Wolff–Kishner reduction: practice and mechanism
Hydrazine forms a hydrazone with the ketone. With KOH and heat N₂ is released and the C=O group becomes a CH₂ group. Not an alcohol, but complete reduction.
Example
Hydrazine forms a hydrazone with the carbonyl group. The strong base deprotonates it, N₂ escapes and the carbanion is protonated. C=O becomes CH₂, under basic conditions (for acid-sensitive molecules).
Mechanism step by step
- 1
Hydrazine adds to the C=O group; after loss of water a hydrazone (C=N–NH₂) forms.
- 2
KOH deprotonates the NH₂; tautomerism shifts the double bond to N=N and the carbon is protonated.
- 3
A second deprotonation releases N₂; the carbanion takes a proton from the solvent: CH₂.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- That is the alcohol from NaBH₄. Wolff–Kishner removes the oxygen completely: C=O becomes CH₂.
Often confused with: Clemmensen reduction · Reduction with sodium borohydride
Frequently asked questions
Wolff–Kishner or Clemmensen?
Same result (C=O → CH₂). Wolff–Kishner is basic, good for acid-sensitive molecules; Clemmensen is acidic, good for base-sensitive ones.
What drives the reaction?
The release of N₂, a very stable gas that escapes. It needs high temperatures (about 200 °C in ethylene glycol).