Química Orgânica 2 · Enolatos: alquilação, condensações e adições conjugadas
Claisen condensation of ethyl propanoate
Condições: Ethyl propanoate, NaOEt (1 equiv), EtOH, reflux; then dilute aqueous HCl
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Ethoxide forms a small amount of ester enolate
Ethoxide removes an α-hydrogen from ethyl propanoate. The ester (pKa ≈ 25) is far less acidic than ethanol (pKa ≈ 16), so very little enolate is present at any moment.
As setas curvas
- The ethoxide lone pair takes an α-hydrogen.
- The C–H pair forms the enolate C=C π bond.
- The C=O π pair moves onto oxygen.
The enolate adds to a second ester
The enolate carbon attacks the carbonyl carbon of another ethyl propanoate molecule, giving a tetrahedral alkoxide intermediate.
As setas curvas
- The oxygen lone pair re-forms the ester C=O.
- The enolate π pair forms the C–C bond to the second ester's carbonyl carbon.
- The acceptor's C=O π pair moves onto oxygen.
Ethoxide is expelled
The alkoxide re-forms the C=O bond and pushes out ethoxide, completing a nucleophilic acyl substitution. The product is a β-keto ester.
As setas curvas
- The oxygen lone pair re-forms the C=O bond.
- The C–O bond breaks and ethoxide leaves.
Ethoxide deprotonates the β-keto ester
The hydrogen between the two carbonyls (pKa ≈ 11) is removed by ethoxide (ethanol pKa ≈ 16), a step favoured by about 10⁵. This deprotonation pulls every earlier, unfavourable equilibrium toward product, which is why a full equivalent of base is used.
As setas curvas
- The ethoxide lone pair takes the doubly activated proton.
- The C–H pair forms a C=C π bond toward the ketone carbon.
- The ketone π pair moves onto oxygen, giving the stabilised enolate.
Acid workup gives the neutral β-keto ester
Hydronium, added after the reaction is complete, protonates the stabilised enolate on carbon while the oxygen lone pair re-forms the ketone.
As setas curvas
- The oxygen lone pair re-forms the ketone C=O.
- The C=C π pair takes a proton from hydronium at the central carbon.
- The O–H pair stays on oxygen as water.
Ethyl 2-methyl-3-oxopentanoate
A propanoyl group from one ester is now attached to the α-carbon of the other. The α-carbon keeps its methyl group, so the product is the 2-methyl β-keto ester.
O que você vai praticar
Draw the Claisen condensation of ethyl propanoate: enolate formation, addition to a second ester, loss of ethoxide, the deprotonation that drives the reaction, and acid workup.
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