Química Orgánica 2 · Adición y condensación aldólica
(E,E)-Dibenzalacetone: the second condensation
Condiciones: NaOH, EtOH/H2O, benzaldehyde (2 equiv)
Dibuja este problema tú mismo
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 methyl enolate
With two equivalents of benzaldehyde, the benzalacetone formed first still has a methyl group α to the carbonyl. Hydroxide removes one of its hydrogens; the vinylic hydrogens cannot form an enolate.
Las flechas curvas
- Hydroxide takes a methyl α-hydrogen.
- The C–H pair forms the enolate C=C bond.
- The C=O π pair moves onto oxygen.
Addition to the second benzaldehyde
The enolate carbon attacks the second benzaldehyde, forming a 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.
Protonation of the alkoxide
Water protonates the alkoxide, giving the β-hydroxy ketone and regenerating hydroxide.
Las flechas curvas
- The alkoxide takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
Second enolization
Hydroxide removes the remaining hydrogen on the carbon between the carbonyl and the carbinol carbon, forming the enolate that will expel hydroxide.
Las flechas curvas
- Hydroxide takes the α-hydrogen.
- The C–H pair forms the enolate C=C bond.
- The C=O π pair moves onto oxygen.
Why stop at two?
Each condensation consumes the two α-hydrogens on one side of the carbonyl.
E1cB loss of hydroxide
The enolate re-forms C=O and expels hydroxide from the benzylic carbon. The new C=C completes a fully conjugated cross-conjugated dienone, and the E geometry is favoured on both sides.
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,E)-Dibenzalacetone
(E,E)-Dibenzalacetone (1,5-diphenylpenta-1,4-dien-3-one) precipitates as a yellow solid; hydroxide is regenerated.
Qué vas a practicar
Draw the second Claisen–Schmidt condensation: the methyl enolate of (E)-benzalacetone adds to a second benzaldehyde and loses hydroxide by E1cB to give (E,E)-dibenzalacetone.
Acabas de dibujar cada flecha de este problema.
Cada uno de los 5349 problemas de Orgánica 1 y 2 funciona así: comentarios en cada flecha, pistas cuando te atascas y un modo examen sin pistas. Gratis con una cuenta, sin tarjeta.
¿Atascado con tu propia tarea?
Tómale una foto: el primer escaneo es gratis, sin registrarte
¿Escaneas cada semana? Scan Standard: por unos problemas