2-hexyne gives trans-2-hexene on treatment with

• (A) Pt/H₂
• (B) Li/NH₃
• (C) Pd/BaSO₄
• (D) LiAlH₄

To determine which reagent converts 2-hexyne to trans-2-hexene, we need to consider the mechanisms of hydrogenation and reduction reactions. Each of the options provided has distinct characteristics that influence the outcome of the reaction.

Understanding the Reagents

Let's break down the options:

• A) Pt/H₂: Platinum in the presence of hydrogen is a catalyst for hydrogenation. This reaction typically leads to the formation of alkenes and can produce both cis and trans isomers, but it does not have a preference for one over the other.
• B) Li/NH₃: Lithium in liquid ammonia is known for its ability to reduce alkynes to trans alkenes. This reagent selectively forms the trans isomer due to the nature of the radical intermediates formed during the reaction.
• C) Pd/BaSO₄: Palladium on barium sulfate is often used in the Lindlar catalyst, which selectively hydrogenates alkynes to cis alkenes. Therefore, this option would not yield trans-2-hexene.
• D) LiAlH₄: Lithium aluminum hydride is a strong reducing agent, primarily used for reducing carbonyl compounds and esters, not for converting alkynes to alkenes.

Identifying the Correct Answer

Given the analysis above, the reagent that specifically leads to the formation of trans-2-hexene from 2-hexyne is:

B) Li/NH₃

Mechanism of the Reaction

When 2-hexyne is treated with lithium in liquid ammonia, the reaction proceeds through a radical mechanism. Here’s how it works:

1. The lithium donates an electron to the alkyne, generating a radical anion.
2. This radical anion then undergoes protonation by ammonia, leading to the formation of a trans-alkene.
3. The trans configuration is favored because the bulky groups on the alkene are positioned opposite each other, minimizing steric hindrance.

Visualizing the Process

Imagine the alkyne as a straight line with two ends. When you add lithium in ammonia, it’s like introducing a force that pushes one end of the line down while the other end goes up, creating a trans configuration. This spatial arrangement is more stable due to reduced steric interactions compared to the cis configuration.

Conclusion

In summary, the treatment of 2-hexyne with lithium in liquid ammonia effectively yields trans-2-hexene due to the unique reaction pathway that favors the formation of the trans isomer. Understanding the properties of each reagent helps clarify why Li/NH₃ is the correct choice for this transformation.