Química Orgánica 2 · Condensación de Claisen y adición conjugada
Heptan-2-one by acetoacetic ester synthesis
Condiciones: NaOEt, EtOH; then H3O⁺, Δ
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El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
Leer los 13 pasos escritos
Form the acetoacetate enolate
Ethoxide removes a hydrogen from the CH2 between the ketone and ester of ethyl acetoacetate (pKa ≈ 11); this deprotonation is essentially complete.
Las flechas curvas
- Ethoxide's lone pair takes the doubly activated hydrogen.
- The C–H pair stays on carbon, delocalized onto both carbonyls.
SN2 alkylation with 1-bromobutane
The enolate carbon attacks the primary carbon of 1-bromobutane from the back side, displacing bromide.
Las flechas curvas
- The enolate carbon's lone pair forms the C–C bond.
- The C–Br bond breaks and bromide leaves.
Why a primary halide?
SN2 scope.
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.
Las flechas curvas
- A carbonyl oxygen lone pair takes a proton from hydronium.
- The O–H pair returns to oxygen, releasing water.
Water adds to the carbonyl carbon
Water attacks the activated carbonyl carbon, giving a tetrahedral intermediate with a positively charged oxygen.
Las flechas 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.
Las flechas 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).
Las flechas 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.
Las flechas 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.
Las flechas 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.
Las flechas 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.
Las flechas 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.
Las flechas curvas
- Water's lone pair takes the O–H proton.
- The O–H pair stays on oxygen, forming the C=O bond.
Heptan-2-one
The product is heptan-2-one, a methyl ketone: the acetoacetate supplied CH3CO–CH2– and 1-bromobutane supplied the butyl group. Carbon dioxide, ethanol and bromide are by-products.
Qué vas a practicar
Draw the full acetoacetic ester synthesis of heptan-2-one: enolate alkylation with 1-bromobutane, acid hydrolysis of the ester, decarboxylation and tautomerization.
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