Química Orgânica 2 · Adições à carbonila: revisão para a prova
N-Methylimine from pentan-3-one
Condições: Pentan-3-one, MeNH2 (1.5 equiv), AcOH/AcONa buffer (pH ≈ 4.5), 4 Å molecular sieves, 25 °C
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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 8 passos escritos
The amine adds to the carbonyl carbon
The nitrogen lone pair of methylamine attacks the carbonyl carbon and the C=O π pair moves onto oxygen, giving a zwitterion with N⁺ and O⁻.
As setas curvas
- The nitrogen lone pair forms the new C–N bond.
- The C=O π pair moves onto oxygen.
Acetic acid protonates the alkoxide
The negatively charged oxygen takes the proton from acetic acid, giving an OH group and acetate.
As setas curvas
- An alkoxide lone pair takes the acetic acid proton.
- The O–H pair stays on oxygen, giving acetate.
Acetate removes a proton from nitrogen
Acetate takes one N–H proton from the ammonium nitrogen, giving the neutral carbinolamine and acetic acid.
As setas curvas
- An acetate oxygen lone pair takes the N–H proton.
- The N–H pair stays on nitrogen, which becomes neutral.
Why pH 4–5?
The next event needs acid to protonate the OH, but the first event needed a free, unprotonated amine; pH 4–5 balances the two.
Acetic acid protonates the OH
Acetic acid protonates the hydroxyl oxygen of the carbinolamine, turning it into a good leaving group (water).
As setas curvas
- A hydroxyl oxygen lone pair takes the acetic acid proton.
- The O–H pair stays on oxygen, giving acetate.
The nitrogen lone pair pushes out water
The nitrogen lone pair forms a C=N π bond while the C–O bond breaks and water leaves, giving an iminium ion.
As setas curvas
- The nitrogen lone pair forms the C=N π bond.
- The C–O bond breaks and water leaves with the pair.
Acetate removes the N–H proton
The iminium nitrogen still carries one hydrogen. Acetate removes it, giving the neutral imine and regenerating acetic acid.
As setas curvas
- An acetate oxygen lone pair takes the N–H proton.
- The N–H pair becomes the nitrogen lone pair of the imine.
N-Methylpentan-3-imine
The carbonyl oxygen has been replaced by N–CH3 through a C=N double bond, and the oxygen has left as water. Because both groups on the imine carbon are ethyl, there are no E/Z isomers.
O que você vai praticar
Draw imine formation from pentan-3-one and methylamine with acetic acid and acetate as the only proton donor and acceptor, and explain why the reaction is run near pH 4–5.
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