Química Orgânica 2 · Condensação de Claisen e adição conjugada
Crossed Claisen of acetophenone with methyl propanoate
Condições: Acetophenone, methyl propanoate (2 equiv), NaOMe (1.1 equiv), MeOH/THF, reflux; then aq. HCl workup
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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
Methoxide forms the ketone enolate
Methoxide removes a proton from the methyl group next to the ketone carbonyl. The C–H pair becomes a C=C π bond and the C=O π pair moves onto oxygen, giving the enolate.
As setas curvas
- A methoxide lone pair takes an α-hydrogen.
- The C–H pair forms the C=C π bond of the enolate.
- The carbonyl π pair moves onto oxygen.
Which partner becomes the enolate?
The ketone α-hydrogens (pKa about 19) are more acidic than the ester α-hydrogens (pKa about 25), so the acetophenone enolate is the nucleophile and the ester is the acylating agent.
The enolate adds to the ester carbonyl
The enolate carbon attacks the ester carbonyl carbon, forming the new C–C bond and a tetrahedral alkoxide intermediate.
As setas curvas
- The oxygen lone pair reforms the ketone C=O.
- The enolate π pair forms the C–C bond to the ester carbonyl carbon.
- The ester C=O π pair moves onto oxygen.
The tetrahedral intermediate expels methoxide
The alkoxide re-forms the carbonyl and methoxide leaves, completing the acyl substitution and giving the 1,3-diketone.
As setas curvas
- The oxygen lone pair reforms the C=O bond.
- The C–O bond breaks and methoxide leaves.
Methoxide deprotonates the 1,3-diketone
The CH2 between two carbonyls (pKa about 9) is far more acidic than methanol, so methoxide removes it irreversibly. This step pulls the whole sequence of equilibria toward product and is why a full equivalent of base is needed.
As setas curvas
- A methoxide lone pair takes the doubly activated proton.
- The C–H pair forms a C=C π bond toward the propanoyl carbonyl.
- The C=O π pair moves onto oxygen, giving the stabilised enolate.
Acid workup gives the neutral diketone
Hydronium protonates the stabilised enolate at the central carbon while the oxygen lone pair re-forms the C=O bond.
As setas curvas
- The oxygen lone pair re-forms the carbonyl.
- The C=C π pair takes a proton from hydronium at the central carbon.
- The O–H pair stays on water.
1-Phenylpentane-1,3-dione
The benzoyl and propanoyl groups now flank one CH2: 1-phenylpentane-1,3-dione.
O que você vai praticar
Draw the methoxide enolisation of acetophenone, the acyl substitution at methyl propanoate, the deprotonation that drives the reaction, and the acid workup that gives 1-phenylpentane-1,3-dione.
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