Química Orgánica 2 · Acetales, iminas y equilibrios relacionados
N-Methylbenzylamine by reductive amination
Condiciones: NaBH3CN, MeOH, pH 5 (AcOH buffer)
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Arrastra desde los electrones que se mueven hasta donde van, o toca uno y luego el otro. Cada flecha se revisa; Pista te ayuda cuando te atascas. No necesitas cuenta.
El mecanismo, paso a paso
Siguiente reproduce las flechas curvas de cada paso y las explica. Los pasos escritos están abajo.
Leer los 8 pasos escritos
The nitrogen adds to the carbonyl carbon
The methylamine nitrogen lone pair attacks the electrophilic carbonyl carbon of benzaldehyde and the C=O π electrons move onto oxygen. At pH 5 an acetic acid buffer (assumed) supplies the protons; NaBH3CN does not reduce the aldehyde at this pH.
Las flechas 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.
Check
Answer before you continue.
Protonate the alkoxide
The tetrahedral intermediate carries O⁻ next to N⁺. Acetic acid, the acid catalyst, protonates the negative oxygen.
Las flechas 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.
Las flechas 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.
Las flechas 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.
Las flechas curvas
- The nitrogen lone pair forms the C=N π bond.
- The C–O bond pair leaves with the oxygen as neutral water.
Hydride reduces the iminium ion
The B–H bond of cyanoborohydride delivers hydride to the iminium carbon; the C=N π pair returns to nitrogen as a lone pair.
Las flechas curvas
- The B–H bond pair forms the new C–H bond.
- The C=N π pair moves onto nitrogen.
N-Methylbenzylamine
The C=O carbon is now a CH2 bonded to NHCH3: a secondary amine, made without the over-alkylation that alkyl halides cause.
Qué vas a practicar
Draw iminium formation from benzaldehyde and methylamine in a pH 5 buffer, then hydride transfer from NaBH3CN to give N-methylbenzylamine.
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