Química Orgánica 2 · Enolatos: alquilación, condensaciones y adiciones conjugadas
3-Methylcyclohex-2-en-1-one by intramolecular aldol condensation of heptane-2,6-dione
Condiciones: NaOH, H2O, 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
Form the enolate at a terminal methyl
Hydroxide removes an α-hydrogen from one terminal methyl group. Of the possible enolates, only this one can reach the other carbonyl to make a strain-free six-membered ring.
Las flechas curvas
- Hydroxide takes the α-hydrogen.
- The C–H pair stays on the α-carbon, stabilized by the adjacent carbonyl.
Which enolate closes the ring
Which enolate closes the ring.
Ring closure
The enolate carbon attacks the other ketone carbonyl, forming the six-membered ring; the C=O π pair moves onto oxygen.
Las flechas curvas
- The carbanion lone pair forms the new ring C–C bond.
- The C=O π pair moves onto oxygen.
Protonate the alkoxide
Water protonates the alkoxide, giving the β-hydroxy ketone (the aldol) and regenerating hydroxide.
Las flechas curvas
- The alkoxide takes a proton from water.
- The O–H pair stays on oxygen, regenerating hydroxide.
Deprotonate again
On heating, hydroxide removes an α-hydrogen from the ring CH2 that lies between the ketone and the hydroxyl-bearing carbon.
Las flechas curvas
- Hydroxide takes the α-hydrogen.
- The C–H pair stays on carbon as an enolate-type carbanion.
E1cB loss of hydroxide
The carbanion pushes out the neighbouring hydroxide, forming the C=C bond conjugated with the ketone; conjugation drives this dehydration.
Las flechas curvas
- The carbanion lone pair forms the C=C π bond.
- The C–O bond breaks and hydroxide leaves.
3-Methylcyclohex-2-en-1-one
3-Methylcyclohex-2-en-1-one is the product: the former terminal methyl carbon is now the ring CH of the enone and the other ketone carbon carries the methyl group. Hydroxide is regenerated (catalytic base).
Qué vas a practicar
Draw the hydroxide-promoted ring closure of heptane-2,6-dione through its terminal-methyl enolate and the E1cB dehydration that gives 3-methylcyclohex-2-en-1-one.
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