Gattermann-Koch Reaction: Synthesis of Benzaldehyde
The Gattermann-Koch Reaction is an industrial method for the formylation of benzene and alkylbenzenes. It involves treating the aromatic compound with Carbon Monoxide ($CO$) and Hydrogen Chloride ($HCl$) in the presence of a Lewis acid catalyst ($AlCl_3$) and a trace of Copper(I) Chloride ($CuCl$) to produce Benzaldehyde or substituted benzaldehydes.
1. General Reaction
Benzene reacts with a mixture of CO and HCl to introduce a formyl group ($-CHO$) onto the ring.
Conditions:
- Reagents: Carbon Monoxide ($CO$) + Dry $HCl$ gas.
- Catalyst: Anhydrous Aluminium Chloride ($AlCl_3$).
- Co-Catalyst: Cuprous Chloride ($CuCl$) is essential (assists reaction with CO).
- Product: Aromatic Aldehyde.
2. Detailed Mechanism
The reaction is an example of Electrophilic Aromatic Substitution (EAS). The attacking electrophile is the Formyl Cation ($H-C^+=O$).
Step 1: Generation of Electrophile
Technically, Formyl Chloride ($HCOCl$) is unstable at room temperature. The reagents $CO$ and $HCl$ react in the presence of the catalyst to generate the active electrophile in situ.
Note: CuCl helps in absorbing Carbon Monoxide and facilitating its reaction.
Step 2: Electrophilic Attack
The Formyl cation attacks the electron-rich benzene ring to form a sigma complex (Wheland intermediate).
Step 3: Deprotonation
The base ($[AlCl_4]^-$) abstracts a proton from the intermediate to restore aromaticity, yielding Benzaldehyde and regenerating the catalyst.
3. Comparison: Gattermann vs. Gattermann-Koch
Don't Confuse Them!
- Gattermann-Koch: Uses CO + HCl. Product is Benzaldehyde. Catalyst needs $CuCl$.
- Gattermann Reaction: Uses HCN + HCl (Hydrogen Cyanide). Intermediate is an Aldimine ($Ar-CH=NH$), which is hydrolyzed to Benzaldehyde.
4. Limitations
- Not for Phenols/Anilines: This reaction is typically not applicable to Phenol or Aniline. The Lewis acid ($AlCl_3$) coordinates strongly with the lone pairs on Oxygen or Nitrogen, deactivating the ring or forming stable complexes that prevent reaction. (Gattermann reaction using HCN is preferred for phenols).
- Alkylbenzenes: For Toluene, the major product is p-Tolualdehyde due to steric hindrance at the ortho position.
5. Summary Table
| Feature | Reaction Detail |
|---|---|
| Substrate | Benzene, Alkylbenzenes |
| Reagents | $CO, HCl$ |
| Catalyst | $AlCl_3 + CuCl$ |
| Electrophile | Formyl Cation ($HCO^+$) |
Gattermann-Koch Quiz
Test your concepts on Formylation. 10 MCQs with explanations.
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