It becomes clean when one partner has no α-H and can only be attacked, for example benzaldehyde with acetone (Claisen–Schmidt condensation to benzalacetone).
Typical mistakes
Often confused with: Aldol addition
Frequently asked questions
What does E1cB mean?
Elimination via the conjugate base: the anion (enolate) forms first, then the leaving group departs.
Why does OH⁻ leave here although it is a poor leaving group?
Because the enolate is high in energy and the conjugated product very stable; heat helps as well.
What is the difference between aldol reaction and aldol condensation?
The aldol reaction (aldol addition) stops at the β-hydroxy carbonyl. In the aldol condensation water is also lost, usually on heating, giving an α,β-unsaturated carbonyl compound.