Gattermann-Koch reaction
Benzene reacts with carbon monoxide and HCl in the presence of anhydrous AlCl3 and CuCl to give benzaldehyde.
BenzeneC₆H₆Arene - Step 1
CO + HCl, anhydrous AlCl₃ / CuCl
BenzaldehydeC₆H₅CHOAldehyde
How it works
- CO and HCl form the formyl chloride equivalent (HCOCl), which is not isolable.
- AlCl₃ turns it into the formyl cation HCO⁺.
- Electrophilic substitution on benzene gives benzaldehyde.
What it makes
Benzaldehyde and alkylbenzaldehydes.
Where it fails
It does not work on deactivated rings or on phenols and phenol ethers.
Class 12, Chapter 8 Aldehydes, Ketones and Carboxylic Acids
Gattermann-Koch reaction: quick answers
Why is formyl chloride not used directly?
It is unstable and cannot be stored.
What is the electrophile?
The formyl cation, HCO+.
Is this the same as Gattermann?
No. Gattermann replaces N2+ by Cl or Br; Gattermann-Koch formylates an arene.