Bromoethane is heated with a reagent X to give fluoroethane. Which of the following can be X?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \( \ce{Hg2F2} \)
Fluoroethane is made from bromoethane by halogen exchange: the alkyl bromide is heated with a metallic fluoride such as AgF, Hg2F2, CoF2 or SbF3, and the bromine is replaced by fluorine. This is the Swarts reaction.
\( \displaystyle \ce{2C2H5Br + Hg2F2} \xrightarrow{\Delta} \ce{2C2H5F + Hg2Br2} \)
Why the other options are incorrect
Option (1) NaI in dry acetone — Incorrect. This exchanges the halogen for iodine (Finkelstein reaction) and gives iodoethane, not fluoroethane.
Option (2) Sodium in dry ether — Incorrect. Sodium joins two alkyl groups (Wurtz reaction), so bromoethane gives butane.
Option (4) \( \ce{Cu2Cl2} \) in HCl — Incorrect. This reagent replaces the diazonium group of an arenediazonium salt by chlorine (Sandmeyer reaction); it is not used to put fluorine into an alkyl halide.
NCERT (2026-27 edition):
6.4.3 Halogen Exchange – “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.”