Question

Benzene reacts with a mixture of carbon monoxide (\( \text{CO} \)) and hydrogen chloride (\( \text{HCl} \)) gas in the presence of an anhydrous aluminium chloride (\( \text{AlCl}_3 \)) catalyst to yield Benzaldehyde. This organic transformation is officially known as the:

• A. Rosenmund Reduction
• B. Gattermann-Koch Reaction
• C. Etard Reaction
• D. Stephen Reaction

Hint:

To remember the Gattermann-Koch reaction easily, connect the word "Koch" to the formula of its core reagents: Carbon monoxide + hydrogen chloride can be read phonetically as "C-O-H-Cl" (similar to Koch).

Answer 1

The Correct Option is B

Concept: Formylation is an electrophilic aromatic substitution reaction used to introduce a formyl group (\(-\text{CHO}\)) directly into an activated aromatic benzene ring core.

Step 1: Identify the active reagents and chemical conversion.
The reaction uses a mixture of high-pressure \( \text{CO} \) and \( \text{HCl} \) gas, which reacts in situ under the influence of a Lewis acid catalyst (\(\text{AlCl}_3\)) to form a reactive formyl cation intermediate (\([\text{HC}=\text{O}]^+\)). This intermediate attacks the benzene ring to yield 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 specific synthesis pathway is called the Gattermann-Koch Reaction.

Step 2: Differentiate from alternative aldehyde name reactions.

• Rosenmund Reduction: Converts an acyl chloride into an aldehyde using hydrogen gas over a poisoned palladium catalyst (\(\text{H}_2/\text{Pd}-\text{BaSO}_4\)).
• Etard Reaction: Oxidizes toluene to benzaldehyde using chromyl chloride (\(\text{CrO}_2\text{Cl}_2\)).
• Stephen Reaction: Reduces a nitrile to an aldehyde using tin(II) chloride and hydrochloric acid (\(\text{SnCl}_2/\text{HCl}\)).