Química Orgánica 2 · Adición y condensación aldólica
3-Methylcyclohex-2-en-1-one from heptane-2,6-dione
Condiciones: NaOH, H2O/EtOH, Δ
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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 terminal enolate
Hydroxide removes a hydrogen from a terminal methyl group. Enolates form reversibly at both the CH3 and the internal CH2 positions, but only the methyl enolate can close a strain-free ring.
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.
Which ring forms?
The enolate carbon and the other carbonyl carbon must reach each other.
Ring closure
The enolate carbon attacks the other ketone carbonyl in the same molecule, closing a six-membered ring and giving a tertiary alkoxide.
Las flechas curvas
- The oxygen lone pair re-forms the C=O.
- The enolate π pair forms the ring C–C bond.
- The ketone π pair moves onto oxygen.
Protonation of the alkoxide
Water protonates the alkoxide, giving the cyclic 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.
Enolization next to the alcohol
On heating, hydroxide removes the ring hydrogen between the ketone and the carbinol carbon.
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.
E1cB dehydration
The enolate re-forms the C=O and expels hydroxide from the β-carbon, giving the conjugated cyclohexenone.
Las flechas curvas
- The oxygen lone pair re-forms the C=O.
- The enolate π pair forms the ring C=C bond.
- The C–O pair leaves as hydroxide.
3-Methylcyclohex-2-en-1-one
3-Methylcyclohex-2-en-1-one forms; the conjugated enone and the strain-free six-membered ring make the overall reaction favourable.
Qué vas a practicar
Draw the intramolecular aldol condensation of heptane-2,6-dione: methyl enolate formation, six-membered ring closure, protonation and E1cB dehydration to 3-methylcyclohex-2-en-1-one.
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