Question

Which of the following is NOT obtained when a mixture of methyl chloride and n-propyl chloride is treated with sodium metal in dry ether?

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

Hint:

Logic Tip: The Wurtz reaction of mixed alkyl halides with $n$ carbons and $m$ carbons will produce alkanes with $2n$, $2m$, and $n+m$ carbons. Here $n=1$ and $m=3$. The products will have 2, 6, and 4 carbons. Propane has 3 carbons, so it's not a product!

Answer 1

The Correct Option is C

Concept:
This reaction is known as the Wurtz reaction. When a mixture of two different alkyl halides ($R-X$ and $R'-X$) is treated with sodium metal in dry ether, a mixture of three alkanes is generally formed through the coupling of the generated alkyl radicals: $$R-X + 2Na + X-R' \xrightarrow{\text{dry ether R-R + R'-R' + R-R' + 2NaX$$
Step 1: Identify the reacting alkyl halides and their corresponding radicals.
The given reactants are: 1. Methyl chloride ($CH_3Cl$) $\rightarrow$ produces methyl radical ($CH_3^\bullet$) 2. n-propyl chloride ($CH_3CH_2CH_2Cl$) $\rightarrow$ produces n-propyl radical ($CH_3CH_2CH_2^\bullet$)
Step 2: Determine the possible coupling products.
The radicals will couple with themselves and with each other to form alkanes:

• Self-coupling of methyl radicals: $$CH_3^\bullet + CH_3^\bullet \rightarrow CH_3-CH_3 \quad (\text{Ethane})$$
• Self-coupling of n-propyl radicals: $$CH_3CH_2CH_2^\bullet + CH_3CH_2CH_2^\bullet \rightarrow CH_3CH_2CH_2-CH_2CH_2CH_3 \quad (\text{Hexane})$$
• Cross-coupling of methyl and n-propyl radicals: $$CH_3^\bullet + CH_3CH_2CH_2^\bullet \rightarrow CH_3-CH_2CH_2CH_3 \quad (\text{Butane})$$

Step 3: Identify the missing product.
The primary products formed by the coupling reactions are Ethane, Hexane, and Butane. Propane ($CH_3CH_2CH_3$) contains 3 carbon atoms and cannot be formed by coupling a 1-carbon radical with another 1-carbon radical, a 3-carbon radical with a 3-carbon radical, or a 1-carbon radical with a 3-carbon radical. Thus, Propane is NOT obtained.