Química Orgánica 2 · Condensación de Claisen y adición conjugada
Propanoate and acetate by retro-Claisen cleavage of ethyl 2-methyl-3-oxobutanoate
Condiciones: NaOH (conc.), H2O, Δ
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Arrastra desde los electrones que se mueven hasta donde van, o toca uno y luego el otro. Cada flecha se revisa; Pista te ayuda cuando te atascas. No necesitas cuenta.
El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
Leer los 8 pasos escritos
Hydroxide adds to the ketone
Concentrated hydroxide attacks the ketone carbonyl carbon, giving a tetrahedral alkoxide. In dilute base the β-keto ester would mostly be deprotonated instead.
Las flechas curvas
- A hydroxide lone pair forms the C–O bond to the ketone carbon.
- The C=O π pair moves onto oxygen.
Break the C–C bond
The alkoxide re-forms C=O and pushes out the ester-stabilized carbon; water delivers a proton to that carbon as the bond breaks, giving acetic acid and ethyl propanoate.
Las flechas curvas
- An oxygen lone pair re-forms the C=O bond.
- The C–C bond pair goes to the leaving carbon, which takes a proton from water.
- The O–H pair of water stays on oxygen as hydroxide.
Why can a C–C bond break?
Retro-Claisen driving force.
Deprotonate acetic acid
Hydroxide removes the acidic O–H proton of acetic acid, giving acetate; this irreversible step pulls the cleavage forward.
Las flechas curvas
- Hydroxide's lone pair takes the carboxylic acid proton.
- The O–H pair stays on oxygen, giving acetate.
Hydroxide adds to the ester
A second hydroxide attacks the ester carbonyl of ethyl propanoate, beginning its saponification.
Las flechas curvas
- A hydroxide lone pair bonds to the ester carbonyl carbon.
- The C=O π pair moves onto oxygen.
Expel ethoxide
The alkoxide re-forms the C=O bond and ethoxide leaves, giving propanoic acid.
Las flechas curvas
- An oxygen lone pair re-forms the C=O bond.
- The C–O bond breaks and ethoxide leaves.
Deprotonate propanoic acid
Ethoxide removes the carboxylic acid proton, giving propanoate and ethanol; this step makes saponification irreversible.
Las flechas curvas
- Ethoxide's lone pair takes the acid proton.
- The O–H pair stays on oxygen, giving propanoate.
Propanoate and acetate
The β-keto ester is cleaved into propanoate and acetate, with ethanol and water. Concentrated base attacks the ketone (acid cleavage) rather than only deprotonating the α-carbon.
Qué vas a practicar
Draw the retro-Claisen cleavage of ethyl 2-methyl-3-oxobutanoate by concentrated hydroxide and the saponification that gives propanoate and acetate.
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