On treatment with which of the following does benzenediazonium chloride give benzene?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → CH3CH2OH
The diazonium group is a very good leaving group. A mild reducing agent such as ethanol (or hypophosphorous acid) replaces it by hydrogen, giving the arene, while ethanol is itself oxidised to ethanal:
\( \displaystyle \ce{C6H5N2^+Cl^- + CH3CH2OH -> C6H6 + N2 ^ + CH3CHO + HCl} \)
Why the other options are incorrect
Option (1) LiAlH4 — Incorrect. Lithium aluminium hydride is a strong reducing agent used for groups such as –CN, –CONH2 and –COOH; it is not the reagent used to replace the diazonium group by H.
Option (3) CuCN — Incorrect. In the Sandmeyer reaction CuCN replaces the diazonium group by –CN, giving benzonitrile, \( \ce{C6H5CN} \), not benzene.
Option (4) Pyridine — Incorrect. Pyridine is a base, used for example to remove HCl in acylation; it is not a reducing agent and does not replace the diazonium group.
NCERT (2026-27 edition):
9.9 Chemical Reactions – “4. Replacement by H: Certain mild reducing agents like hypophosphorous acid (phosphinic acid) or ethanol reduce diazonium salts to arenes and themselves get oxidised to phosphorous acid and ethanal, respectively.”