Problem 74 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{C}-\mathrm{C}-\mathrm{C} \equiv \mathrm{C} \stackrel{\angle \mathrm{A}}{\mathrm{B}}>\mathrm{C}-\mathrm{C} \equiv \mathrm{C}-\mathrm{C}\) Here, \([\mathrm{A}]\) and \([\mathrm{B}]\) are (a) alc. \(\mathrm{KOH}\) and \(\mathrm{NaNH}_{2}\) (b) \(\mathrm{NaNH}_{2}\) and Lindlar catalyst (c) \(\mathrm{NaNH}_{2}\) and alcoholic \(\mathrm{KOH}\) (d) Lindlar catalyst and \(\mathrm{NaNH}_{2}\)

Step-by-Step Solution

Answer

[A] is Lindlar catalyst, [B] is \(\mathrm{NaNH}_2\); option (d).

Step 1: Identify the Reaction Condition for A

The reaction shows a triple bond being converted to a double bond. This suggests a partial reduction of an alkyne to an alkene. Common reagents for such transformation include Lindlar's catalyst, which selectively reduces alkynes to cis-alkenes.

Step 2: Identify the Reaction Condition for B

In the given reaction, after the formation of a double bond from triple bond using [A], a transformation occurs maintaining the double bond. To ensure the conversion to an alkene, strong bases like \(\mathrm{NaNH}_2\) can be used to initiate deprotonation or elimination reactions to stabilize double bonds.

Step 3: Match the Reagents to the Reactions

Option (d) suggests Lindlar catalyst for [A] and \(\mathrm{NaNH}_2\) for [B]. Using Lindlar's catalyst allows reduction of alkyne to alkene, and a base like \(\mathrm{NaNH}_2\) ensures stability and possible further transformations without isomerizing the newly formed alkene.

Step 4: Evaluate Other Options

Option (a) pairs alcoholic \(\mathrm{KOH}\) with \(\mathrm{NaNH}_2\), which does not facilitate the selective reduction of alkynes to alkenes. Option (b) and (c) also don’t fit as neither correctly pairs the necessary environment for this selective reduction as Lindlar's catalyst does.

Key Concepts

Alkyne Reduction

Alkyne reduction is a chemical process where a carbon-carbon triple bond in alkynes is partially or fully reduced to form a less unsaturated compound, such as an alkene or an alkane. In the context of partial reduction, alkynes are typically transformed into alkenes through selective methods.
For students dive deeper into learning about this reaction, it's crucial to consider the reagents used in the process.

• Catalytic hydrogenation can convert alkynes to alkanes, but special catalysts are necessary for selective conversion to alkenes.
• The type of catalyst or reducing agent will determine whether the resulting compound retains specific characteristics such as stereochemistry.

This partial reduction is particularly useful in organic synthesis where controlled generation of alkenes is required.

Lindlar Catalyst

The Lindlar catalyst is a finely tuned combination of powdered palladium deposited on a calcium carbonate base, treated with various additives like lead salts. It acts as a poison to the catalyst, significantly reducing its activity to enable selective reduction of alkynes to cis-alkenes.
This catalyst is vital when a partial reduction of alkynes is intended without further reducing them to alkanes. Here’s how it works:

• The lead acetate used in the catalyst mixture is important for "poisoning" the palladium surface.
• This effectively controls the catalytic activity to halt the reaction at the alkene stage.
• This controlled process naturally results in the formation of cis-alkenes, which have the same groups on the same side of the double bond.

By using Lindlar's catalyst, chemists achieve exquisite control over the stereochemistry of the resulting alkene.

Deprotonation

Deprotonation is a crucial concept in organic chemistry that involves the removal of a proton ( H^+ ) from a molecule, resulting in the formation of an anion. This process is fundamental in the manipulation and stabilization of carbon chemistry, particularly in forming and maintaining multiple bonds such as double bonds in alkenes.
In reduction reactions following the formation of alkene, deprotonation might be necessary to ensure that the resulting product is stabilized or that further unwanted reactions are minimized.

• Commonly, strong bases such as NaNH_2 are employed to abstract hydrogen atoms from substances.
• This enhances the potential for elimination reactions, which can further stabilize unsaturated systems.
• The NaNH_2 base also effectively maintains the configuration of specific bonds, preserving the desired molecule structure during synthesis.

Grasping how deprotonation works in tandem with reduction reactions is crucial for understanding and predicting the outcomes in organic synthesis.

Alkene Formation

Alkene formation is a fundamental goal in organic chemistry, particularly when converting alkynes to alkenes through selective reductions. During alkyne reduction with precision catalysts like Lindlar’s catalyst, chemists can control the geometry of the resulting alkene.
Successfully producing alkenes is paramount, as they serve as building blocks for more complex reactions and organic molecules.

• In Lindlar catalysis, the partial hydrogenation process ensures the alkene product is a cis-alkene, crucial for many synthetic applications where stereochemistry matters.
• Maintaining the double bond stability through deprotonation and other supportive reactions helps preserve the desired properties of these alkenes.
• Once formed, alkenes undergo further transformations—such as polymerizations or further functional group modifications—highlighting their versatility and importance.

Understanding alkene formation is key for students and researchers focused on precise chemical synthesis and the development of organic materials.