Consider the following statements about the preparation of halogen compounds.
(A) Alkyl iodides are often prepared by the reaction of alkyl chlorides or bromides with NaI in dry acetone.
(B) In the Finkelstein reaction, the NaCl or NaBr formed dissolves in dry acetone, and this facilitates the forward reaction.
(C) Alkyl fluorides are best prepared by heating an alkyl chloride or bromide in the presence of a metallic fluoride such as AgF, Hg2F2, CoF2 or SbF3.
(D) Replacement of the diazonium group of a diazonium salt by iodine requires cuprous iodide, just as replacement by chlorine requires cuprous chloride.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A) and (C) only
In halogen exchange, the halogen of an alkyl chloride or bromide is replaced by iodine or fluorine. Aryl iodides, on the other hand, are made from diazonium salts.
(A) Alkyl iodides are often prepared by the reaction of alkyl chlorides or bromides with NaI in dry acetone. — True. This is the Finkelstein reaction:
\( \displaystyle \ce{R-X + NaI} \xrightarrow{\text{dry acetone}} \ce{R-I + NaX} \qquad (\ce{X = Cl, Br}) \)
(B) In the Finkelstein reaction, the NaCl or NaBr formed dissolves in dry acetone, and this facilitates the forward reaction. — False. The NaCl or NaBr is precipitated in dry acetone. Its removal from the solution pushes the equilibrium forward, by Le Chatelier’s principle.
(C) Alkyl fluorides are best prepared by heating an alkyl chloride or bromide in the presence of a metallic fluoride such as AgF, Hg2F2, CoF2 or SbF3. — True. This is the Swarts reaction, for example:
\( \displaystyle \ce{CH3Br + AgF} \xrightarrow{\Delta} \ce{CH3F + AgBr} \)
(D) Replacement of the diazonium group of a diazonium salt by iodine requires cuprous iodide, just as replacement by chlorine requires cuprous chloride. — False. Cuprous chloride or bromide is needed for –Cl or –Br (Sandmeyer’s reaction), but iodine is introduced without any cuprous halide, by shaking the diazonium salt with potassium iodide:
\( \displaystyle \ce{C6H5N2+Cl- + KI -> C6H5I + KCl + N2 ^} \)
NCERT (2026-27 edition):
6.4.3 Halogen Exchange – “Alkyl iodides are often prepared by the reaction of alkyl chlorides/ bromides with NaI in dry acetone. This reaction is known as Finkelstein reaction.”
“NaCl or NaBr thus formed is precipitated in dry acetone. It facilitates the forward reaction according to Le Chatelier’s Principle.”
“The synthesis of alkyl fluorides is best accomplished by heating an alkyl chloride/bromide in the presence of a metallic fluoride such as AgF, Hg2F2, CoF2 or SbF3. The reaction is termed as Swarts reaction.”
6.5 Preparation of Haloarenes – “Replacement of the diazonium group by iodine does not require the presence of cuprous halide and is done simply by shaking the diazonium salt with potassium iodide.”