Problem 7 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is used for the conversion of 2-hexyne into trans-2-hexane? (a) \(\mathrm{H}_{2} / \mathrm{Pd} / \mathrm{BaSO}_{4}\) (b) \(\mathrm{H}_{2}, \mathrm{PtO}_{2}\) (c) \(\mathrm{NaBH}_{4}\) (d) \(\mathrm{Li}-\mathrm{NH}_{3} / \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\)

Step-by-Step Solution

Answer

Option (d): Li-NH3 / C2H5OH ] is used for converting 2-hexyne to trans-2-hexane.

Step 1: Identify the Reaction Type

The problem is asking for the conversion of 2-hexyne, which is a triple-bonded alkyne, into trans-2-hexane, which is a double-bonded alkene with trans configuration. This suggests a selective reduction process of the alkyne to form the trans alkene.

Step 2: Understand the Role of Reagents

Each reagent listed serves a different purpose in organic chemistry: -  [ 202 / Pd / BaSO4 ] catalyzes hydrogenation of alkynes to form alkenes and is known as the Lindlar's catalyst, which gives cis alkenes. - H2, PtO2 ] is used for complete reduction of alkynes to alkanes. - NaBH4 ] is used for the reduction of carbonyl compounds, not typically for alkynes. - Li-NH3 / C2H5OH ] is known for the reduction of alkynes to trans alkenes via a dissolving metal reduction.

Step 3: Select the Correct Option

To convert 2-hexyne to trans-2-hexane, a reagent that promotes transformation to trans alkenes is needed. The reagent Li-NH3 / C2H5OH ] is used in this type of transformation because it provides a stereo-selective reduction, converting the alkyne to a trans alkene.

Step 4: Conclusion and Choice

Based on understanding the role of reagents, the correct option for converting 2-hexyne into trans-2-hexane is option (d), Li-NH3 / C2H5OH ], which effectively transforms the alkyne into a trans alkene.

Key Concepts

Stereoselective Reduction

Stereoselective reduction is an important concept in organic chemistry, especially when converting alkynes to alkenes. This process controls the configuration (cis or trans) of the resulting double bond. For instance, when reducing alkynes like 2-hexyne to trans-2-hexane, the goal is not just to break down bonds, but to do so in a way that promotes the formation of a specific geometric isomer. This is what stereoselective reduction is all about. In a stereoselective reduction, specific reagents are used to favor the formation of one stereoisomer over another when reducing multiple bonds. This means the atoms or groups in the newly formed molecule are arranged in a particular spatial orientation. For trans alkenes, this results in opposite sides of the double bond being populated by substituents, creating a more linear molecule. Understanding the nature of stereoselectivity helps chemists choose the right reagents and reaction conditions to achieve the desired product selectively.

Dissolving Metal Reduction

Dissolving metal reduction is a special technique used in organic synthesis to reduce alkynes to trans alkenes. This method is highly useful when the trans configuration is specifically desired. The process involves the use of metals like lithium (Li) in ammonia (NH₃). Here's how it works:

• The metallic lithium dissolves in ammonia, creating solvated electrons.
• These electrons add to the alkyne carbon-carbon triple bond, forming a radical anion intermediate.
• Protonation (often using a solvent like C₂H₅OH) then leads to the formation of the trans alkene.

This sequence of steps allows the selective formation of the trans isomer by offering a stepwise electron and hydrogen transfer mechanism, which stabilizes the formation of trans-2-hexane from 2-hexyne. The use of this technique showcases the precision with which chemists can manipulate molecular structures through specific reaction pathways.

Organic Chemistry Reagents

Organic chemistry reagents are the building blocks or tools used to carry out chemical transformations. Each reagent has unique properties and capabilities, determining its role in chemical reactions. Understanding how to choose the correct reagent for a specific reaction is crucial in organic synthesis. For example, the transformation of alkynes to alkenes can be done using different reagents, depending on the desired stereochemistry:

• a.) H₂ / Pd / BaSO₄ : Known as Lindlar's catalyst, this helps achieve the cis configuration of alkenes.
• b.) H₂, PtO₂ : Used for complete reduction of alkynes to alkanes without stereochemistry control.
• c.) NaBH₄ : Commonly used for reducing carbonyl compounds, not ideal for alkynes.
• d.) Li-NH₃ / C₂H₅OH : Offers dissolving metal reduction to produce trans alkenes selectively.

Choosing the right reagent is like picking the right tool from a toolbox; each has a specific function and is best used for certain types of tasks or reactions. Understanding these functionalities allows chemists to design and implement reactions more effectively.