Organic Chemistry 2 · Aldol addition and condensation
(E)-Chalcone from acetophenone and benzaldehyde
Conditions: NaOH, EtOH, rt
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The mechanism, step by step
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Hydroxide forms the enolate of acetophenone
Hydroxide removes an α-hydrogen of acetophenone. Only acetophenone has α-hydrogens; benzaldehyde cannot form an enolate, so it can only be the electrophile.
The curved arrows
- Hydroxide takes an α-hydrogen.
- The C–H pair forms the enolate C=C bond.
- The C=O π pair moves onto oxygen.
Why does dehydration happen so easily?
Base-mediated dehydration of an aldol goes through the enolate, not through a carbocation.
The enolate attacks benzaldehyde
The enolate α-carbon adds to the aldehyde carbon of benzaldehyde, which is more electrophilic than the ketone carbonyl, forming the new C–C bond and an alkoxide.
The curved arrows
- The oxygen lone pair re-forms the C=O.
- The enolate π pair forms the new C–C bond.
- The aldehyde π pair moves onto oxygen.
Water protonates the alkoxide
The alkoxide takes a proton from water, giving the β-hydroxy ketone (aldol) and regenerating hydroxide.
The curved arrows
- The alkoxide takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
Hydroxide forms the enolate of the aldol
The remaining α-hydrogen, now flanked by the carbonyl and the benzylic carbinol carbon, is removed by hydroxide to give a new enolate.
The curved arrows
- Hydroxide takes the second α-hydrogen.
- The C–H pair forms the enolate C=C bond.
- The C=O π pair moves onto oxygen.
E1cB loss of hydroxide
The enolate re-forms C=O and expels hydroxide from the β-carbon (E1cB). The new C=C is conjugated with both the carbonyl and the benzene ring, which makes this dehydration fast and irreversible under the conditions; the E isomer forms because it keeps the aryl ring and the carbonyl apart.
The curved arrows
- The oxygen lone pair re-forms the C=O.
- The enolate π pair forms the new C=C bond.
- The C–O pair leaves as hydroxide.
(E)-chalcone
The (E)-chalcone forms as the E isomer, with hydroxide regenerated and one water released overall.
What you'll practice
Draw the base-catalysed condensation of acetophenone with benzaldehyde to (E)-chalcone, including the aldol addition, the second enolization and the E1cB loss of hydroxide.
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