The Gatterman-Koch reaction is used in the industrial preparation of benzaldehyde. The electrophile involved in this reaction is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → HCO+
In the Gattermann–Koch reaction, benzene is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride, and benzaldehyde is formed.
It is an electrophilic substitution. CO and HCl combine (as formyl chloride, HCOCl), and the Lewis acid \( \ce{AlCl3} \) removes \( \ce{Cl-} \) from it to give the formyl cation, which is the electrophile:
\( \displaystyle \ce{CO + HCl + AlCl3 -> HCO+ + AlCl4-} \)
The formyl cation attacks the benzene ring; loss of \( \ce{H+} \) gives benzaldehyde and regenerates the catalyst. So the electrophile is HCO+.
Why the other options are incorrect
Option (1) CO+ — Incorrect. Carbon monoxide is not ionised to \( \ce{CO+} \) in this reaction; it must first combine with HCl to give the HCO group that is put on the ring.
Option (2) HCl + CO2 + anhydrous AlCl3 — Incorrect. The reagent is carbon monoxide, not carbon dioxide; and a mixture of reagents is not the electrophile itself.
Option (4) CO + anhydrous AlCl3 — Incorrect. These are part of the reagents (HCl is missing), not the species that attacks the ring.
NCERT (2026-27 edition):
By Gatterman – Koch Reaction
