Question

What are the primary reagents and catalysts required to carry out the Gattermann-Koch reaction?

• A. $\text{CO, HCl}$, and anhydrous $\text{AlCl}_3\text{ / CuCl}$
• B. $\text{CHCl}_3$ and aqueous $\text{NaOH}$
• C. $\text{HCN, HCl}$, and anhydrous $\text{AlCl}_3$
• D. $\text{CH}_3\text{Cl}$ and anhydrous $\text{AlCl}_3$

Hint:

Distinction: Gattermann-Koch uses $\text{CO} + \text{HCl}$, whereas the standard Gattermann formylation utilizes $\text{HCN} + \text{HCl}$.

Answer 1

The Correct Option is A

Concept: The Gattermann-Koch reaction is a classic organic named reaction used to introduce a formyl functional group ($-\text{CHO}$) directly onto an aromatic ring. This transformation converts benzene or activated benzene derivatives into aromatic aldehydes. Mechanistically, it operates as a variation of a Friedel-Crafts acylation, requiring a gaseous source to generate a highly reactive, short-lived formyl cation intermediate ($[\text{HC}=\text{O}]^+$).

Step 1: Understanding the generation of the active electrophilic formyl intermediate.
Formyl chloride ($\text{HCOCl}$) is highly unstable and decomposes rapidly at room temperature, meaning it cannot be stored or added directly to a reaction flask. To circumvent this, the Gattermann-Koch reaction generates the necessary electrophilic species in situ. A gaseous mixture of carbon monoxide ($\text{CO}$) and hydrogen chloride ($\text{HCl}$) is bubbled into the reaction mixture. In the presence of a strong Lewis acid catalyst like anhydrous aluminum chloride ($\text{AlCl}_3$), these components interact to form the transient formyl electrophile.

Step 2: Evaluating the role of the co-catalyst during aromatic substitution.
Because carbon monoxide reactions can be slow under standard conditions, cuprous chloride ($\text{CuCl}$) is added as a vital co-catalyst. The copper(I) center binds temporarily to carbon monoxide, increasing its local concentration and facilitating its reaction with $\text{HCl}$ and $\text{AlCl}_3$. The resulting formyl cation undergoes an electrophilic aromatic substitution attack on the benzene ring, yielding benzaldehyde: \[ \text{C}_6\text{H}_6 + \text{CO} + \text{HCl} \xrightarrow{\text{anhydrous AlCl}_3 / \text{CuCl}} \text{C}_6\text{H}_5\text{CHO} + \text{HCl} \] This confirms option (A) as the correct reagent combination.