Química Orgânica 2 · Síntese e transformação de aminas
Hofmann rearrangement — N-bromoamide to a shorter amine
Condições: Preformed N-bromoacetamide in aqueous hydroxide; the initial N-bromination is not shown. Follow the Hofmann rearrangement and amine-forming hydrolysis, ending with bicarbonate capture of CO₂. Two hydroxide equivalents are explicit; countercations are omitted spectators. Carbonate equilibria beyond bicarbonate are outside the endpoint.
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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.
Ler todos os 9 passos escritos
N-haloamide starting material
Hydroxide accepts the N-bound proton. The N–H pair remains on nitrogen.
As setas curvas
- Hydroxide accepts the N-bound proton.
- The N–H pair remains on nitrogen.
Carbon-skeleton reorganization
The carbon–methyl sigma pair migrates with methyl to nitrogen. A nitrogen lone pair supplies the C=N pi bond. The N–Br pair leaves as bromide in the coupled rearrangement.
As setas curvas
- The carbon–methyl sigma pair migrates with methyl to nitrogen.
- A nitrogen lone pair supplies the C=N pi bond.
- The N–Br pair leaves as bromide in the coupled rearrangement.
Aqueous encounter
Hydroxide adds to the electrophilic central carbon. The C=N pi pair moves onto nitrogen.
As setas curvas
- Hydroxide adds to the electrophilic central carbon.
- The C=N pi pair moves onto nitrogen.
First proton balance
The nitrogen anion accepts a water proton. Water gives its bonding pair to oxygen, regenerating hydroxide.
As setas curvas
- The nitrogen anion accepts a water proton.
- Water gives its bonding pair to oxygen, regenerating hydroxide.
Second proton balance
Hydroxide accepts the carbamic acid proton. The O–H pair gives the carbamate anion.
As setas curvas
- Hydroxide accepts the carbamic acid proton.
- The O–H pair gives the carbamate anion.
Carbon loss
The oxygen pair forms the second C=O bond of carbon dioxide. The C–N sigma pair stays with nitrogen as carbon dioxide separates. Decarboxylation and protonation are drawn separately to show the electron flow. This does not mean an amide anion accumulates in water.
As setas curvas
- The oxygen pair forms the second C=O bond of carbon dioxide.
- The C–N sigma pair stays with nitrogen as carbon dioxide separates.
Complete the amine proton count
Nitrogen takes a proton from the explicit water. The O–H pair restores hydroxide.
As setas curvas
- Nitrogen takes a proton from the explicit water.
- The O–H pair restores hydroxide.
Specified carbon dioxide capture endpoint
Hydroxide adds to carbon dioxide. A C=O pi pair moves onto oxygen, giving the bicarbonate oxygen anion.
As setas curvas
- Hydroxide adds to carbon dioxide.
- A C=O pi pair moves onto oxygen, giving the bicarbonate oxygen anion.
Methylamine, bicarbonate, and bromide
The original methyl group is now bonded to nitrogen rather than carbonyl carbon. Nitrogen has acquired two hydrogens and is neutral. The carbonyl carbon is retained in bicarbonate. Bicarbonate and bromide carry the original total charge of −2; further carbonate speciation is not modeled.
O que você vai praticar
Track carbon-to-nitrogen migration, hydrolytic proton transfers, and carbon dioxide loss in the Hofmann rearrangement.
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