Question

In Kolbe's electrolytic method, when sodium acetate is electrolysed, the gases generated at anode are

• A. ethane and \( H_2 \)
• B. \( H_2 \) and \( CO_2 \)
• C. methane and ethane
• D. ethane and \( CO_2 \)
• E. methane and \( H_2 \)

Hint:

Kolbe → alkane formation + CO\(_2\).

Answer 1

The Correct Option is D

Step 1: Understanding Kolbe's Electrolysis:
Kolbe's electrolysis is an organic reaction used to synthesize alkanes by the electrolysis of an aqueous solution of a sodium or potassium salt of a carboxylic acid. The reaction proceeds via a free-radical mechanism.
Step 2: Identifying Reactants and Electrodes:
The reactant is sodium acetate (CH\(_3\)COONa). In aqueous solution, it dissociates into acetate ions (CH\(_3\)COO\(^-\)) and sodium ions (Na\(^+\)). Water (H\(_2\)O) is also present.
Anode (Oxidation): Anions (CH\(_3\)COO\(^-\) and OH\(^-\) from water) migrate to the anode.
Cathode (Reduction): Cations (Na\(^+\) and H\(^+\) from water) migrate to the cathode.
Step 3: Reaction at the Anode (Oxidation):
The acetate ion has a lower discharge potential than the hydroxide ion, so it gets oxidized at the anode.
The acetate ion loses an electron to form an acetoxy free radical: \[ 2\text{CH}_3\text{COO}^- \rightarrow 2\text{CH}_3\text{COO}\cdot + 2e^- \]
This acetoxy radical is unstable and rapidly undergoes decarboxylation (loses CO\(_2\)) to form a methyl free radical: \[ 2\text{CH}_3\text{COO}\cdot \rightarrow 2\text{CH}_3\cdot + 2\text{CO}_2 \]
Two methyl free radicals combine (dimerize) to form ethane: \[ \text{CH}_3\cdot + \cdot\text{CH}_3 \rightarrow \text{CH}_3\text{-}\text{CH}_3 \quad (\text{Ethane}) \]
Therefore, the gases generated at the anode are ethane (C\(_2\)H\(_6\)) and carbon dioxide (CO\(_2\)).
Step 4: Reaction at the Cathode (Reduction):
At the cathode, H\(^+\) ions (from water) have a lower discharge potential than Na\(^+\) ions, so they are reduced to form hydrogen gas.
\[ 2\text{H}_2\text{O} + 2e^- \rightarrow \text{H}_2\text{(g)} + 2\text{OH}^- \] So, hydrogen gas is evolved at the cathode.
Step 5: Final Answer:
The gases generated at the anode during the Kolbe's electrolysis of sodium acetate are ethane and carbon dioxide.