Question

In order to obtain trans \(\mathrm{Me}-\mathrm{CH}=\mathrm{CH}-\mathrm{Me}\) from \(\mathrm{MeC} \equiv \mathrm{CMe}\), the reagent(s) to be used would includes: (a) \(\mathrm{LiAlH}_{4}\) in \(\mathrm{EtO}_{2}\) (b) \(\mathrm{H}_{2} / \mathrm{Pd}, \mathrm{BaSO}_{4}\) (c) \(\mathrm{H}_{2} / \mathrm{Pd}, \mathrm{Pb}(\mathrm{OAc})_{2}\) (d) Sodium in liquid ammonia or sodium in \(\mathrm{EtOH}\)

Short answer

Use sodium in liquid ammonia to convert the alkyne to a trans alkene.

Step-by-step solution

Understanding the Goal

The problem asks us to convert \( \mathrm{MeC} \equiv \mathrm{CMe} \) to trans-\( \mathrm{Me}-\mathrm{CH}=\mathrm{CH}-\mathrm{Me} \). The initial compound is an alkyne and the target compound is an alkene, specifically the trans isomer. We need a reagent that can selectively convert an alkyne to a trans alkene.

Reagent Analysis

We analyze the reagent options: \( \mathrm{LiAlH}_{4} \) in \( \mathrm{EtO}_{2} \) reduces carbonyls; it does not affect alkynes or alkenes directly. \( \mathrm{H}_{2} / \mathrm{Pd}, \mathrm{BaSO}_{4} \) and \( \mathrm{H}_{2} / \mathrm{Pd}, \mathrm{Pb}(\mathrm{OAc})_{2} \) are both forms of Lindlar's catalysts, which usually lead to cis alkenes, not trans. Sodium in liquid ammonia is known for its use in converting alkynes to trans alkenes.

Decision on Suitable Reagent

Sodium in liquid ammonia best fits the requirement because it is used in the dissolving metal reduction that selectively converts alkynes to trans alkenes. None of the other reagents are specifically used for forming trans alkenes from alkynes.

Key concepts

Alkyne to Alkene Reduction

Alkynes, which contain a carbon-carbon triple bond, are often reduced to alkenes, which contain a carbon-carbon double bond. This process is not random; it requires a specific set of conditions and reagents to ensure the desired product. The challenge in alkyne reduction lies in selectively obtaining either a cis or a trans alkene. Depending on the reagents employed, the reduction can be controlled to yield one stereoisomer over the other.

This conversion is crucial in synthetic organic chemistry due to the different chemical properties and uses of cis and trans alkenes. Obtaining the correct isomer is important in the synthesis of certain pharmaceuticals and specialty chemicals. By choosing the appropriate reagent, you ensure that the triple bond becomes a double bond with the preferred spatial orientation of substituents.

Dissolving Metal Reduction

Dissolving metal reduction is a powerful technique in organic chemistry. It is specifically used to reduce alkynes to alkenes with trans configuration. In this process, a metal, commonly sodium or lithium, is dissolved in liquid ammonia to create solvated electrons, which are highly reactive.

The mechanism involves these solvated electrons helping to partially "break" the strong alkyne π-bonds, transforming them into a double bond. The intermediate formed during this reaction relaxes back into the more stable trans-alkene. This method is selective due to the specific way electrons interact with the alkyne molecule. Students should remember that the solvent also plays a crucial role in this reaction, as liquid ammonia provides the necessary environment for solvated electrons to function effectively.

Trans Alkenes

Trans alkenes, which have substituents on opposite sides of the double bond, are typically more stable than their cis counterparts. The stability arises from reduced steric hindrance between the substituents, which are farther apart in a trans configuration than in a cis configuration.

In organic synthesis, obtaining a trans alkene can be desirable when the goal is to minimize steric interference or when aiming for compounds with particular physical properties. Trans alkenes often exhibit different boiling points, and reactivity patterns compared to cis alkenes, which can be pivotal in subsequent reactions.

Understanding these differences helps chemists manage synthesis pathways effectively, ensuring the desired product forms without unwanted side reactions or byproducts.

Reagent Selection

Choosing the right reagent is key in chemical transformations, especially in achieving specific isomeric forms such as trans alkenes. In the exercise provided, several options were considered, but not all are suitable for forming trans alkenes from alkynes.

The provided reagents included lithium aluminum hydride, which does not typically affect alkynes, and various forms of hydrogenation catalysts, which favor the formation of cis alkenes. Sodium in liquid ammonia stood out as the best choice due to its unique ability to yield trans alkenes via dissolving metal reduction.

This demonstrates the importance of understanding reagent capabilities to apply them effectively in synthesis. Knowledge of how different reagents act allows chemists to meticulously plan pathways to achieve their desired chemical transformations efficiently.