Aldol addition: practice and mechanism
The enolate of one aldehyde attacks the C=O of a second one: a β-hydroxy aldehyde forms; in the cold no water is lost.
Example
The base turns one aldehyde into an enolate (at the α carbon), which attacks the carbonyl group of a second aldehyde molecule. A β-hydroxy aldehyde (aldol) forms.
Mechanism step by step
- 1
Hydroxide removes an α-H (next to the C=O group), giving a resonance-stabilised enolate.
- 2
The α carbon of the enolate attacks the carbonyl carbon of a second aldehyde: a new C–C bond.
- 3
Water protonates the alkoxide: β-hydroxy aldehyde (aldol), with hydroxide regenerated.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: Aldol condensation · Reduction with sodium borohydride
Frequently asked questions
Why doesn't benzaldehyde react with itself?
Benzaldehyde has no α-H and cannot form an enolate; it only works as the electrophile (crossed aldol reaction).
What does β-hydroxy mean?
The OH group is on the second carbon after the carbonyl group (α is the first).