Only bromobenzene and chlorobenzene can be prepared by Sandmeyer’s reaction. Fluoro-, iodo-, and astatobenzene cannot be synthesized using this method.
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Step 1: Recall the concept of Sandmeyer’s reaction
Sandmeyer’s reaction involves the conversion of an aryl diazonium salt into an aryl halide using cuprous salts. It is mainly applicable for introducing Cl, Br, and CN groups on an aromatic ring. The general form is:
\[
C_6H_5N_2^+Cl^- \xrightarrow{CuCl} C_6H_5Cl, \quad
C_6H_5N_2^+Br^- \xrightarrow{CuBr} C_6H_5Br.
\]
This reaction proceeds smoothly with CuCl and CuBr but not with CuF or CuI.
Step 2: Analyze each compound
I. Fluorobenzene (C6H5F): Cannot be prepared by Sandmeyer’s reaction because CuF is unstable. It is prepared by the Balz–Schiemann reaction (using BF₄⁻ salts).
II. Chlorobenzene (C6H5Cl): Formed by Sandmeyer’s reaction using CuCl.
III. Bromobenzene (C6H5Br): Formed by Sandmeyer’s reaction using CuBr.
IV. Iodobenzene (C6H5I): Formed by simply treating diazonium salt with KI (not Sandmeyer’s reaction since no copper salt is used).
V. Atodobenzene (C6H5At): Not practical and not prepared by Sandmeyer’s method.
Step 3: Count the applicable halobenzenes
Only Chlorobenzene (II) and Bromobenzene (III) are prepared by Sandmeyer’s reaction.
