Química Orgânica 2 · Enolatos: alquilação, condensações e adições conjugadas
Self-aldol condensation of butanal
Condições: Butanal, aqueous NaOH (catalytic), heat
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Hydroxide forms a small amount of enolate
Hydroxide removes an α-hydrogen from C2 of one butanal molecule. Water (pKa 15.7) is a slightly stronger acid than the aldehyde (pKa ≈ 17), so only a small fraction of enolate is present, but it is surrounded by unreacted aldehyde.
As setas curvas
- A hydroxide 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 butanal
The enolate carbon attacks the carbonyl carbon of a neutral butanal molecule, forming the new C–C bond and an alkoxide.
As setas curvas
- The oxygen lone pair re-forms the aldehyde C=O.
- The enolate π pair forms the C–C bond to the other carbonyl carbon.
- The acceptor's C=O π pair moves onto oxygen.
Water protonates the alkoxide
Water gives a proton to the alkoxide, giving the aldol addition product, 2-ethyl-3-hydroxyhexanal, and regenerating hydroxide.
As setas curvas
- The alkoxide lone pair takes a proton from water.
- The O–H pair stays on oxygen as hydroxide.
On heating, hydroxide forms the enolate of the aldol
Hydroxide removes the remaining α-hydrogen, on the carbon between the aldehyde and the CH–OH carbon.
As setas curvas
- A hydroxide lone pair takes the α-hydrogen.
- The C–H pair forms the enolate C=C π bond.
- The C=O π pair moves onto oxygen.
E1cB loss of hydroxide
The enolate re-forms the C=O bond and expels hydroxide from the β-carbon. Hydroxide is a poor leaving group, but the conjugated product makes this elimination favourable on heating.
As setas curvas
- The oxygen lone pair re-forms the C=O bond.
- The C=C π pair shifts to form the new C=C bond toward the β-carbon.
- The C–O bond breaks and hydroxide leaves.
(E)-2-Ethylhex-2-enal
The C=C is conjugated with the aldehyde C=O. The ethyl group on C2 came from the enolate molecule, and the propyl group on C3 from the acceptor molecule.
O que você vai praticar
Draw the hydroxide-catalysed aldol addition of two butanal molecules and the E1cB dehydration that gives (E)-2-ethylhex-2-enal.
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