Química Orgánica 2 · Adición y condensación aldólica
3-Hydroxy-2-methylpentanal from propanal
Condiciones: NaOH (cat.), H2O, 0 °C
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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 5 pasos escritos
Hydroxide forms the enolate
Hydroxide removes a hydrogen from the α-carbon (C2) of propanal. Only a small fraction of propanal is deprotonated (pKa about 17 vs 15.7 for water), so plenty of unreacted aldehyde remains to act as the electrophile.
Las flechas curvas
- A hydroxide lone pair takes the α-hydrogen.
- The C–H pair becomes the C=C π bond of the enolate.
- The C=O π pair moves onto oxygen, which carries the negative charge.
Which atom is the nucleophile?
The enolate is an ambident nucleophile, but carbon–carbon bond formation through the α-carbon gives the stable aldol.
The enolate adds to a second propanal
The nucleophilic α-carbon attacks the carbonyl carbon of a second propanal molecule, forming the new C2–C3 bond of the product and an alkoxide.
Las flechas curvas
- The oxygen lone pair re-forms the aldehyde C=O.
- The enolate π pair forms the new C–C bond to the carbonyl carbon.
- The C=O π pair moves onto oxygen as an alkoxide.
Water protonates the alkoxide
The alkoxide takes a proton from water, giving the neutral β-hydroxy aldehyde and regenerating hydroxide, so the base is a catalyst.
Las flechas curvas
- The alkoxide lone pair takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
3-Hydroxy-2-methylpentanal
3-Hydroxy-2-methylpentanal forms with two new stereocentres; both faces of the planar enolate and carbonyl react, so it is a racemic mixture of diastereomers. At 0 °C it does not dehydrate.
Qué vas a practicar
Draw the hydroxide-catalysed self-aldol addition of propanal: enolate formation, attack on a second propanal and protonation to 3-hydroxy-2-methylpentanal.
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