Electrophilic aromatic substitution
Gattermann-Koch formylation
How the Gattermann-Koch formylation works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: CO, HCl, AlCl₃ / CuCl. Conditions: Benzene or alkylbenzene, high CO pressure, 25–50 °C. Bonds formed: Ar–CHO (C–C σ); bonds broken: Ar–H; H–Cl. Stereochemistry: No stereocentre; para to alkyl groups. Rate and selectivity: Limited to benzene/alkylbenzenes; not for phenols or deactivated rings.
At a glance
Step 1: CO + HCl → formyl cation
CO and HCl combine under AlCl₃/CuCl catalysis to generate a formyl cation (HC≡O⁺) — the same acylium-type electrophile as Friedel-Crafts acylation, just built from carbon monoxide instead of an acid chloride.
Step 2: Ring π attacks HC≡O⁺
The ring's π electrons attack this formyl cation, forming the familiar arenium ion intermediate.
Step 3: Loss of H⁺ gives benzaldehyde
Loss of a proton restores aromaticity, installing a CHO group directly on the ring — a rare way to formylate benzene, since simple aldehydes can't otherwise act as Friedel-Crafts electrophiles.
Abbreviations and terms
- Ar
- Aryl group. Any aromatic ring substituent such as phenyl.
- AlCl3
- Aluminium chloride. Strong Lewis acid for Friedel–Crafts reactions.
- HCl
- Hydrochloric acid / hydrogen chloride. Strong acid; chloride is the nucleophile in additions/substitutions.
- CO
- Carbon monoxide (carbonyl ligand). σ-donor / strong π-acceptor ligand; also a C1 building block.
- Wheland
- Wheland intermediate (arenium ion). The cyclohexadienyl cation formed when an electrophile adds to a benzene ring.
How it compares with related reactions
| Gattermann-Koch formylation | Bromination of benzene (FeBr₃-catalyzed) | Duff reaction | |
|---|---|---|---|
| Reagents | CO, HCl, AlCl₃ / CuCl | Br₂ with FeBr₃ (Lewis-acid catalyst) | Hexamethylenetetramine (HMTA), acid (AcOH/glycerol or TFA); then H₃O⁺ |
| Conditions | Benzene or alkylbenzene, high CO pressure, 25–50 °C | Anhydrous, 25 °C, dark | Heat, 100–150 °C; aqueous hydrolysis |
| Stereochemistry | No stereocentre; para to alkyl groups | None (planar, achiral) | Ortho-selective (OH directs; H-bonded iminium) |
| Rate / selectivity | Limited to benzene/alkylbenzenes; not for phenols or deactivated rings | Slow step = arenium formation; deactivated rings react poorly | Moderate; electron-rich phenols only |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Gattermann-Koch formylation?
CO, HCl, AlCl₃ / CuCl. Conditions: Benzene or alkylbenzene, high CO pressure, 25–50 °C.
Which bonds are formed and which are broken?
Formed: Ar–CHO (C–C σ). Broken: Ar–H; H–Cl.
What is the stereochemical outcome?
No stereocentre; para to alkyl groups
What controls the rate or the selectivity?
Limited to benzene/alkylbenzenes; not for phenols or deactivated rings
Study guides that help with this reaction
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go
- How to answer mechanism questions in an exam