Step 1: Understanding the Concept:
The Gatterman-Koch reaction is an electrophilic aromatic substitution reaction utilized to introduce a formyl group ($-\text{CHO}$) onto an aromatic ring, effectively synthesizing aromatic aldehydes from arenes.
Step 2: Key Formula or Approach:
Approach: Recall the specific reagents and catalyst required for the Gatterman-Koch reaction and distinguish them from reagents used in other named reactions (like Étard or DIBAL-H reductions).
Step 3: Detailed Explanation:
In the Gatterman-Koch reaction, benzene or its derivative is treated with carbon monoxide ($\text{CO}$) and hydrogen chloride ($\text{HCl}$) in the presence of a Lewis acid catalyst such as anhydrous aluminum chloride ($\text{AlCl}_3$) and a trace of cuprous chloride ($\text{CuCl}$).
The reagents $\text{CO}$ and $\text{HCl}$ act together to generate the reactive formyl cation ($\text{CHO}^+$) in situ, which subsequently attacks the aromatic ring.
Evaluating the other options:
- $\text{AlH}(\text{i}-\text{Bu})_2$ and $\text{DIBAl}-\text{H}$ (options A and D are the same reagent) are reducing agents used to convert esters or nitriles to aldehydes.
- $\text{CrO}_2\text{Cl}_2$ in $\text{CS}_2$ (option C) is the reagent for the Étard reaction, which oxidizes methyl groups on aromatic rings to aldehydes.
Step 4: Final Answer:
The correct reagent combination is $\text{CO}, \text{HCl} \, (\text{anhyd. AlCl}_3)$.