Química Orgânica 2 · Condensação de Claisen e adição conjugada
Lose CO₂ from a β-keto acid
Condições: 2-Benzyl-3-oxobutanoic acid, the free acid obtained after ester hydrolysis in an acetoacetic ester synthesis, heated in dilute aqueous acid. Follow loss of CO₂ through the internally hydrogen-bonded arrangement, then acid-catalyzed tautomerization to 4-phenylbutan-2-one. One hydronium ion is explicit; its counterion is omitted.
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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 4 passos escritos
Release carbon dioxide
The acid proton is held near the ketone oxygen. Six atoms form a ring: the ketone oxygen, the ketone carbon, the α carbon, the carboxyl carbon, the hydroxyl oxygen and its hydrogen. Three electron pairs move at once around this ring. The ketone oxygen is essential: it accepts the pair that would otherwise be left on the α carbon as a carbanion.
As setas curvas
- The ketone π pair forms a bond between the ketone oxygen and the acid proton.
- The acid O–H pair forms a second C=O π bond at the carboxyl carbon, completing CO₂.
- The σ pair joining the α carbon to the carboxyl carbon forms a π bond between the α carbon and the ketone carbonyl carbon, giving an enol.
Protonate the enol at carbon
Hydronium (pKa approximately −1.7) delivers a proton to the enol carbon while the oxygen lone pair builds the C=O bond, giving a protonated ketone.
As setas curvas
- The enol oxygen lone pair forms a C=O π bond.
- The enol π pair bonds the carbon not bearing oxygen to a hydronium proton.
- The hydronium O–H pair stays on oxygen, giving water.
Release the oxygen proton
A protonated ketone (pKa approximately −7) is a much stronger acid than hydronium, so water removes the proton easily and the acid catalyst is regenerated.
As setas curvas
- The water formed in the previous step accepts the proton from the protonated ketone oxygen.
- The O–H pair stays on the ketone oxygen, leaving a neutral ketone.
Ketone, carbon dioxide and recovered acid
The carboxyl carbon and its two oxygens formed CO₂. The α carbon gained a proton from hydronium, and the ketone carbonyl was restored: 4-phenylbutan-2-one. The original carboxylic acid proton ends on the regenerated hydronium ion. An ordinary carboxylic acid lacks the β carbonyl that accepts the electrons here, which is why it does not lose CO₂ under the same mild heating.
O que você vai praticar
Draw the cyclic six-atom decarboxylation of a β-keto acid and finish the acid-catalyzed conversion of the enol to a ketone.
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