Química Orgánica 2 · Adición y condensación aldólica
2-Cyclohexylidenemalononitrile by Knoevenagel condensation
Condiciones: NH4OAc, AcOH, toluene, Δ (−H2O)
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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 8 pasos escritos
Acetate deprotonates malononitrile
Acetate, the base supplied by ammonium acetate, removes a CH2 hydrogen from malononitrile. The two nitrile groups make this C–H acidic (pKa about 11), so even this weak base forms the carbanion.
Las flechas curvas
- An acetate oxygen lone pair takes a CH2 hydrogen.
- The C–H pair stays on carbon as the dicyanomethyl anion.
Addition to cyclohexanone
The carbanion attacks the ketone carbonyl carbon, forming the new C–C bond and an alkoxide.
Las flechas curvas
- The carbanion lone pair forms the new C–C bond.
- The C=O π pair moves onto oxygen.
Acetic acid protonates the alkoxide
Acetic acid, the strongest acid in the buffer, protonates the alkoxide to give the tertiary alcohol.
Las flechas curvas
- The alkoxide takes the acetic acid proton.
- The O–H pair stays on oxygen as acetate.
Acetate removes the second C–H
Acetate removes the remaining hydrogen between the two nitriles, giving a carbanion next to the C–OH carbon.
Las flechas curvas
- Acetate takes the C–H proton.
- The C–H pair stays on carbon.
E1cB loss of hydroxide
The carbanion pushes its lone pair into a C=C bond and expels hydroxide from the neighbouring carbon (E1cB); the alkylidene is conjugated with both nitriles.
Las flechas curvas
- The carbanion lone pair forms the C=C bond.
- The C–O pair leaves as hydroxide.
Acetic acid neutralizes hydroxide
In the acetic acid buffer the hydroxide released is immediately protonated, giving water.
Las flechas curvas
- Hydroxide takes the acetic acid proton.
- The O–H pair stays on oxygen as acetate.
Why does this work on a ketone?
Ketones are poor aldol acceptors, but the Knoevenagel product is strongly favoured.
2-Cyclohexylidenemalononitrile
2-Cyclohexylidenemalononitrile forms with water; acetate and acetic acid are regenerated.
Qué vas a practicar
Draw the acetate-catalysed Knoevenagel condensation of malononitrile with cyclohexanone: deprotonation, addition to the ketone, protonation, and E1cB dehydration to 2-cyclohexylidenemalononitrile.
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