Problem 209 - A Complete Resource Book in Chemistry for JEE Main

Question

2-Hexyne gives trans \(-2\)-Hexene on treatment with [2012] (a) \(\mathrm{Pd} / \mathrm{BaSO}_{4}\) (b) \(\mathrm{Li} / \mathrm{NH}_{3}\) (c) \(\mathrm{Pt} / \mathrm{H}_{2}\) (d) \(\mathrm{LiAlH}_{4}\)

Step-by-Step Solution

Answer

Option (b) \\(\mathrm{Li} / \mathrm{NH}_{3}\\) gives trans-2-Hexene from 2-Hexyne.

Step 1: Understanding the Reaction Type

2-Hexyne, an alkyne, is being transformed into trans-2-Hexene, an alkene. Reactants that are known to selectively reduce alkynes to trans-alkenes need to be considered.

Step 2: Evaluating Option (a)

The option (a) \(\mathrm{Pd} / \mathrm{BaSO}_{4}\) is known as the Lindlar Catalyst, which typically reduces alkynes to cis-alkenes, not trans-alkenes. Therefore, this is not suitable for obtaining trans-2-Hexene.

Step 3: Evaluating Option (b)

Option (b) \(\mathrm{Li} / \mathrm{NH}_{3}\) is known as the Birch Reduction. This reagent is known for converting alkynes to trans-alkenes, as it performs partial hydrogenation. This is the correct reagent for producing trans-2-Hexene from 2-Hexyne.

Step 4: Evaluating Option (c)

In option (c) \(\mathrm{Pt} / \mathrm{H}_{2}\), platinum with hydrogen will fully reduce alkynes to alkanes, rather than stopping at alkene formation. This makes it incorrect for this transformation.

Step 5: Evaluating Option (d)

Option (d) \(\mathrm{LiAlH}_{4}\) is a strong reducing agent used primarily for reducing carbonyl compounds to alcohols. It is not typically used on alkynes or to achieve selective alkene formation, making it not suitable for the given reaction.

Step 6: Selecting the Correct Reagent

Based on the analysis, option (b) \(\mathrm{Li} / \mathrm{NH}_{3}\) is the correct option for producing trans-2-Hexene from 2-Hexyne.

Key Concepts

Alkyne Reduction

Alkyne reduction is a fundamental process in organic chemistry where triple bonds within alkynes are converted to double or single bonds. Broadly speaking, chemists perform this transformation to create alkenes or alkanes, depending on the choice of reagent. Alkenes derived from alkynes retain a specific geometry — either cis or trans. This is significant in various applications in materials science and pharmaceuticals.

The process can be full or partial:

• Full Reduction: Alkynes are fully reduced to alkanes, resulting in the complete removal of pi bonds.
• Partial Reduction: This stops the reduction at the alkene stage. The triple bond becomes a double bond, allowing for preservation of particular stereochemistry, such as cis or trans orientations.

Understanding the type of reduction is key, as different chemical reagents and conditions lead to different types of products.

Birch Reduction

Birch reduction is a unique method used to reduce alkynes to trans-alkenes. Unlike other reduction methods, Birch reduction employs a combination of lithium in liquid ammonia \(\text{(Li/NH}_3)\) and is thus considered a partial hydrogenation technique.

The method works through the addition of electrons to the alkyne, followed by proton addition from ammonia:

• Electron Addition: Lithium donates electrons to the alkyne, creating an alkenyllithium intermediate.
• Proton Donation: Liquid ammonia donates protons to the intermediate, resulting in a trans-alkene.

The process selectively breaks one of the triple bonds to form the double bond of a trans-alkene. It's a valuable technique in the synthetic chemistry toolkit, especially when the specific geometrical form matters.

Lindlar Catalyst

The Lindlar catalyst is specifically used for partial hydrogenation of alkynes to produce cis-alkenes. It consists of palladium deposited on calcium carbonate and is often "poisoned" with lead or quinoline. This significantly decreases its activity, preventing over-reduction to single-bonded alkanes.

It works through adsorption, where the alkyne temporarily adheres to the catalyst surface, allowing controlled hydrogen addition:

• Adsorption: The alkyne binds to the catalyst's surface, positioning it for reduction.
• Hydrogen Addition: Hydrogen is added in a stereospecific manner, producing cis-alkenes.

Because of the controlled reaction, Lindlar catalysts are perfect for producing cis-alkenes from alkynes, making them indispensable in organic synthesis for stereospecific hydrogenation.

Partial Hydrogenation

Partial hydrogenation is the technique of adding hydrogen to an alkyne to convert it into an alkene rather than an alkane. When only one of the two pi bonds in an alkyne is converted, the method is known as partial.

This process is critical for maintaining desired chemical properties and selective reactivity. It plays a substantial role in creating a wide array of products, especially in the synthesis of trans or cis alkenes.

This technique allows for:

• Selective Bond Formation: Only one pi bond is reduced, crucial for forming alkenes with specific geometrical arrangements.
• Retention of Unsaturated Features: The alkene produced still possesses reactive sites ideal for subsequent chemical modifications.

Technology like Lindlar catalysts and Birch reduction serve as the backbone for achieving such selectivity, highlighting the significance of partial hydrogenation in organic reactions.