Química Orgânica 2 · Substituição acílica e escolha do derivado
Butanal from butanenitrile with DIBAL-H
Condições: DIBAL-H, −78 °C; then H3O⁺
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O mecanismo, passo a passo
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Nitrogen binds aluminium
DIBAL-H is a neutral Lewis acid with an empty orbital on aluminium. The nitrile nitrogen lone pair coordinates to it first, which makes the nitrile carbon more electrophilic.
As setas curvas
- The nitrile nitrogen lone pair forms a bond to the empty orbital on aluminium.
Hydride moves to carbon
The Al–H bond delivers hydride to the nitrile carbon while one C≡N π pair moves onto nitrogen. The product is a neutral N-aluminium imine, which is stable at −78 °C.
As setas curvas
- The Al–H bond pair forms the new C–H bond at the nitrile carbon.
- One C≡N π pair moves onto nitrogen, which keeps its bond to aluminium.
Why only one hydride?
The imine nitrogen carries aluminium and a negative-charge-like lone pair.
Protonate the imine nitrogen
Aqueous acid is added in the workup. The imine nitrogen lone pair takes a proton from hydronium, giving an iminium ion whose carbon is now strongly electrophilic.
As setas curvas
- The nitrogen lone pair takes a proton from hydronium.
- The O–H bond pair stays on oxygen, releasing water.
Water adds to the iminium carbon
Water attacks the iminium carbon and the C=N π pair moves onto nitrogen, giving a tetrahedral intermediate with an oxonium group.
As setas curvas
- A lone pair on water bonds to the iminium carbon.
- The C=N π pair moves onto nitrogen, neutralising it.
Deprotonate the oxonium
A water molecule removes a proton from the oxonium oxygen, leaving a neutral carbinolamine.
As setas curvas
- Water uses a lone pair to take the oxonium proton.
- The O–H bond pair stays on the oxygen now bonded to carbon.
Protonate nitrogen
Hydronium protonates the nitrogen so that it can leave as a neutral amine–aluminium species rather than as an anion.
As setas curvas
- The nitrogen lone pair takes a proton from hydronium.
- The O–H bond pair returns to the water oxygen.
Oxygen pushes nitrogen out
The OH lone pair forms a C=O π bond as the C–N bond breaks, expelling the neutral aminoaluminium fragment and giving a protonated aldehyde.
As setas curvas
- An oxygen lone pair forms the new C=O π bond.
- The C–N bond pair leaves with nitrogen.
Deprotonate to butanal
Water removes the proton on the carbonyl oxygen, releasing neutral butanal and regenerating hydronium.
As setas curvas
- Water takes the proton from the protonated carbonyl.
- The O–H pair becomes a lone pair on the aldehyde oxygen.
Butanal
The nitrile carbon has gained exactly one hydrogen and the nitrogen has been replaced by oxygen from water: butanal. Nitrogen leaves with aluminium as an amine salt in the workup.
O que você vai praticar
Draw the DIBAL-H reduction of butanenitrile to the aluminium imine and its aqueous hydrolysis to butanal.
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