Clemmensen reduction
The carbonyl group of an aldehyde or ketone is reduced to CH2 by zinc amalgam and concentrated HCl.
AcetophenoneC₆H₅COCH₃Ketone - Step 1
Zn-Hg, conc. HCl
EthylbenzeneC₆H₅CH₂CH₃Arene
How it works
- Zinc amalgam supplies electrons at the metal surface.
- The C=O is reduced in the acid medium.
- Water is lost and the CH₂ compound forms.
What it makes
Alkanes from aldehydes and ketones; straight-chain alkylbenzenes from acylbenzenes.
Where it fails
The substrate must tolerate strong acid. For acid-sensitive molecules use the Wolff-Kishner reduction.
Class 12, Chapter 8 Aldehydes, Ketones and Carboxylic Acids
Clemmensen reduction: quick answers
When do you choose Clemmensen over Wolff-Kishner?
When the compound is stable in acid but sensitive to base.
Why is this useful with Friedel-Crafts?
Acylation then Clemmensen puts an unrearranged straight chain on the ring.
What is Zn-Hg?
Zinc amalgam.