Química Orgânica 2 · Ferramentas de oxidação e redução
DIBAL-H partial reduction of methyl hexanoate to hexanal
Condições: DIBAL-H (1.0 equiv), CH2Cl2, −78 °C, 1 h; then aqueous HCl workup at −78 °C → 25 °C
Desenhe este problema você mesmo
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 9 passos escritos
The carbonyl oxygen coordinates aluminium
Neutral DIBAL-H has an empty orbital on aluminium. A lone pair on the ester carbonyl oxygen forms an O–Al bond, activating the carbonyl carbon and placing the Al–H bond next to it.
As setas curvas
- A carbonyl oxygen lone pair forms a bond to the electron-poor aluminium.
Hydride moves from aluminium to carbon
The Al–H pair forms the new C–H bond at the carbonyl carbon while the C=O π pair returns to oxygen. The result is a neutral aluminium hemiacetal: the oxygen stays bonded to aluminium.
As setas curvas
- The Al–H pair carries the hydrogen to the carbonyl carbon.
- The C=O π pair moves onto oxygen, cancelling its positive charge.
Why the reduction stops
The aluminium hemiacetal does not expel methoxide at −78 °C, so no aldehyde exists while DIBAL-H is still active.
Workup: hydronium protonates the oxygen on aluminium
Aqueous HCl is added. Hydronium protonates the hemiacetal oxygen that is bonded to aluminium.
As setas curvas
- An oxygen lone pair takes a proton from hydronium.
- The hydronium O–H pair stays on oxygen, leaving water.
Water displaces the hemiacetal from aluminium
Water bonds to aluminium as the O–Al bond breaks with its pair going to the hemiacetal oxygen. The free hemiacetal, 1-methoxyhexan-1-ol, is released.
As setas curvas
- A water lone pair forms a bond to aluminium.
- The O–Al pair returns to the hemiacetal oxygen.
Hydronium protonates the methoxy oxygen
In aqueous acid the hemiacetal is unstable. Hydronium protonates the methoxy oxygen, turning methanol into a leaving group.
As setas curvas
- A methoxy oxygen lone pair takes a proton from hydronium.
- The hydronium O–H pair stays on oxygen, leaving water.
Methanol leaves
The OH lone pair pushes in to form C=O⁺ as the C–O bond to the protonated methoxy group breaks, releasing methanol and the protonated aldehyde.
As setas curvas
- An oxygen lone pair forms the C=O π bond.
- The C–O bond to the protonated methoxy group breaks, releasing methanol.
Water deprotonates the protonated aldehyde
Water removes the proton from the carbonyl oxygen, giving neutral hexanal and hydronium.
As setas curvas
- A water lone pair takes the proton from the carbonyl oxygen.
- The O–H pair stays on the carbonyl oxygen.
Hexanal
The ester has been reduced by exactly one hydride to the aldehyde oxidation level: hexanal and methanol are released after workup.
O que você vai praticar
Draw coordination of DIBAL-H to methyl hexanoate, hydride transfer to the stable aluminium hemiacetal, and the acidic workup that releases hexanal.
Você acabou de desenhar cada seta deste problema.
Cada um dos 5349 problemas de Orgânica 1 e 2 funciona assim: feedback em cada seta, dicas quando você travar e um modo prova sem dicas. Grátis com uma conta, sem cartão.
Travou na sua própria lição?
Tire uma foto: o primeiro scan é grátis, sem cadastro
Você escaneia toda semana? Scan Standard: por cerca de problemas