Intramolecular aldol condensation: practice and mechanism
A diketone condenses with itself: the enolate on one side attacks the other carbonyl group. Low-strain five- or six-membered rings are favoured; loss of water gives a cyclic enone.
Example
If a molecule contains two carbonyl groups, the enolate of one attacks the other within the same molecule. The low-strain five- and six-membered rings are favoured; after loss of water a cyclic enone forms.
Mechanism step by step
- 1
NaOH forms an enolate at one of the α-carbons.
- 2
The enolate attacks the other carbonyl group in the same molecule; the least strained ring forms (five or six atoms).
- 3
On warming water is eliminated: a cyclic enone.
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 · Robinson annulation
Frequently asked questions
Which ring forms from hexane-2,5-dione?
The enolate at the methyl group attacks the other C=O: five-membered ring, 3-methylcyclopent-2-en-1-one. A three-membered ring would be too strained.
And from heptane-2,6-dione?
A six-membered ring: 3-methylcyclohex-2-en-1-one.