Imines, Enamines, and Related Derivatives

What forms

• 1° amine + aldehyde/ketone → imine (Schiff base) (C=N–R) + H₂O.
• 2° amine + aldehyde/ketone → enamine (C=C–NR₂) + H₂O.
• NH₃ → imine (often unstable/polymerizes unless controlled).

Mechanism (imine formation, acid-catalyzed, reversible)

1. Nucleophilic attack: amine adds to C=O → carbinolamine (hemiaminal).
2. Proton transfers (pH ~4–5 best): protonate OH, deprotonate NH₂⁺ to set up loss.
3. Loss of water → iminium ion.
4. Deprotonate N → imine. Removing water drives equilibrium forward.

Mechanism (enamine formation)

• Same up to iminium; since 2° amine cannot lose N–H, a β-H is removed instead, forming C=C and regenerating neutral amine → enamine.

Related derivatives

• Oxime: + NH₂OH → R₂C=NOH.
• Hydrazone: + NH₂NH₂ → R₂C=NNH₂ (Wolff–Kishner precursor).
• 2,4-DNPH: diagnostic hydrazone (colored solid).
• Semicarbazone: + semicarbazide → R₂C=N–NH–C(O)NH₂.
• Biochem: PLP forms imines (aldimines) with amino acids in enzymes (electron sink).

Reactivity highlights

• Hydrolysis: Imines/enamines revert to carbonyl + amine in aqueous acid.
• Reduction: Imines → amines (H₂/Ni or NaBH₃CN in reductive amination).
• Enamine alkylation/acylation: Enamines attack electrophiles at β-carbon; hydrolysis regenerates carbonyl with α-substitution.

Summary: Mild acid, water removal, and amine class dictate imine vs enamine. These C=N derivatives are key for carbonyl protection/derivatization, C–N bond formation, and α-functionalization via enamines.