Overall reaction
R₂C=O + N₂H₄ → R₂C=NNH₂ → (KOH, 200 °C) R₂CH₂ + N₂
Hydrazone + strong base at high temperature: C=O becomes CH₂ with loss of N₂.
Name reaction · mechanism
Wolff–Kishner Reduction
The Wolff–Kishner reduction converts the C=O of an aldehyde or ketone into CH₂. The carbonyl compound forms a hydrazone with hydrazine; strong base at about 200 °C then removes N–H protons, the molecule loses nitrogen gas, and the carbanion is protonated.
Mechanism step by step
- Deprotonation. The hydrazone (from C=O + N₂H₄) loses an N–H; the charge moves to C
- C-Protonation. Water protonates carbon: a diimide
- Loss of N₂. Deprotonation releases N₂ and a carbanion
- Protonation. Water gives the alkane
Each step is checked electron by electron: the curly arrows give exactly the structures drawn.
Key points
- Loss of N₂ gas makes the reduction irreversible.
- Strongly basic: the complement to the acidic Clemmensen reduction (Zn/Hg, HCl).
- Huang-Minlon: one pot in ethylene glycol, water distilled off.
- Base-sensitive groups (esters, alkyl halides) do not survive.
For JEE and NEET
- Basic conditions: use it for acid-sensitive molecules; Clemmensen (Zn–Hg, HCl) is the acidic alternative.
- Loss of N₂ gas drives the reaction.
- Huang-Minlon: one pot, water and excess hydrazine distilled off to reach 200 °C.
- Esters and alkyl halides are attacked by the strong base.
FAQ
Wolff–Kishner or Clemmensen?
Both reduce C=O to CH₂. Choose Wolff–Kishner for acid-sensitive substrates and Clemmensen for base-sensitive ones.
Why is a high temperature needed?
Deprotonation of the diimide and loss of N₂ have a high barrier; ethylene glycol (b.p. 197 °C) allows the temperature.
What is the role of hydrazine?
It forms the hydrazone (R₂C=N–NH₂), whose N–H protons and N=N bond provide the route to lose N₂.
Draw it in MolDraw
The button above opens the complete figure — overall reaction, numbered mechanism and key points — as ordinary editable structures. Change substituents, recolour atoms, or export a PNG, SVG or chemfig code for notes and slides. The same figure is in the Templates panel under Name reactions.