Clemmensen reduction: practice and mechanism
Amalgamated zinc in concentrated hydrochloric acid reduces the carbonyl group completely to a CH₂ group. Same result as Wolff–Kishner, but under acidic conditions.
Example
Amalgamated zinc in concentrated hydrochloric acid reduces the carbonyl group completely to CH₂, under acidic conditions. Typical after a Friedel–Crafts acylation: acylation plus Clemmensen gives an alkylbenzene without rearrangement.
Mechanism step by step
- 1
The carbonyl group is protonated and bound at the zinc surface.
- 2
Zinc transfers electrons stepwise; via zinc carbenoid intermediates the oxygen is removed as water.
- 3
Further electrons and protons give the CH₂ group. The exact mechanism is still not fully understood.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Background
Conditions
Amalgamated zinc (Zn/Hg) in concentrated hydrochloric acid under reflux. The C=O group of a ketone or aldehyde becomes a CH₂ group. It works especially well with aryl ketones.
What it is used for
The classic in syntheses: Friedel–Crafts acylation first, then Clemmensen reduction. This gives unbranched alkylbenzenes such as propylbenzene, which direct Friedel–Crafts alkylation cannot make because the carbocation rearranges.
Clemmensen or Wolff–Kishner?
Both turn C=O into CH₂. Clemmensen is strongly acidic, Wolff–Kishner (hydrazine, KOH, heat) strongly basic. The other groups in the molecule decide: acid-sensitive molecules go Wolff–Kishner, base-sensitive ones Clemmensen.
Typical mistakes
Often confused with: Wolff–Kishner reduction · Reduction with sodium borohydride
Frequently asked questions
What is the Clemmensen reduction typically used for?
After a Friedel–Crafts acylation: the aryl ketone becomes an alkyl chain. This gives alkylbenzenes without rearrangement or multiple alkylation.
Is an alcohol formed as an intermediate?
No. Alcohols are not reduced to alkanes under Clemmensen conditions, so they are not an intermediate.
What is the Clemmensen reduction?
The reduction of a ketone or aldehyde with zinc amalgam and concentrated hydrochloric acid to the alkane: C=O becomes CH₂. Typically used after a Friedel–Crafts acylation.