Benzenediazonium chloride is treated separately with each of the following reagents. In how many cases is an aryl halide obtained as the main organic product?
(i) \( \ce{Cu2Cl2} \)/HCl
(ii) \( \ce{Cu2Br2} \)/HBr
(iii) CuCN/KCN
(iv) Copper powder and HBr
(v) Potassium iodide
(vi) Hypophosphorous acid (phosphinic acid) and water
(vii) Water, with the temperature allowed to rise up to 283 K
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 4
The diazonium group is a very good leaving group. In each of these reactions it is replaced by another group and nitrogen escapes as a gas. The product depends on the group that comes in; an aryl halide is formed only when a halogen comes in:
\( \displaystyle \ce{C6H5N2^+Cl^-} \xrightarrow{\ce{Cu2Cl2/HCl}} \ce{C6H5Cl + N2 ^} \)
\( \displaystyle \ce{C6H5N2^+Cl^-} \xrightarrow{\ce{Cu2Br2/HBr}} \ce{C6H5Br + N2 ^} \)
\( \displaystyle \ce{C6H5N2^+Cl^-} \xrightarrow{\ce{Cu/HBr}} \ce{C6H5Br + N2 ^} \)
\( \displaystyle \ce{C6H5N2^+Cl^- + KI -> C6H5I + KCl + N2 ^} \)
Aryl halide:
(i) \( \ce{Cu2Cl2} \)/HCl (chlorobenzene, Sandmeyer reaction)
(ii) \( \ce{Cu2Br2} \)/HBr (bromobenzene, Sandmeyer reaction)
(iv) Copper powder and HBr (bromobenzene, Gattermann reaction)
(v) Potassium iodide (iodobenzene)
No aryl halide:
(iii) CuCN/KCN — the cyano group comes in; benzonitrile is formed.
(vi) Hypophosphorous acid and water — this mild reducing agent gives benzene.
(vii) Water at 283 K — the salt is hydrolysed to phenol.
Aryl halide: 4 cases.
NCERT (2026-27 edition):
9.9 Chemical Reactions – “1. Replacement by halide or cyanide ion: The Cl–, Br– and CN– nucleophiles can easily be introduced in the benzene ring in the presence of Cu(I) ion. This reaction is called Sandmeyer reaction.”
“Alternatively, chlorine or bromine can also be introduced in the benzene ring by treating the diazonium salt solution with corresponding halogen acid in the presence of copper powder. This is referred as Gatterman reaction.”
“2. Replacement by iodide ion: Iodine is not easily introduced into the benzene ring directly, but, when the diazonium salt solution is treated with potassium iodide, iodobenzene is formed.”
“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.”
“5. Replacement by hydroxyl group: If the temperature of the diazonium salt solution is allowed to rise upto 283 K, the salt gets hydrolysed to phenol.”