What happens when a mixture of carbon monoxide and hydrogen chloride is passed through benzene in the presence of anhydrous aluminium chloride containing a trace of cuprous chloride?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Benzaldehyde is produced
This is the Gatterman-Koch reaction. Carbon monoxide and hydrogen chloride, with anhydrous \( \ce{AlCl3} \) and a trace of \( \ce{CuCl} \), put a –CHO group on the benzene ring:
\( \displaystyle \ce{C6H6 + CO} \xrightarrow[\text{anhyd. }\ce{AlCl3}/\ce{CuCl}]{\ce{HCl}} \ce{C6H5CHO} \)
Why the other options are incorrect
Option (2) Benzophenone is produced — Incorrect. Benzophenone, \( \ce{C6H5COC6H5} \), is a ketone; it is made by Friedel-Crafts acylation of benzene with benzoyl chloride, not from CO and HCl.
Option (3) Acetophenone is produced — Incorrect. Acetophenone, \( \ce{C6H5COCH3} \), needs an acetyl group; it comes from Friedel-Crafts acylation with ethanoyl chloride and anhydrous \( \ce{AlCl3} \).
Option (4) Benzoyl chloride is produced — Incorrect. CO and HCl add only a formyl (–CHO) group to the ring; no acyl chloride is formed.
NCERT (2026-27 edition):
8.2.2 Preparation of Aldehydes – “(iii) By Gatterman – Koch reaction
When benzene or its derivative is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride, it gives benzaldehyde or substituted benzaldehyde.”