Question
What is the Wurtz reaction? How does the Wurtz-Fittig reaction differ from it? Give the mechanism and discuss the limitations of the Wurtz reaction.
(NCERT Class 12, Chapter 10)
Solution — Step by Step
The Wurtz reaction couples two alkyl halides in the presence of sodium metal in dry ether to form a higher alkane:
Example:
Two ethyl bromide molecules join to form butane.
The reaction proceeds via an organosodium intermediate:
- Na metal donates an electron to R-X, forming a radical R and NaX
- The radical R reacts with another Na to form R-Na (organosodium)
- R-Na attacks another molecule of R-X in an S2 fashion: R-Na + R-X → R-R + NaX
The coupling happens between the organosodium compound and the alkyl halide.
This is a modification where one alkyl halide and one aryl halide react with Na:
Example:
Chlorobenzene + methyl chloride → toluene. The aryl and alkyl groups are joined.
- Mixture of products with two different alkyl halides: R-R, R’-R’, and R-R’ all form, making separation difficult.
- Methane cannot be prepared — you would need two “half-carbon” fragments, which is impossible.
- Works best only with primary alkyl halides. Secondary and tertiary halides prefer elimination (giving alkenes) over coupling.
- Aryl halides alone do not undergo the Wurtz reaction — they need the Fittig modification.
Why This Works
Sodium is a strong reducing agent that strips the halogen from the alkyl halide, generating a highly reactive organosodium species. This species is nucleophilic enough to attack another alkyl halide, forming the C-C bond. The reaction works in dry ether because ether is an inert solvent that doesn’t react with sodium.
The Fittig modification works because aryl halides are less reactive than alkyl halides towards Na. The alkyl halide forms the organosodium first, which then attacks the aryl halide.
Alternative Method — Using Organolithium (Corey-House Synthesis)
For selective cross-coupling without mixtures, the Corey-House synthesis is better: R-Li + CuI → RCuLi, then RCuLi + R’-X → R-R’. This gives only the desired cross-coupled product.
Board exam trick: “Prepare ethane from methyl bromide.” Answer: Wurtz reaction — 2\text{CH}_3\text{Br} + 2\text{Na} \to \text{C}_2\text{H}_6} + 2\text{NaBr}. This is a 1-mark question that appears almost every year in CBSE.
Common Mistake
Students attempt to prepare an odd-carbon alkane (like propane) from two different alkyl halides using the Wurtz reaction. This gives a mixture of three products (R-R, R’-R’, and R-R’), not a single product. The Wurtz reaction gives a clean product only when both alkyl halides are the same (symmetric coupling).