Química Orgânica 2 · Acetais, iminas e equilíbrios relacionados
Cyclohexanone oxime from cyclohexanone
Condições: NH2OH·HCl, NaOAc, EtOH
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The nitrogen adds to the carbonyl carbon
The hydroxylamine nitrogen lone pair attacks the electrophilic carbonyl carbon of cyclohexanone and the C=O π electrons move onto oxygen. Acetic acid here comes from NaOAc buffering the NH2OH·HCl (assumed buffer). Nitrogen, not oxygen, attacks: it is the less electronegative, more nucleophilic atom (alpha effect).
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⁺. Acetic acid, 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 acetate.
Remove the N–H proton
The acetate 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 acetate 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 acetate.
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 acetate removes the last N–H proton, giving the neutral C=N product and regenerating the acid catalyst. The oxime keeps its N–OH group.
As setas curvas
- A lone pair on acetate takes the N–H proton.
- The N–H bond pair becomes the nitrogen lone pair of the C=N product.
Cyclohexanone oxime
The C=N–OH group replaces C=O. Oximes are crystalline derivatives and the substrates of the Beckmann rearrangement.
O que você vai praticar
Draw addition of hydroxylamine nitrogen to cyclohexanone, proton transfers, acid-assisted loss of water and deprotonation to cyclohexanone oxime.
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