Alkyl halides react with sodium to give alkanes with double number of carbon atoms in
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Wurtz reaction
Three named reactions join two carbon groups by heating halides with sodium in dry ether; they differ in what is joined:
Wurtz reaction: two alkyl halides give an alkane with double the number of carbon atoms: \( \ce{2R-X + 2Na -> R-R + 2NaX} \)
Fittig reaction: two aryl halides give a diaryl: \( \ce{2Ar-X + 2Na -> Ar-Ar + 2NaX} \)
Wurtz-Fittig reaction: an alkyl halide and an aryl halide give an alkylarene: \( \ce{Ar-X + R-X + 2Na -> Ar-R + 2NaX} \)
For example, methyl bromide gives ethane:
\( \displaystyle \ce{2CH3Br + 2Na} \xrightarrow{\text{dry ether}} \ce{CH3-CH3 + 2NaBr} \)
The ether must be dry because sodium reacts with water. So the reaction described is the Wurtz reaction.
Why the other options are incorrect
Option (1) Sandmeyer’s reaction — Incorrect. The Sandmeyer reaction replaces the diazonium group of an arenediazonium salt by a halogen using copper(I) halide: \( \ce{ArN2^+Cl^-} \xrightarrow{\ce{Cu2Cl2/HCl}} \ce{ArCl + N2} \). No sodium and no alkane are involved.
Option (3) Wurtz-Fittig reaction — Incorrect. It couples an alkyl halide with an aryl halide, giving an alkylbenzene such as toluene, not an alkane with double the carbon atoms.
Option (4) Williamson synthesis — Incorrect. An alkyl halide reacts with a sodium alkoxide, not sodium metal, and gives an ether: \( \ce{R-X + R'-ONa -> R-O-R' + NaX} \).
NCERT (2026-27 edition):
Wurtz Reaction
