Química Orgânica 2 · Enóis, enolatos e reações em alfa
Ethyl 2,2,2-trideuterioacetate by α-H/D exchange
Condições: NaOEt (cat.), EtOD (excess)
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O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 8 passos escritos
Ethoxide forms a small amount of enolate
Ethoxide removes an α-hydrogen of the ester. The ester α-C–H (pKa about 25) is far less acidic than ethanol (pKa 16), so only a tiny equilibrium concentration of enolate forms, but that is enough for exchange.
As setas curvas
- The ethoxide lone pair takes an α-proton.
- The C–H electrons form a C=C bond.
- The C=O π electrons move onto oxygen.
The enolate takes a deuteron
Solvent EtOD is present in large excess, so the enolate α-carbon is far more likely to pick up deuterium than to recover its original proton.
As setas curvas
- An oxygen lone pair re-forms the C=O bond.
- The C=C π electrons take the deuteron from EtOD.
- The O–D electrons stay on oxygen as ethoxide.
Second exchange: deprotonation
The newly formed ethoxide removes another α-hydrogen. Removing H is faster than removing D (kinetic isotope effect), which also favours complete exchange.
As setas curvas
- Ethoxide takes a second α-proton.
- The C–H electrons form a C=C bond.
- The C=O π electrons move onto oxygen.
Second deuteration
The enolate again takes a deuteron from the solvent.
As setas curvas
- An oxygen lone pair re-forms the C=O bond.
- The C=C π electrons take a deuteron.
- The O–D electrons stay on oxygen.
Where does exchange stop?
Only hydrogens that can be removed to form an enolate exchange.
Third exchange: deprotonation
Ethoxide removes the last α-hydrogen.
As setas curvas
- Ethoxide takes the last α-proton.
- The C–H electrons form a C=C bond.
- The C=O π electrons move onto oxygen.
Third deuteration
The last enolate takes a deuteron, completing the exchange and regenerating the catalytic ethoxide.
As setas curvas
- An oxygen lone pair re-forms the C=O bond.
- The C=C π electrons take a deuteron.
- The O–D electrons stay on oxygen.
Ethyl 2,2,2-trideuterioacetate
All three α-hydrogens are now deuterium; ethoxide is regenerated, so it is a catalyst. Counting the deuteriums incorporated (by mass spectrometry or NMR) is classic evidence that a reaction goes through an enolate.
O que você vai praticar
Draw three rounds of ethoxide deprotonation and O-deuterated ethanol deuteration that exchange every α-hydrogen of ethyl acetate for deuterium.
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