Problem 12 - Chemistry in 30 days

Question

2-Butyne is treated with sodium in liquid \(\mathrm{NH}_{3}\). The major product formed is (a) cis-2-butene (b) trans-2-butene (c) butane (d) \(1.3\)-butadiene

Step-by-Step Solution

Answer

The major product is trans-2-butene.

Step 1: Identify the Reaction Type

The reaction of an alkyne with sodium in liquid ammonia is a partial hydrogenation reaction known as the Birch reduction. This typically results in the conversion of an alkyne to a trans-alkene.

Step 2: Predict the Reaction Outcome

Given the nature of the Birch reduction, 2-butyne, when treated with sodium in liquid ammonia, will form trans-2-butene. Sodium in liquid ammonia provides an electron which facilitates the formation of a trans-alkene from the alkyne.

Step 3: Determine the Major Product

Based on the Birch reduction process, the major product of treating 2-butyne with sodium in liquid ammonia is trans-2-butene, as the reaction favors the formation of the trans configuration over the cis configuration or other products. Therefore, option (b) is correct.

Key Concepts

Alkyne Reduction

Alkyne reduction is a fascinating chemical process that alters the structure of a molecule. An alkyne is a hydrocarbon that contains at least one carbon-carbon triple bond. These triple bonds are very reactive, making alkynes a prime choice for various chemical reactions.
An essential aspect of alkyne reduction is the transformation of this triple bond into a double bond, typically resulting in an alkene. Through this transformation, the molecule's overall reactivity and properties can change significantly.

• Alkynes with more than one triple bond can undergo multiple reductions.
• Some reducing agents can convert the triple bonds all the way to single bonds.
• Others, like the Birch reduction reagents, will stop at forming an alkene.

The choice of reducing agent and conditions determines the final product of the reduction process.

Trans-Alkene Formation

Forming a trans-alkene from an alkyne is a unique reaction pathway. In organic chemistry, a trans-alkene has two substituents on opposite sides of the double bond, which impacts the molecule's geometry and properties.
The formation of a trans-alkene is often achieved using the Birch reduction method, where one electron at a time is added to the alkyne. This slow and controlled approach allows for precise formation of the double bond configurations:

• Trans-alkenes are often more stable due to less steric hindrance compared to their cis counterparts.
• The electron-rich environment helps keep the substituents apart, favoring a trans configuration.

Trans-alkenes are prevalent in many natural products and are a significant focus in synthetic chemistry due to their stability and unique functionality.

Partial Hydrogenation

Partial hydrogenation speaks to the selective addition of hydrogen atoms to a compound. In the context of alkyne reduction, this process is strategic, stopping at producing an alkene rather than fully saturating the molecule.
In typical reactions, reagents are crucial for controlling the step of hydrogen addition.

• With complete hydrogenation, an alkyne could become an alkane.
• Partial hydrogenation retains the double bond, yielding an alkene.

The Birch reduction scenario, using sodium in liquid ammonia, embodies partial hydrogenation by precisely tuning this reaction to halt at the alkene stage, showcasing control over molecular changes.

Sodium in Liquid Ammonia

The use of sodium in liquid ammonia is a hallmark of the Birch reduction process. This reagent combination is notable for its ability to donate electrons effectively, enabling controlled reductions like transforming alkynes to alkenes.
When sodium is dissolved in liquid ammonia, a beautiful blue solution forms due to solvated electrons—free electrons that enhance the reduction potential.

• The solvated electrons are pivotal in adding to the alkyne to form a radical anion.
• Protonation steps follow to stabilize the formed intermediates, eventually yielding the desired alkene.

Sodium in liquid ammonia is chosen because it selectively reduces triple bonds to double bonds without over-reducing to single bonds, making it an invaluable tool in synthetic chemistry.