The Gattermann–Koch reaction is an electrophilic aromatic substitution reaction used to introduce a formyl group (–CHO) into a suitable aromatic ring. The aromatic substrate is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminum chloride and copper(I) chloride. The reaction produces an aromatic aldehyde. ^[1–4]

The reaction is named after Ludwig Gattermann and Julius Arnold Koch, who first reported it in 1897. It works best with benzene, alkylbenzenes, and other sufficiently reactive arenes; strongly deactivated aromatic rings generally react poorly.

General Reaction

The general reaction can be represented as: ^[1–3]

Ar–H + CO –[HCl , anhydrous AlCl₃, CuCl ]→ Ar–CHO

where: 

Ar–H = aromatic compound

Ar–CHO = corresponding aromatic aldehyde 

A formyl group replaces a hydrogen atom on the aromatic ring.

The reaction is generally carried out under anhydrous conditions because aluminum chloride reacts readily with water. Carbon monoxide is commonly supplied under pressure. Copper(I) chloride acts as a co-catalyst, facilitating the formation of the reactive formylating species.

Example

Benzene reacts with carbon monoxide in the presence of hydrogen chloride, anhydrous aluminum chloride, and copper(I) chloride to form benzaldehyde. ^[1–4]

C₆H₆ + CO –[HCl , anhydrous AlCl₃, CuCl ]→ C₆H₅–CHO

In this reaction, one hydrogen atom of benzene is replaced by a formyl group, producing benzaldehyde.

Mechanism

The Gattermann–Koch reaction can be explained using a simplified electrophilic aromatic substitution mechanism. ^[2–4]

1. Formation of the formyl electrophile: Carbon monoxide is activated by the HCl–AlCl₃–CuCl system, producing a reactive formylating electrophile commonly represented as the formyl cation, HCO⁺. It can be represented by the resonance forms H–C≡O⁺ and H–C⁺=O.
2. Attack by the aromatic ring: The π electrons of benzene attack the electrophilic carbon, forming a carbon–carbon bond and producing an arenium ion, also called a sigma complex.
3. Restoration of aromaticity: AlCl₄^– removes a proton from the substituted carbon. The electrons from the C–H bond restore the aromatic ring, producing the aromatic aldehyde while regenerating HCl and AlCl₃.

The Gattermann–Koch reaction provides a direct method for introducing a formyl group into suitable aromatic rings. The resulting aromatic aldehydes are valuable intermediates in the synthesis of pharmaceuticals, dyes, fragrances, and other organic compounds.