Química Orgânica 2 · Síntese e transformação de aminas
2,5-Dimethylpyrrole by the Paal–Knorr synthesis
Condições: NH4OAc, AcOH, Δ
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Arraste dos elétrons que se movem até onde eles vão, ou toque em um e depois no outro. Cada seta é verificada; Dica ajuda quando você travar. Não precisa de conta.
O mecanismo, passo a passo
Próximo mostra as setas curvas de cada passo e as explica. Os passos escritos estão abaixo.
Ler todos os 14 passos escritos
Ammonia adds to a ketone
Ammonium acetate supplies ammonia and acetic acid. Ammonia attacks one ketone carbon and the C=O π pair moves onto oxygen.
As setas curvas
- The ammonia lone pair forms the C–N bond.
- The C=O π pair moves onto oxygen.
Acetic acid protonates the alkoxide
Acetic acid, the acid present, protonates the alkoxide.
As setas curvas
- The alkoxide takes the proton.
- The O–H pair stays on oxygen as acetate.
Acetate removes an N–H proton
Acetate deprotonates the ammonium nitrogen, giving a neutral amino alcohol whose nitrogen is free to attack again.
As setas curvas
- Acetate takes the N–H proton.
- The N–H pair returns to nitrogen.
Nitrogen closes the ring
The nitrogen attacks the second ketone carbon, forming a five-membered ring, the favoured ring size.
As setas curvas
- The nitrogen lone pair forms the ring C–N bond.
- The C=O π pair moves onto oxygen.
Protonate the new alkoxide
Acetic acid protonates the alkoxide.
As setas curvas
- The alkoxide takes the proton.
- The O–H pair stays on oxygen.
Acetate removes the N–H proton
Acetate deprotonates the ring nitrogen, giving the neutral 2,5-dihydroxypyrrolidine.
As setas curvas
- Acetate takes the N–H proton.
- The N–H pair returns to nitrogen.
Protonate the first OH
Acetic acid protonates one OH so it can leave as water.
As setas curvas
- The OH oxygen takes the proton.
- The O–H pair stays on oxygen.
Lose water: first iminium
The ring nitrogen lone pair pushes out water, giving an iminium ion.
As setas curvas
- The nitrogen lone pair forms the C=N π bond.
- The C–O pair leaves with water.
Deprotonate α to the iminium
Acetate removes a ring C–H next to the iminium carbon, giving an enamine (the first ring C=C).
As setas curvas
- Acetate takes the C–H proton.
- The C–H pair forms the C=C π bond.
- The C=N π pair moves onto nitrogen.
Protonate the second OH
Acetic acid protonates the remaining OH.
As setas curvas
- The OH oxygen takes the proton.
- The O–H pair stays on oxygen.
Lose water: second iminium
The nitrogen lone pair pushes out the second water.
As setas curvas
- The nitrogen lone pair forms the C=N π bond.
- The C–O pair leaves with water.
Driving force
The last step creates something special.
Deprotonate: aromatic pyrrole
Acetate removes the last ring C–H next to the iminium carbon; the new C=C completes the aromatic ring.
As setas curvas
- Acetate takes the C–H proton.
- The C–H pair forms the C=C π bond.
- The C=N π pair moves onto nitrogen.
2,5-Dimethyl-1H-pyrrole
2,5-Dimethyl-1H-pyrrole forms with two waters; acetic acid is regenerated.
O que você vai praticar
Draw ammonia adding to one ketone, ring closure of the nitrogen onto the second ketone, and the two dehydrations and deprotonations that give 2,5-dimethyl-1H-pyrrole.
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