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) \(\mathrm{Li}-\mathrm{NH}_{3} / \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\)

Short answer

(d) \mathrm{Li}-\mathrm{NH}_{3} / \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH} is used for this conversion.

Step-by-step solution

Identify the Reaction Goal

The task is to convert 2-hexyne, which is an alkyne, into trans-2-hexane, which is an alkene.

Understand the Characteristics of Reagents

Among the given options, (a) \( \mathrm{H}_{2} / \mathrm{Pd} / \mathrm{BaSO}_{4} \) is known for partial hydrogenation and usually leads to a cis-alkene. (b) \( \mathrm{H}_{2}, \mathrm{PtO}_{2} \) is a strong catalyst that fully reduces alkynes to alkanes. (c) \( \mathrm{NaBH}_{4} \) is generally used for the reduction of carbonyl compounds, not alkynes. (d) \( \mathrm{Li}-\mathrm{NH}_{3} / \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH} \), also known as the Birch reduction, selectively reduces alkynes to trans-alkenes.

Select the Appropriate Reagent

The goal is to convert 2-hexyne into a trans-alkene. Based on the reagents' characteristics, \( \mathrm{Li}-\mathrm{NH}_{3} / \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH} \) is the reagent that achieves this, leading to the formation of trans-2-hexane.

Key concepts

Birch reduction

Birch reduction is a fascinating and important reaction in organic chemistry. It's particularly known for transforming alkynes, which are molecules containing carbon-carbon triple bonds, into alkenes, with double bonds. What makes Birch reduction unique is that it selectively produces trans-alkenes. This selectivity is uncommon and very useful. The reaction uses a combination of alkali metals, like lithium (Li), and liquid ammonia (NH₃), often with an alcohol such as ethanol (C₂H₅OH) to act as a proton source. Here’s why Birch reduction works so well:

• It relies on the unique solvating properties of liquid ammonia, which helps stabilize the intermediate radical anion formed during the reduction process.
• The process leads to the removal of one π-bond in alkynes, converting them to alkenes.
• Because it forms trans-alkenes, it's highly valued in synthetic chemistry for creating geometrically controlled products.

This reaction is especially useful when chemists need to convert alkynes into trans-alkenes specifically, without over-reducing to alkanes.

hydrogenation

Hydrogenation is a chemical process that introduces hydrogen (H₂) into a compound, typically with the aid of a catalyst. This process is extensively used in organic chemistry to convert unsaturated compounds into saturated ones. When applied to alkynes, hydrogenation can reduce them to either alkenes or further to alkanes, depending on the conditions and catalysts used.

• Complete hydrogenation involves the conversion of multiple bonds to single bonds, producing fully saturated alkanes.
• Partial hydrogenation aims to stop the reaction at the alkene stage without going all the way to alkanes, allowing control over the degree of saturation.
• Catalysts often used include metals like platinum, palladium, and nickel, which facilitate the breaking of hydrogen molecules and their addition to the target compound.

By adjusting the catalyst and reaction conditions, chemists can control whether an alkyne is reduced to an alkene or an alkane. This reaction is a powerhouse in the synthetic chemistry toolbox, providing flexibility whether the desired product is a cis or trans configuration.

alkyne to alkene conversion

Alkyne to alkene conversion is a core transformation in the realm of organic chemistry. It involves reducing the triple bond of an alkyne to a double bond, producing an alkene. The challenge here is to achieve selectivity between cis and trans forms. Various methods and reagents are available to accomplish this conversion: - **Hydrogenation with a Lindlar catalyst**: Typically results in cis-alkenes by using a partially deactivated palladium catalyst. This controls the reduction to one π-bond only. - **Birch reduction**: As previously mentioned, this uniquely produces trans-alkenes using lithium, ammonia, and an alcohol in the reaction mix. This transformation is fundamental for creating specific stereoisomers in chemical syntheses. It allows chemists to obtain alkenes with precise geometries, essential for developing drugs and complex organic molecules with specific functional properties.

transition metals in catalysis

Transition metals are indispensable in catalysis, especially when dealing with organic reactions like hydrogenation. These elements, which include platinum, palladium, and nickel, have d-orbitals that enable them to activate diatomic molecules like hydrogen effectively. With their catalytic presence, transition metals facilitate the addition of hydrogen to unsaturated bonds, significantly lowering the energy required for these reactions to occur. - **Role as Catalysts**: Transition metals provide surface sites for hydrogen molecules to adsorb and dissociate into active atoms, which then add across the bonds of an unsaturated organic molecule. - **Selectivity and Control**: By choosing the right metal and modifying reaction conditions, chemists can steer reactions to produce different isomers or selectively hydrogenate particular bonds in complex molecules. - **Reusability**: These catalysts are not consumed in the reaction, making them highly valuable in industrial applications for efficiency and cost-effectiveness. The versatility of transition metals in catalysis is a cornerstone of modern organic synthesis, enabling precise and efficient transformations of organic compounds.