Match List-I (reagent or condition used with benzenediazonium chloride) with List-II (product).
| List-I (Reagent or condition) | List-II (Product) |
|---|---|
| (A) Potassium iodide | (I) Benzene |
| (B) Hypophosphorous acid and water | (II) Phenol |
| (C) Fluoroboric acid, then heating the precipitate | (III) Iodobenzene |
| (D) Solution allowed to warm up to 283 K | (IV) Fluorobenzene |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A-III, B-I, C-IV, D-II
The diazonium group is a very good leaving group. In all four reactions it leaves as nitrogen gas and another group takes its place on the ring.
(A) Potassium iodide — This matches (III) Iodobenzene. Iodine is not easily put into the ring directly, but the iodide ion replaces the diazonium group:
\( \displaystyle \ce{C6H5N2^+Cl^- + KI -> C6H5I + KCl + N2 ^} \)
(B) Hypophosphorous acid and water — This matches (I) Benzene. \( \ce{H3PO2} \) is a mild reducing agent; it replaces the diazonium group by H and is itself oxidised to phosphorous acid:
\( \displaystyle \ce{C6H5N2^+Cl^- + H3PO2 + H2O -> C6H6 + N2 ^ + H3PO3 + HCl} \)
(C) Fluoroboric acid, then heating the precipitate — This matches (IV) Fluorobenzene. Benzenediazonium fluoroborate is precipitated first; on heating it decomposes to the aryl fluoride:
\( \displaystyle \ce{C6H5N2^+Cl^- + HBF4 -> C6H5N2^+BF4^- + HCl} \)
\( \displaystyle \ce{C6H5N2^+BF4^-} \xrightarrow{\Delta} \ce{C6H5F + BF3 + N2 ^} \)
(D) Solution allowed to warm up to 283 K — This matches (II) Phenol. The salt is hydrolysed by the water of the solution:
\( \displaystyle \ce{C6H5N2^+Cl^- + H2O -> C6H5OH + N2 ^ + HCl} \)
NCERT (2026-27 edition):
9.9 Chemical Reactions – “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.”
“3. Replacement by fluoride ion: When arenediazonium chloride is treated with fluoroboric acid, arene diazonium fluoroborate is precipitated which on heating decomposes to yield aryl fluoride.”
“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.”