Química Orgánica 2 · Acetales, iminas y equilibrios relacionados
Acetone semicarbazone
Condiciones: semicarbazide·HCl, NaOAc, H2O/EtOH
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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 semicarbazide nitrogen lone pair attacks the electrophilic carbonyl carbon of acetone and the C=O π electrons move onto oxygen. Acetic acid comes from NaOAc buffering semicarbazide hydrochloride (assumed buffer). Only the terminal NH2 attacks; the two amide nitrogens donate their lone pairs into the C=O.
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.
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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.
Deprotonate the iminium ion
The acetate removes the last N–H proton, giving the neutral C=N product and regenerating the acid catalyst. Semicarbazones are crystalline derivatives used to identify ketones.
Las flechas 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.
Acetone semicarbazone
The C=N–NH–C(=O)NH2 group replaces C=O; acetic acid is regenerated.
Qué vas a practicar
Draw addition of the hydrazine-type NH2 of semicarbazide to acetone, proton transfers, loss of water and deprotonation to acetone semicarbazone.
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