Question

The alkane which is next to methane in the homologous series can be prepared from which of the following reactions? 

• A. I, IV only
• B. II, III only
• C. I, III only
• D. II, IV only

Hint:

The Wurtz reaction is a reliable method for synthesizing alkanes by coupling alkyl halides with sodium metal in dry ether. The reduction of alkyl halides using zinc and acid also effectively produces alkanes.

Answer 1

The Correct Option is A

Step 1: Understanding the homologous series

The homologous series of alkanes consists of compounds differing by a -CH₂- group. The alkane next to methane is ethane (C₂H₆).

Step 2: Evaluating the given reactions

1. Reaction I: Wurtz Reaction

2CH₃Br + 2Na → _{dry ether} C₂H₆ + 2NaBr

This reaction successfully forms ethane, making it a correct method.

2. Reaction II: Decarboxylation of Carboxylic Acids

CH₃COOH + NaOH → _{CaO, Δ} CH₄ + Na₂CO₃

This reaction produces methane, not ethane, so it is incorrect.

3. Reaction III: Hydrogenation of Ethene

CH₂CH₂ + H₂ → _Pt C₂H₆

This reaction forms ethane, but it is not the best preparative method for homologous series alkane synthesis.

4. Reaction IV: Reduction of Alkyl Halides

CH₃CH₂Br + 2[H] → _Zn/H⁺ C₂H₆ + HBr

This reaction also produces ethane, making it a valid method.

Step 3: Evaluating the Given Options

• Option (1): Correct, as I (Wurtz reaction) and IV (alkyl halide reduction) are the valid methods.
• Option (2): Incorrect, as II produces methane instead of ethane.
• Option (3): Incorrect, as I is correct but III is not a conventional method for preparing alkanes in a homologous series.
• Option (4): Incorrect, as II does not yield ethane.

Thus, the correct answer is Option (1).