Química Orgânica 2 · Condensação de Claisen e adição conjugada
Butan-2-one by ketonic cleavage of ethyl 2-methyl-3-oxobutanoate
Condições: H3O⁺, Δ
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O mecanismo, passo a passo
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Protonate the carbonyl of the β-keto ester
Hydronium protonates the carbonyl oxygen of the β-keto ester, making its carbon much more electrophilic toward weakly nucleophilic water.
As setas curvas
- A carbonyl oxygen lone pair takes a proton from hydronium.
- The O–H pair returns to oxygen, releasing water.
Why heat in acid?
Plan of the ketonic cleavage.
Water adds to the carbonyl carbon
Water attacks the activated carbonyl carbon, giving a tetrahedral intermediate with a positively charged oxygen.
As setas curvas
- A water lone pair forms the new C–O bond.
- The C=O π pair moves onto the protonated oxygen, neutralizing it.
Deprotonate the added water
Water removes a proton from the oxonium oxygen, giving a neutral tetrahedral intermediate.
As setas curvas
- Water's lone pair takes the oxonium proton.
- The O–H pair stays on oxygen.
Protonate the ethoxy oxygen
Hydronium protonates the ethoxy oxygen, turning it into a good leaving group (neutral ethanol).
As setas curvas
- The ethoxy oxygen lone pair takes a proton from hydronium.
- The O–H pair returns to oxygen as water.
Expel ethanol
An OH lone pair re-forms the C=O bond and pushes out ethanol.
As setas curvas
- An oxygen lone pair forms the C=O π bond.
- The C–O bond breaks and ethanol leaves.
Deprotonate to the carboxylic acid
Water removes the proton from the protonated carbonyl, giving the neutral carboxylic acid and regenerating hydronium.
As setas curvas
- Water's lone pair takes the proton.
- The O–H pair stays on oxygen, giving C=O.
Decarboxylate through a cyclic transition state
On heating, the β-carbonyl oxygen takes the carboxyl proton while CO2 leaves, all in one six-membered cyclic step. This gives carbon dioxide and an enol.
As setas curvas
- The C=O π pair of the β-carbonyl bonds to the carboxyl hydrogen.
- The O–H pair becomes the second C=O of carbon dioxide.
- The C–C bond pair becomes the C=C π bond of the enol.
Protonate the enol on carbon
Hydronium protonates the enol at its α-carbon, giving a protonated carbonyl.
As setas curvas
- The enol oxygen lone pair forms a C=O π bond as the C=C π pair leaves.
- The enol C=C π pair bonds to a proton from hydronium.
- The O–H pair returns to oxygen as water.
Deprotonate oxygen
Water removes the O–H proton, completing the tautomerization to the ketone.
As setas curvas
- Water's lone pair takes the O–H proton.
- The O–H pair stays on oxygen, forming the C=O bond.
Butan-2-one
The product is butan-2-one (ethyl methyl ketone): the ester carbon has been lost as CO2, leaving a ketone with the α-methyl group from the β-keto ester. Ethanol is the other by-product.
O que você vai praticar
Draw the acid-catalysed hydrolysis of ethyl 2-methyl-3-oxobutanoate and the decarboxylation and tautomerization that give butan-2-one.
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