Consider the following statements about the reactions of benzenediazonium chloride.
(A) Chlorine or bromine can be introduced in the benzene ring by treating the diazonium salt solution with the corresponding halogen acid in the presence of copper powder.
(B) The yield in the Gattermann reaction is found to be better than that in the Sandmeyer reaction.
(C) Hypophosphorous acid reduces the diazonium salt to benzene and is itself oxidised to phosphorous acid.
(D) If the temperature of the diazonium salt solution is allowed to rise up to 283 K, the salt gets hydrolysed to phenol.
(E) Ethanol reduces the diazonium salt to benzene and is itself oxidised to ethanoic acid.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (B) and (E) only
In these reactions the diazonium group is displaced by another atom or group and nitrogen escapes as a gas. The reagent decides what takes its place.
(A) Chlorine or bromine can be introduced in the benzene ring by treating the diazonium salt solution with the corresponding halogen acid in the presence of copper powder. — Correct statement. This is the Gattermann reaction:
\( \displaystyle \ce{C6H5N2^+Cl^-} \xrightarrow{\ce{Cu/HCl}} \ce{C6H5Cl + N2 ^} \)
(B) The yield in the Gattermann reaction is found to be better than that in the Sandmeyer reaction. — Incorrect statement. It is the other way round: the yield in the Sandmeyer reaction (Cu(I) salt with the halogen acid) is better than in the Gattermann reaction.
(C) Hypophosphorous acid reduces the diazonium salt to benzene and is itself oxidised to phosphorous acid. — Correct statement. H3PO2 is a mild reducing agent:
\( \displaystyle \ce{C6H5N2^+Cl^- + H3PO2 + H2O -> C6H6 + N2 ^ + H3PO3 + HCl} \)
(D) If the temperature of the diazonium salt solution is allowed to rise up to 283 K, the salt gets hydrolysed to phenol. — Correct statement.
\( \displaystyle \ce{C6H5N2^+Cl^- + H2O -> C6H5OH + N2 ^ + HCl} \)
(E) Ethanol reduces the diazonium salt to benzene and is itself oxidised to ethanoic acid. — Incorrect statement. Ethanol does reduce the salt to benzene, but it is oxidised only as far as ethanal:
\( \displaystyle \ce{C6H5N2^+Cl^- + CH3CH2OH -> C6H6 + N2 ^ + CH3CHO + HCl} \)
NCERT (2026-27 edition):
9.9 Chemical Reactions – “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.”
“The yield in Sandmeyer reaction is found to be better than Gattermann reaction.”
“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.”