Problem 72 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{CHCOONa} \stackrel{\text { Electolysis }}{\longrightarrow}[\mathrm{A}] \frac{\mathrm{L}_{2}}{\text { liquid } \mathrm{NH}_{\mathrm{I}}}[\mathrm{B}]\) Here, [B] is (a) \(\mathrm{CH} \equiv \mathrm{CH}\) (b) \(\mathrm{NaC} \equiv \mathrm{CNa}\) (c) \(\mathrm{I}-\mathrm{C} \equiv \mathrm{C}-\mathrm{I}\) (d) \(\mathrm{IC} \equiv \mathrm{CNa}\)

Step-by-Step Solution

Answer

[B] is CH≡CH, choice (a).

Step 1: Understanding the Reaction

The electrolysis of sodium acetate (CH3COONa) in aqueous solution leads to the formation of a carboxyl radical, which decarboxylates to give methyl radicals. These methyl radicals combine to form ethane (C2H6). The specific reaction is: 2 CH3COONa + 2 H2O → C2H6 + 2 CO2 + H2 + 2 NaOH.

Step 2: Subsequent Reaction with Lithium in Liquid Ammonia

In the presence of lithium in liquid ammonia, ethane typically does not react further. However, the problem suggests that an additional product [B] is formed. The formation of acetylene (C2H2, choice (a)) is common from carbyl reactions under these conditions.

Step 3: Final Conclusion

Given that After electrolysis and subsequent reaction in liquid ammonia, acetylene (C2H2), or CH≡CH is formed, [B] is indeed option (a) CH≡CH.

Key Concepts

Sodium Acetate Electrolysis

Sodium acetate electrolysis is a key process in organic chemistry where sodium acetate is decomposed through electrolysis. During this procedure, electricity is used to cause a chemical reaction that wouldn’t occur otherwise. The electrolysis of sodium acetate usually takes place in an aqueous solution, meaning the electrolyte, sodium acetate ( CH_3COONa ), is dissolved in water.

• When electricity passes through the solution, sodium acetate dissociates into acetate ions (CH_3COO⁻) and sodium ions (Na⁺).
• The acetate ions undergo decarboxylation, which means the removal of a carbon dioxide (CO₂) molecule.
• This results in the formation of a carboxyl radical.

Through this step, methyl radicals are formed, which combine to create ethane ( C_2H_6 ), carbon dioxide, and hydrogen. The overall reaction can be represented simply as: 2 CH₃COONa + 2 H₂O → C₂H₆ + 2 CO₂ + H₂ + 2 NaOH.

The presence of water in the reaction is crucial as it facilitates the electrolytic dissociation of sodium acetate which is essential for the subsequent formation of the intermediate radicals.

Formation of Acetylene

The formation of acetylene, C_2H_2 , occurs through specific reactions which are the result of additional transformations following the electrolysis of sodium acetate. After ethane is formed from the initial electrolysis, further reactions can occur under certain conditions.

In this particular exercise, the presence of lithium in liquid ammonia potentially influences the transformation of ethane into acetylene:

• Liquid ammonia acts as a solvent which can deprotonate compounds, allowing transformations under milder conditions.
• It is speculated that under these conditions, or similar, acetylene can be formed even though ethane does not typically react further on its own.

The hypothesis is that, through processes like oxidative coupling or partial dehydrogenation facilitated by lithium in this solvent, the ethane could convert to acetylene. Although not directly reacting under normal conditions, the environment provided could introduce a new reaction pathway that leads to acetylene as product (B) in the problem.

Carboxyl Radical Decarboxylation

Carboxyl radical decarboxylation is an essential process in organic reactions involving radicals. It is crucial in the electrolysis of sodium acetate as it triggers the formation of smaller radicals such as the methyl radical.

The process begins with the formation of a carboxyl radical from acetate ions due to the electric current. Once formed, this radical undergoes decarboxylation – effectively losing a carbon dioxide molecule.

• Decarboxylation is a mechanism that removes the carboxylic group, simplifying the molecule.
• This removal leads to more stable and smaller radicals, such as methyl radicals, CH₃·.
• These radicals are highly reactive and quickly combine to create stable compounds like ethane.

Decarboxylation is vital as it provides a way to convert larger molecules into smaller, more manageable ones, paving the way for newer synthetic routes in organic chemistry.