Química Orgânica 2 · Ferramentas de oxidação e redução
LiAlH4 reduction of hexanoic acid to hexan-1-ol
Condições: LiAlH4 (excess, 1.5 equiv), THF, reflux; then aqueous H2SO4 workup
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O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
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Hydride removes the acidic proton
Before any reduction, the acidic O–H proton reacts with an Al–H bond. Hydrogen gas bubbles off and the acid becomes a carboxylate.
As setas curvas
- The Al–H pair forms an H–H bond to the acidic hydrogen.
- The O–H pair stays on oxygen, giving the carboxylate.
Where did the first hydride go?
A carboxylic acid is far more acidic than H2, so the first hydride acts as a base and is lost as hydrogen gas.
The carboxylate oxygen binds aluminium
A carboxylate oxygen lone pair bonds to AlH3. Tying that oxygen to aluminium both activates the carbonyl and turns it into a leaving group.
As setas curvas
- An oxygen lone pair forms the O–Al bond.
Hydride adds to the carbonyl carbon
A second AlH4− delivers hydride to the carbonyl carbon while the C=O π pair moves onto oxygen, giving a tetrahedral intermediate.
As setas curvas
- The Al–H pair carries the hydrogen to the carbonyl carbon.
- The C=O π pair moves onto oxygen.
Collapse to hexanal
The alkoxide lone pair re-forms the C=O bond and the oxygen bound to aluminium leaves, giving hexanal inside the reaction mixture.
As setas curvas
- An oxygen lone pair rebuilds the C=O π bond.
- The C–O bond to the aluminium-bound oxygen breaks.
Hexanal is reduced at once
Hexanal is much more electrophilic than the carboxylate it came from, so it takes the next hydride immediately, giving hexan-1-olate.
As setas curvas
- The Al–H pair forms the second new C–H bond at the aldehyde carbon.
- The C=O π pair moves onto oxygen.
Acidic workup protonates the alkoxide
Dilute aqueous acid destroys the excess hydride and protonates the alkoxide, giving hexan-1-ol.
As setas curvas
- An alkoxide lone pair takes a proton from hydronium.
- The hydronium O–H pair stays on oxygen, leaving water.
Hexan-1-ol
The carboxyl carbon now carries two hydrogens and one OH: hexanoic acid has been reduced to the primary alcohol hexan-1-ol.
O que você vai praticar
Draw the LiAlH4 reduction of hexanoic acid: deprotonation of the acid with loss of H2, activation of the carboxylate by aluminium, hydride addition, collapse to hexanal, a second hydride addition and acidic workup to hexan-1-ol.
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