Chlorobenzene can be obtained from benzenediazonium chloride either by the Sandmeyer reaction or by the Gattermann reaction. Which of the following is correct?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The Sandmeyer reaction uses Cu(I) ion, and its yield is better than that of the Gattermann reaction.
Both reactions replace the diazonium group by chlorine (or bromine). They differ in the form of copper used.
In the Sandmeyer reaction, Cl− is introduced in the presence of Cu(I) ion, as copper(I) chloride in HCl:
\( \displaystyle \ce{C6H5N2^+Cl^-} \xrightarrow{\ce{Cu2Cl2/HCl}} \ce{C6H5Cl + N2 ^} \)
In the Gattermann reaction, the diazonium salt solution is treated with the halogen acid in the presence of copper powder:
\( \displaystyle \ce{C6H5N2^+Cl^-} \xrightarrow{\ce{Cu/HCl}} \ce{C6H5Cl + N2 ^} \)
The yield is better in the Sandmeyer reaction.
Why the other options are incorrect
Option (1) The Gattermann reaction uses Cu(I) ion, and its yield is better than that of the Sandmeyer reaction. — Incorrect. Both parts are reversed: Cu(I) ion belongs to the Sandmeyer reaction, and the Gattermann reaction gives the lower yield.
Option (2) The Sandmeyer reaction uses hydrochloric acid with copper powder, and its yield is better than that of the Gattermann reaction. — Incorrect. The yield part is right, but copper powder with the halogen acid is the Gattermann reagent, not the Sandmeyer reagent.
Option (3) The Gattermann reaction uses hydrochloric acid with copper powder, and its yield is better than that of the Sandmeyer reaction. — Incorrect. The reagent part is right, but the Gattermann reaction gives a lower yield than the Sandmeyer reaction.
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.”
“The yield in Sandmeyer reaction is found to be better than Gattermann reaction.”