Química Orgânica 2 · Síntese e transformação de aminas
Reduce an amide all the way to an amine
Condições: N,N-Dimethylbenzamide with lithium aluminum hydride in dry ether, followed by aqueous workup. Two AlH₄⁻ equivalents each supply one hydrogen; each AlH₃ group includes its three hydrogens that are not transferred. Li⁺ counterions are omitted. The aluminum–oxygen fragment is a simplified drawing of the bonds and charges, not the full structure of the real aggregated salt.
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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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Deliver the first hydride
Amides are the least electrophilic acyl derivatives, so this needs the powerful donor LiAlH₄; NaBH₄ does not reduce amides under ordinary conditions.
As setas curvas
- The Al–H pair forms a bond between hydrogen and the carbonyl carbon; no free hydride ion is released.
- The carbonyl π pair becomes a lone pair on oxygen.
Bind oxygen to aluminum
Coordination to aluminum turns the oxygen into part of a leaving group.
As setas curvas
- An alkoxide oxygen lone pair bonds to the aluminum Lewis acid.
Push out oxygen with nitrogen
This is the step that differs from ester reduction. In an ester the tetrahedral intermediate expels an alkoxide and a carbonyl is rebuilt; here nitrogen, the better π donor, pushes out oxygen and an iminium ion forms instead.
As setas curvas
- The nitrogen lone pair forms a C=N π bond.
- The C–O pair leaves with the aluminum-bound oxygen.
Deliver the second hydride
An iminium ion is more electrophilic than the starting amide, so it is reduced at once.
As setas curvas
- The Al–H pair forms a bond between hydrogen and the iminium carbon.
- The C=N π pair returns to nitrogen as its lone pair.
Tertiary amine
The former carbonyl carbon now carries two hydrogens and is bonded to nitrogen: N,N-dimethylbenzylamine. The carbonyl oxygen left entirely, ending in the aluminum salts that the aqueous workup converts to aluminum hydroxides. The amine needs no protonation step, because nitrogen already has its lone pair back.
O que você vai praticar
Explain why LiAlH₄ turns an amide into an amine rather than an alcohol by following the iminium ion that forms after the first hydride.
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