More practice questions

3. Q3. Which reagent would you use to turn benzenediazonium chloride into (a) fluorobenzene, (b) bromobenzene, (c) nitrobenzene?

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   (a) Fluoroboric acid, then heat the precipitated fluoroborate. (b) Cu₂Br₂/HBr (Sandmeyer), or copper powder and HBr (Gattermann). (c) Make the fluoroborate with HBF₄, then heat it with aqueous NaNO₂ in the presence of copper (NCERT pages 275–276).

4. Q4. What do ethanol and hypophosphorous acid become when they reduce benzenediazonium chloride?

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   Ethanol is oxidised to ethanal, CH₃CHO, and hypophosphorous acid to phosphorous acid, H₃PO₃. Both reactions also give benzene, nitrogen and HCl (NCERT pages 275–276).

5. Q5. Why is coupling of benzenediazonium chloride with phenol called electrophilic substitution?

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   The diazonium ion is the electrophile: it replaces a hydrogen at the para position of the phenol ring, which the –OH group activates (NCERT page 276; reasoned).

6. Q6. Show with oxidation numbers that hypophosphorous acid, H₃PO₂, is oxidised when it reduces benzenediazonium chloride to benzene.

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   It becomes phosphorous acid, H₃PO₃ (NCERT page 275). In H₃PO₂, 3(+1) + x + 2(−2) = 0, so phosphorus is +1; in H₃PO₃, 3(+1) + x + 3(−2) = 0, so it is +3. The oxidation number of phosphorus rises by 2, so H₃PO₂ is oxidised, while the diazonium ion is reduced: –N₂⁺ is replaced by H (reasoned).