Química Orgânica 2 · Síntese e transformação de aminas
Ethylamine by the Curtius rearrangement
Condições: NaN3, acetone; then Δ (toluene); then H2O, Δ
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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 12 passos escritos
Azide adds to the acyl chloride
An azide nitrogen attacks the very electrophilic carbonyl carbon of the acid chloride and the C=O π pair moves onto oxygen.
As setas curvas
- The azide lone pair forms the C–N bond.
- The C=O π pair moves onto oxygen.
Chloride leaves
The alkoxide re-forms C=O and expels chloride, the best leaving group, giving propanoyl azide.
As setas curvas
- The oxygen lone pair re-forms the C=O bond.
- The C–Cl pair leaves as chloride.
Acyl azide contributor
Shifting the azide π electrons puts the negative charge on the carbonyl-bound nitrogen and makes N2 a ready-made leaving group; this contributor shows the migration that follows.
As setas curvas
- The terminal lone pair forms the N≡N bond.
- The N=N π pair moves onto the carbonyl-bound nitrogen.
Ethyl migrates as N2 leaves
On heating, the ethyl group moves from carbon to nitrogen with its bonding pair while N2 departs, in one concerted step; the nitrogen lone pair forms the C=N bond of ethyl isocyanate. No free nitrene or cation forms, so a migrating stereocentre would keep its configuration.
As setas curvas
- The C–C pair carries the ethyl group to nitrogen.
- The nitrogen lone pair forms the C=N π bond.
- The N–N pair leaves with N2.
Carbon count
The rearrangement changes the skeleton.
Water adds to the isocyanate
Water attacks the central isocyanate carbon and the C=N π pair moves onto nitrogen.
As setas curvas
- A water lone pair forms the C–O bond.
- The C=N π pair moves onto nitrogen.
Water removes the oxonium proton
A second water deprotonates the positive oxygen.
As setas curvas
- Water takes the O–H proton.
- The O–H pair stays on oxygen.
Nitrogen takes a proton
The nitrogen anion takes a proton from hydronium, giving the carbamic acid.
As setas curvas
- The nitrogen lone pair takes the proton.
- The O–H pair stays on oxygen.
Deprotonate the carbamic acid
Water removes the acidic carbamic acid O–H proton, giving the carbamate ion.
As setas curvas
- Water takes the O–H proton.
- The O–H pair stays on oxygen.
Protonate nitrogen
Hydronium protonates the nitrogen, which makes it a neutral-amine leaving group.
As setas curvas
- The nitrogen lone pair takes the proton.
- The O–H pair stays on oxygen.
Lose CO2
The carboxylate oxygen forms a second C=O bond and the C–N bond breaks onto nitrogen, releasing CO2 and ethylamine.
As setas curvas
- The oxygen lone pair forms the C=O bond.
- The C–N pair stays on nitrogen.
Ethylamine
Ethylamine forms with CO2 and N2; the former carbonyl carbon left as CO2.
O que você vai praticar
Draw formation of propanoyl azide, the concerted migration of ethyl to nitrogen with loss of N2, water addition to ethyl isocyanate, and decarboxylation to ethylamine.
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