Química Orgánica 2 · Adición y condensación aldólica
(E)-Benzalacetone from benzaldehyde and acetone
Condiciones: NaOH, EtOH, rt
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Arrastra desde los electrones que se mueven hasta donde van, o toca uno y luego el otro. Cada flecha se revisa; Pista te ayuda cuando te atascas. No necesitas cuenta.
El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
Leer los 7 pasos escritos
Hydroxide forms the enolate of acetone
Hydroxide removes an α-hydrogen of acetone. Only acetone has α-hydrogens; benzaldehyde cannot form an enolate, so it can only be the electrophile.
Las flechas curvas
- Hydroxide takes an α-hydrogen.
- The C–H pair forms the enolate C=C bond.
- The C=O π pair moves onto oxygen.
Who is the donor?
A crossed aldol is clean only when one partner cannot form an enolate.
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.
Las flechas curvas
- 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.
Las flechas curvas
- 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.
Las flechas curvas
- 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.
Las flechas curvas
- 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)-benzalacetone
The (E)-benzalacetone forms as the E isomer, with hydroxide regenerated and one water released overall.
Qué vas a practicar
Draw the Claisen–Schmidt condensation of acetone with benzaldehyde: enolate formation, aldol addition, protonation, second enolization and E1cB loss of hydroxide to (E)-benzalacetone.
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