Química Orgânica 2 · Adição e condensação aldólica
Intramolecular aldol condensation of 5-oxohexanal
Condições: 5-Oxohexanal, NaOH (0.2 equiv), H2O/EtOH, reflux
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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 methyl-ketone enolate
Hydroxide removes a proton from the methyl group of the ketone, giving the enolate whose carbon end will attack the aldehyde.
As setas curvas
- A hydroxide lone pair takes a methyl hydrogen.
- The C–H pair forms the enolate C=C π bond.
- The ketone π pair moves onto oxygen.
Which enolate closes the ring?
Counting from the enolate carbon to the aldehyde carbon decides the ring size; only the methyl-ketone enolate gives a six-membered ring.
Ring closure onto the aldehyde
The enolate carbon attacks the aldehyde carbonyl carbon at the other end of the chain, forming a six-membered ring and an alkoxide.
As setas curvas
- The oxygen lone pair reforms the ketone C=O.
- The enolate π pair forms the ring C–C bond to the aldehyde carbon.
- The aldehyde π pair moves onto oxygen.
Water protonates the alkoxide
Water gives a proton to the alkoxide, forming 3-hydroxycyclohexan-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 between the carbonyl and the alcohol
On heating, hydroxide removes a proton from the ring CH2 between the ketone and the CHOH carbon, giving an enolate with the hydroxyl on the β-carbon.
As setas curvas
- A hydroxide lone pair takes the α-hydrogen.
- The C–H pair forms the enolate C=C π bond.
- The ketone π pair moves onto oxygen.
E1cB loss of hydroxide
The enolate re-forms the C=O bond and pushes out hydroxide from the β-carbon, creating the C=C bond conjugated with the ketone. Conjugation makes this dehydration favourable.
As setas curvas
- The oxygen lone pair reforms the ketone C=O.
- The C=C π pair shifts to form the new ring C=C bond.
- The C–O bond breaks and hydroxide leaves.
Cyclohex-2-en-1-one
The six-membered ring now carries a ketone conjugated with a ring C=C bond, and hydroxide is regenerated.
O que você vai praticar
Draw the base-catalysed intramolecular aldol addition of 5-oxohexanal that closes a six-membered ring, then the E1cB dehydration that gives cyclohex-2-en-1-one.
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