SolutionsAldol Condensation
1. Self-condensations
Draw the products of the aldol condensation of the following compounds with themselves: acetophenone, cyclohexanone and phenylacetaldehyde.
- Acetophenone: the aldol 3-hydroxy-1,3-diphenyl-1-butanone, which dehydrates to 1,3-diphenyl-2-buten-1-one.
- Cyclohexanone: 2-(1-hydroxycyclohexyl)cyclohexanone and, after dehydration, 2-cyclohexylidenecyclohexanone.
- Phenylacetaldehyde: 3-hydroxy-2,4-diphenylbutanal and, after dehydration, 2,4-diphenyl-2-butenal, conjugated with the carbonyl and with a ring.
In all three, the α carbon of one molecule (as an enolate) attacks the carbonyl of another. Dehydration gives an α,β-unsaturated carbonyl compound and shifts the equilibria.
2. Crossed aldol condensation
What is obtained when a mixture of benzaldehyde and acetone is treated with NaOH in ethanol? And with excess benzaldehyde?
Benzaldehyde has no α hydrogens and acts only as the electrophile; acetone forms the enolate. 4-Phenyl-3-buten-2-one (benzylideneacetone) is obtained. With excess benzaldehyde both methyl groups of acetone react and 1,5-diphenyl-1,4-pentadien-3-one (dibenzylideneacetone) forms.
3. Intramolecular condensation
Which product is obtained when 2,5-hexanedione is treated with base? And 2,6-heptanedione?
2,5-Hexanedione gives 3-methyl-2-cyclopentenone and 2,6-heptanedione gives 3-methyl-2-cyclohexenone. The enolate of one methyl group attacks the other carbonyl and closes five- or six-membered rings, which form easily.