What is it?

Introduction of a formyl group (–CHO) into an aromatic ring using:

CO + HCl in presence of Aluminium chloride and Cuprous chloride

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Mechanism (Stepwise)

Formation of Electrophile

• CO + HCl react in presence of AlCl₃/CuCl
• Generate formyl cation

$$CO + HCl \rightarrow HCO^+ \ (\text{electrophile})$$

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Electrophilic Attack (EAS Step)

• Benzene ring attacks HCO⁺
• Forms sigma complex (arenium ion)

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Deprotonation

• Loss of H⁺ restores aromaticity
• Forms benzaldehyde complex

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Hydrolysis

• Final hydrolysis gives aldehyde (–CHO)

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Key Points for Exams

✔ Type of Reaction

Electrophilic Aromatic Substitution (EAS)

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✔ Important Conditions

• CO must be dry
• Presence of both AlCl₃ and CuCl is essential

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❌ Limitations

• Does NOT work with strongly deactivated rings
  (e.g., nitrobenzene ❌)

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🔁 Comparison Trick

Reaction	Reagent	Product
Gattermann–Koch	CO + HCl	–CHO
Friedel–Crafts	R–Cl / RCOCl	–R / –COR

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🔹 One-Line Revision

Gattermann–Koch introduces –CHO on benzene via formyl cation (HCO⁺) using CO + HCl / AlCl₃–CuCl