Química Orgânica 2 · Condensação de Claisen e adição conjugada
Malonic ester synthesis — two staged C-alkylations
Condições: Diethyl malonate with ethoxide/ethyl bromide in ethanol; only after the first alkylation add a second ethoxide equivalent and methyl iodide. Follow C-alkylation in both stages. Counterions omitted; no hydrolysis or decarboxylation is specified.
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Arraste dos elétrons que se movem até onde eles vão, ou toque em um e depois no outro. Cada seta é verificada; Dica ajuda quando você travar. Não precisa de conta.
O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 5 passos escritos
Remove an alpha hydrogen
One of the two explicit active-methylene hydrogens moves to ethoxide; the other remains available for a later, separately staged alkylation.
Nota: The two drawn alpha hydrogens are equivalent.
As setas curvas
- The base uses a lone pair to bond to the drawn hydrogen.
- The bond to hydrogen breaks, leaving its electrons on the atom that lost H.
Build the specified carbon bond
The primary ethyl carbon is the SN2 electrophile; the ester groups are unchanged and the product still has a hydrogen between the two carbonyls.
As setas curvas
- The carbon-centered enolate supplies the new sigma bond.
- The leaving group takes the carbon–halogen bond pair in the same SN2 event.
Remove an alpha hydrogen
A second ethoxide equivalent and methyl iodide have now been introduced. Remove the remaining alpha hydrogen before the next substitution; neither old coproduct disappears.
As setas curvas
- The base uses a lone pair to bond to the drawn hydrogen.
- The bond to hydrogen breaks, leaving its electrons on the atom that lost H.
Build the specified carbon bond
Methyl iodide receives the second carbon nucleophile. The central carbon reaches four C–C sigma bonds and can no longer be deprotonated at this site.
As setas curvas
- The carbon-centered enolate supplies the new sigma bond.
- The leaving group takes the carbon–halogen bond pair in the same SN2 event.
Products
Both original alpha hydrogens are now in separate ethanol molecules. Ethyl and methyl groups occupy the two former C–H positions, bromide and iodide retain both net negative charges, and all ester bonds remain intact. This completes the specified dialkylation without implying later malonic-ester hydrolysis.
O que você vai praticar
Complete two separately staged malonate C-alkylations and recognize when the central carbon has no alpha hydrogen left.
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