Problem 31 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

In the given reaction, \(\mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3} \stackrel{\mathrm{Na} / \mathrm{NH}_{3}(\mathrm{I})}{\longrightarrow}[\mathrm{X}],[\mathrm{X}]\) will be (a) 1-phenyl propane (b) 1-phenyl propene (c) trans-1-phenyl propene (d) cis-1-phenyl propene

Step-by-Step Solution

Answer

The product is (c) trans-1-phenyl propene.

Step 1: Understand the Reaction

The given reaction involves the compound \( \mathrm{C}_{6} \mathrm{H}_{5}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3} \), which is a triple-bond alkene (alkyne) with a phenyl group attached. It undergoes a reaction with sodium (Na) in liquid ammonia (\( \mathrm{NH}_{3} \)) which is known to reduce alkynes to alkenes through trans addition of hydrogen.

Step 2: Identify the Reducing Agent's Mechanism

The reaction condition, Na in liquid ammonia (\( \mathrm{Na/NH}_3 \)), is a reducing system known as the Birch reduction. It is specifically used to convert alkynes to trans-alkenes. This means the triple bond in the compound will be converted into a double bond with hydrogen atoms added in a trans configuration.

Step 3: Deduce the Product's Structure

The triple bond between the two central carbon atoms \( \mathrm{C} \equiv \mathrm{C} \) is reduced to a double bond \( \mathrm{C} = \mathrm{C} \). Due to the trans addition of hydrogen, the two carbon segments attached to this double bond (\( \mathrm{C}_6\mathrm{H}_5\) and \( \mathrm{CH}_3\)) will be opposite to each other. This gives us trans-1-phenyl propene.

Step 4: Match the Product to the Options

The product deduced from the Birch reduction is trans-1-phenyl propene. This matches option (c) from the given choices.

Key Concepts

Trans-Alkenes

Trans-alkenes are a type of alkene where the substituents on either side of the double bond are on opposite sides. This is different from cis-alkenes, where the substituents are on the same side. Trans-alkenes are generally more stable due to reduced steric hindrance.
When alkynes undergo a Birch reduction, the resulting alkenes usually have a trans configuration. This means the added hydrogen atoms are on opposite sides of the double bond.
Key characteristics of trans-alkenes include:

• Stability: More stable than cis alkenes due to lower steric strain.
• Formation: Often formed through specific reduction reactions such as the Birch reduction.
• Physical properties: Generally have higher melting points compared to their cis counterparts.

Understanding the trans-configurations is crucial to predicting the outcomes of reactions involving alkyne reductions.

Reduction Mechanism

The reduction mechanism is a step-by-step process by which a chemical compound is reduced. During a reduction, the molecule gains electrons, often manifested by the addition of hydrogen or the removal of oxygen.
The Birch reduction is a specific reduction method involving sodium (Na) in liquid ammonia (\(\mathrm{NH}_3\)). It effectively converts alkynes to trans-alkenes by adding hydrogens in a trans configuration.
Steps involved in Birch reduction can include:

• Electron transfer from sodium to the alkyne forming a radical anion.
• Protonation by ammonia, adding a hydrogen atom to the alkyne.
• Further electron transfer and protonation leading to the trans-alkene formation.

This method is especially useful for selectively reducing alkynes while maintaining the alkene intermediate with a trans configuration.

Alkynes to Alkenes

Alkynes are hydrocarbons with one or more carbon-carbon triple bonds. To convert them into alkenes, where carbon atoms are connected by double bonds, reduction methods are employed.
The transformation from alkynes to alkenes typically requires a reducing agent like sodium in liquid ammonia. The goal is to add hydrogen atoms across the triple bond, forming a double bond.
The two primary ways alkynes can be reduced include:

• Stereoselective reduction methods: These lead to either cis or trans alkenes. The Birch reduction is a classic example for forming trans-alkenes.
• Partial hydrogenation: Another method, where a catalyst helps reduce an alkyne to an alkene but might not control the stereochemistry as effectively as Birch's method.

Understand that the choice of reagent and conditions determines the type of alkene product formed, and mastering these methods is vital for synthetic chemistry.