Química Orgânica 2 · Acetais, iminas e equilíbrios relacionados
Acetone 2,4-dinitrophenylhydrazone
Condições: H2SO4 (cat.), EtOH
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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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The nitrogen adds to the carbonyl carbon
The 2,4-dinitrophenylhydrazine nitrogen lone pair attacks the electrophilic carbonyl carbon of acetone and the C=O π electrons move onto oxygen. Only the terminal NH2 attacks: the other nitrogen's lone pair is delocalized into the electron-poor dinitrophenyl ring.
As setas curvas
- The nitrogen lone pair forms the new C–N bond to the partially positive carbonyl carbon.
- The C=O π pair moves onto oxygen, which becomes an alkoxide.
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Protonate the alkoxide
The tetrahedral intermediate carries O⁻ next to N⁺. Protonated ethanol (from H2SO4 in ethanol), the acid catalyst, protonates the negative oxygen.
As setas curvas
- The alkoxide lone pair takes the acidic proton.
- The O–H bond pair stays on ethanol.
Remove the N–H proton
The ethanol removes a proton from the positively charged nitrogen, giving the neutral carbinolamine: one carbon carrying both OH and N.
As setas curvas
- A lone pair on ethanol takes the N–H proton.
- The N–H bond pair returns to nitrogen as its lone pair.
Protonate the OH group
OH⁻ is a poor leaving group in an acidic medium, so the acid protonates the hydroxyl oxygen to make water a leaving group.
As setas curvas
- A hydroxyl lone pair takes the proton from the acid.
- The O–H bond pair stays on ethanol.
Water leaves as the C=N⁺ forms
The nitrogen lone pair pushes into the C–N bond while water departs, so no free carbocation forms; the result is an iminium ion with a full octet on every atom.
As setas curvas
- The nitrogen lone pair forms the C=N π bond.
- The C–O bond pair leaves with the oxygen as neutral water.
Deprotonate the iminium ion
The ethanol removes the last N–H proton, giving the neutral C=N product and regenerating the acid catalyst. The orange crystalline hydrazone is the classic 2,4-DNP test for aldehydes and ketones.
As setas curvas
- A lone pair on ethanol takes the N–H proton.
- The N–H bond pair becomes the nitrogen lone pair of the C=N product.
Acetone 2,4-dinitrophenylhydrazone
The C=N–NH–Ar group replaces C=O; sulfuric acid is regenerated.
O que você vai praticar
Draw addition of the terminal NH2 of 2,4-dinitrophenylhydrazine to acetone, proton transfers, loss of water and deprotonation to acetone 2,4-dinitrophenylhydrazone.
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