Question

Which of the following is NOT obtained when a mixture of bromomethane and bromoethane is treated with sodium in dry ether?

• A. Propane
• B. Butane
• C. Methane
• D. Ethane

Hint:

The Wurtz reaction is strictly a coupling reaction that links two alkyl fragments together by forming a new carbon-carbon bond. Because it joins two carbon chains together, it is completely impossible to synthesize methane ($\text{CH}_4$), which contains only one single carbon atom, using this method.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks us to identify which of the listed alkanes cannot be produced when a mixture of two different alkyl halides (bromomethane and bromoethane) reacts with metallic sodium in dry ether.

Step 2: Key Formula or Approach:
This reaction is a classic example of the Wurtz reaction. When a mixture of two different alkyl halides (R-X and R'-X) is treated with sodium in dry ether, a mixture of three different alkanes is formed due to both self-coupling and cross-coupling of the intermediate alkyl free radicals: $$\text{R-X} + \text{R'-X} \xrightarrow{\text{Na / Dry Ether}} \text{R-R} + \text{R-R'} + \text{R'-R'}$$ The smallest alkane that can be formed via this radical mechanism must contain at least two carbon atoms.

Step 3: Detailed Explanation:
In the given mixture, we have bromomethane ($\text{CH}_3\text{Br}$) and bromoethane ($\text{CH}_3\text{CH}_2\text{Br}$). When treated with sodium, two distinct types of alkyl free radicals are generated in the solution: methyl radicals ($\text{CH}_3^{\bullet}$) and ethyl radicals ($\text{CH}_3\text{CH}_2^{\bullet}$). These radicals combine in three possible ways: 1.

Self-coupling of methyl radicals yields ethane: $$\text{CH}_3^{\bullet} + \text{CH}_3^{\bullet} \rightarrow \text{CH}_3\text{-CH}_3$$ 2.

Self-coupling of ethyl radicals yields butane: $$\text{CH}_3\text{CH}_2^{\bullet} + \text{CH}_3\text{CH}_2^{\bullet} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3$$ 3.

Cross-coupling between a methyl and an ethyl radical yields propane: $$\text{CH}_3^{\bullet} + \text{CH}_3\text{CH}_2^{\bullet} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_3$$ Methane ($\text{CH}_4$), being a single-carbon alkane, requires a combination of a methyl radical with a hydrogen atom, which does not occur as a primary pathway in the coupling mechanism of the Wurtz reaction. Therefore, methane cannot be obtained.

Step 4: Final Answer:
The alkane that is NOT obtained is methane, which corresponds to option (C).