Química Orgânica 2 · Enolatos: alquilação, condensações e adições conjugadas
Intramolecular aldol condensation of hexane-2,5-dione
Condições: Hexane-2,5-dione, aqueous NaOH (catalytic), heat
Desenhe este problema você mesmo
Arraste dos elétrons que se movem até onde eles vão, ou toque em um e depois no outro. Cada seta é verificada; Dica ajuda quando você travar. Não precisa de conta.
O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 7 passos escritos
Hydroxide forms the terminal enolate
Hydroxide removes a proton from one terminal methyl group. The molecule is symmetrical, so either methyl gives the same result.
As setas curvas
- A hydroxide lone pair takes a methyl hydrogen.
- The C–H pair forms the enolate C=C π bond.
- The C=O π pair moves onto oxygen.
Which enolate closes the ring?
Count the ring each possible enolate would close onto the other carbonyl carbon; the enolate carbon and the attacked carbon are both ring atoms.
Ring closure onto the other ketone
The enolate carbon attacks the carbonyl carbon at the other end of the chain, forming a five-membered ring and an alkoxide.
As setas curvas
- The oxygen lone pair re-forms the C=O bond.
- The enolate π pair forms the ring C–C bond.
- The attacked C=O π pair moves onto oxygen.
Water protonates the alkoxide
Water gives a proton to the tertiary alkoxide, giving 3-hydroxy-3-methylcyclopentan-1-one and regenerating hydroxide.
As setas curvas
- The alkoxide lone pair takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
Hydroxide forms the enolate next to the C–OH carbon
On heating, hydroxide removes a hydrogen from the ring CH₂ between the ketone and the carbon carrying the OH group.
As setas curvas
- A hydroxide lone pair takes the α-hydrogen.
- The C–H pair forms the enolate C=C π bond.
- The C=O π pair moves onto oxygen.
E1cB loss of hydroxide
The enolate re-forms the C=O bond and pushes out hydroxide, creating a ring C=C bond conjugated with the ketone.
As setas curvas
- The oxygen lone pair re-forms the C=O bond.
- The C=C π pair shifts to the bond toward the methyl-bearing carbon.
- The C–O bond breaks and hydroxide leaves.
3-Methylcyclopent-2-en-1-one
The five-membered ring carries a ketone conjugated with a C=C bond; the methyl group sits on the β-carbon of the enone.
O que você vai praticar
Draw the hydroxide-catalysed ring closure of hexane-2,5-dione onto its own second ketone and the E1cB dehydration that gives 3-methylcyclopent-2-en-1-one.
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