Química Orgânica 2 · Condensação de Claisen e adição conjugada
Ethyl 2-methyl-3-oxopropanoate by formylation of ethyl propanoate
Condições: NaOEt, EtOH; then H3O⁺ 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
Form the enolate
Ethoxide removes an α-hydrogen from ethyl propanoate. 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 ethyl formate, 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.
Why is the crossed Claisen clean?
Donor and acceptor roles.
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-methyl-3-oxopropanoate
The product is ethyl 2-methyl-3-oxopropanoate, an α-formyl ester: a CHO group has been installed on the α-carbon of ethyl propanoate. Ethanol and water are by-products.
O que você vai praticar
Draw the crossed Claisen condensation in which the ethyl propanoate enolate is formylated by ethyl formate, through product trapping and workup.
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