Química Orgânica 2 · Condensação de Claisen e adição conjugada
Ethyl acetoacetate from ethyl acetate (Claisen condensation)
Condições: NaOEt (1 equiv), EtOH; then H3O⁺
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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
Form the ester enolate
Ethoxide removes an α-hydrogen from ethyl acetate. The ester α C–H (pKa ≈ 25) is much less acidic than ethanol (pKa ≈ 16), so only a small amount of enolate is present at any moment.
As setas curvas
- A lone pair on ethoxide oxygen bonds to the α-hydrogen.
- The C–H pair stays on the α-carbon, giving the enolate carbanion stabilized by the carbonyl.
Enolate adds to a second ester
The nucleophilic α-carbon attacks the carbonyl carbon of a second ethyl acetate molecule. The C=O π electrons move onto oxygen, giving a tetrahedral alkoxide intermediate.
As setas curvas
- The enolate carbon's lone pair forms the new C–C bond to the ester carbonyl carbon.
- The C=O π pair moves onto oxygen so the carbonyl carbon keeps an octet.
Why does ethoxide leave?
Acyl substitution, not addition.
Collapse and lose ethoxide
The alkoxide oxygen re-forms the C=O bond and pushes out ethoxide. This gives the β-keto ester ethyl acetoacetate.
As setas curvas
- An oxygen lone pair re-forms the ketone C=O π bond.
- The C–O bond to the ethoxy group breaks and ethoxide leaves.
Deprotonate the β-keto ester
Ethoxide removes a hydrogen from the CH2 between the two carbonyls (pKa ≈ 11). This favorable step uses up the base and pulls the reversible earlier steps toward product, which is why a full equivalent of ethoxide is needed.
As setas curvas
- Ethoxide's lone pair takes the doubly activated hydrogen.
- The C–H pair stays on carbon, delocalized over both carbonyls.
Acidic workup
Hydronium, added after the reaction, protonates the stabilized anion on carbon to give the neutral β-keto ester.
As setas curvas
- The carbanion lone pair bonds to a proton of hydronium.
- The O–H pair returns to oxygen, releasing water.
Ethyl acetoacetate
The product is ethyl acetoacetate, a β-keto ester, with two ethanol molecules and water. One new C–C bond joins the α-carbon of one ester to the carbonyl carbon of the other.
O que você vai praticar
Draw every step of the self-Claisen condensation of ethyl acetate, including the deprotonation that drives it and the acidic workup, to give ethyl acetoacetate.
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