Química Orgânica 2 · Condensação de Claisen e adição conjugada
Ethyl 2-ethyl-3-oxohexanoate by Claisen condensation of ethyl butanoate
Condições: NaOEt, EtOH; then H3O⁺
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
Form the enolate
Ethoxide removes an α-hydrogen from ethyl butanoate. The resulting carbanion is stabilized by delocalization onto the carbonyl oxygen (an enolate).
As setas curvas
- A lone pair on the ethoxide takes the α-hydrogen.
- The C–H bonding pair stays on the α-carbon, forming the enolate.
Enolate attacks the acceptor carbonyl
The nucleophilic α-carbon adds to the carbonyl carbon of a second molecule of ethyl butanoate, and the C=O π electrons move onto oxygen to give a tetrahedral alkoxide.
As setas curvas
- The enolate carbon's lone pair forms the new C–C bond.
- The C=O π pair moves onto oxygen, keeping the carbonyl carbon at an octet.
Where does the branch end up?
Connectivity check.
Collapse and expel ethoxide
The alkoxide re-forms the C=O bond and pushes out ethoxide, completing the acyl substitution and giving the β-dicarbonyl compound.
As setas curvas
- An oxygen lone pair re-forms the C=O π bond.
- The C–O bond breaks and ethoxide leaves with the bonding pair.
Deprotonate the β-dicarbonyl product
The hydrogen on the carbon between the two carbonyls (pKa ≈ 9–11) is far more acidic than ethanol, so ethoxide removes it. This favorable step consumes the base and drives the reversible condensation to completion.
As setas curvas
- The alkoxide lone pair takes the doubly activated hydrogen.
- The C–H pair stays on carbon, delocalized over both carbonyls.
Acidic workup
Hydronium, added in a separate workup, protonates the stabilized anion on carbon to give the neutral β-dicarbonyl product.
As setas curvas
- The carbanion lone pair bonds to a proton of hydronium.
- The O–H pair returns to oxygen, releasing water.
Ethyl 2-ethyl-3-oxohexanoate
The product is ethyl 2-ethyl-3-oxohexanoate: the α-carbon (C2) of one ethyl butanoate is now bonded to the carbonyl carbon of the other, so the ethyl branch sits between the ketone and the ester. Ethanol and water are by-products.
O que você vai praticar
Draw the self-Claisen condensation of ethyl butanoate and track which carbon ends up between the two carbonyls of ethyl 2-ethyl-3-oxohexanoate.
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