Química Orgânica 2 · Adição e condensação aldólica
Crossed aldol addition — propanal hydroxymethylation
Condições: Propanal and formaldehyde with catalytic NaOH in water; trace one crossed addition at propanal alpha carbon. Further hydroxymethylation, dehydration and other base-promoted pathways are outside the specified branch. Counterions are not drawn; no stereochemistry is assigned without bond markers.
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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
Move the electrons
The explicit donor alpha proton is removed in a reversible base-catalyzed encounter.
As setas curvas
- The base removes an alpha hydrogen next to the carbonyl.
- The alpha C–H electrons form a π bond to the carbonyl carbon.
- The carbonyl π electrons move onto oxygen.
Move the electrons
These are two resonance forms of the same enolate, not separate molecules. No proton moves between them.
As setas curvas
- An oxygen lone pair forms the carbonyl π bond.
- The π electrons move onto the alpha carbon as a lone pair.
Move the electrons
Enolate carbon attacks the acceptor carbonyl; the donor carbonyl remains intact.
As setas curvas
- The nucleophile uses a lone pair to form the new single bond.
- The π electrons move off carbon, keeping eight electrons around it.
Move the electrons
The retained conjugate water protonates the acceptor-derived alkoxide and regenerates hydroxide.
As setas curvas
- The base uses a lone pair to accept a proton from the O–H bond.
- The bond to hydrogen breaks, and its electrons stay on the atom losing the proton.
Products
The acceptor carbon keeps both hydrogens and becomes CH₂OH. The donor retains its aldehyde C=O and gains a hydroxymethyl group at alpha carbon; the explicit alpha hydrogen, relayed through water, becomes the hydrogen of the new OH (the oxygen is formaldehyde’s). Propanal’s α C–H (pKa ≈ 17) is slightly less acidic than water (pKa ≈ 15.7), so hydroxide keeps only a small, reversibly formed enolate population (K ≈ 0.05); formaldehyde has no α hydrogen and can only accept.
O que você vai praticar
Use formaldehyde as a non-enolizable acceptor while preserving the donor aldehyde.
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