Question

The conversion of 3 -hexyne into trans- 3 -hexene can be effected by (1) \(\mathrm{Na} /\) liquid \(\mathrm{NH}_{3}\) (2) \(\mathrm{H}_{2} /\) Lindlar's catalyst (3) Clemmensen reduction (4) \(\mathrm{LiNH}_{2}\)

Short answer

Na / liquid NH3.

Step-by-step solution

Identify the Goal

Determine which reagent converts 3-hexyne into trans-3-hexene.

Understand Each Reagent

Analyze the action of each reagent: (1) Na / liquid NH3 (2) H2 / Lindlar's catalyst (3) Clemmensen reduction (4) LiNH2

Examine the Reaction

Na / liquid NH3 typically converts alkynes to trans-alkenes.

Confirm the Correct Reagent

Examine if other reagents can also achieve the same conversion. (2) H2 / Lindlar's catalyst converts alkynes to cis-alkenes. (3) Clemmensen reduction reduces aldehydes or ketones to alkanes. (4) LiNH2 is a strong base, but not useful for hydrogenation.

Choose the Correct Option

Since Na / liquid NH3 converts alkynes to trans-alkenes, it is the suitable reagent here.

Key concepts

trans-alkene formation

Forming a trans-alkene from an alkyne involves specific reactions that ensure the addition of hydrogen atoms. When converting alkynes to alkenes, the aim is to achieve a specific configuration: either 'cis' (both hydrogen atoms on the same side) or 'trans' (hydrogen atoms on opposite sides). In the conversion of 3-hexyne to trans-3-hexene, the goal is to ensure a trans configuration. This structure provides different properties and stability compared to the cis form. The preferred method for creating trans-alkenes involves using a sodium (Na) in liquid ammonia (NH3) reaction, ensuring anti addition of hydrogen atoms, leading to the trans formation.

reagent selection

Choosing the right reagent is crucial in chemical reactions. For alkyne to trans-alkene conversion, different reagents play specific roles. When we look at converting 3-hexyne to trans-3-hexene:

• Na / liquid NH3: This reagent combination is highly effective for producing trans-alkenes. It performs an anti addition, where hydrogen atoms add on opposite sides.
• H2 / Lindlar's catalyst: This combination is tailored for producing cis-alkenes, not trans. Here, hydrogen adds syn (on the same side).
• Clemmensen reduction: Primarily used for reducing aldehydes and ketones to alkanes. It's not suitable for hydrogenation of alkynes.
• LiNH2: A strong base, useful in different reactions but not specifically in hydrogenation processes required here.

Understanding the role and outcome of each reagent ensures the correct selection for desired product formation.

Na/liquid NH3 reaction mechanism

The reaction mechanism involving sodium (Na) in liquid ammonia (NH3) for converting alkynes to trans-alkenes operates through a distinctive pathway. Here's how it works:

• First, Na donates electrons to the alkyne, creating a radial anion and a sodium cation.
• This intermediate then receives a proton from NH3, creating a trans-alkene partial structure.
• A second electron addition by another Na converts the intermediate to a full trans alkene.

This anti addition mechanism ensures hydrogen atoms attach on opposite sides, confirming the trans configuration crucial for applications in synthetic chemistry.

Lindlar's catalyst

Lindlar's catalyst is a specially designed catalyst for selective hydrogenation of alkynes to cis-alkenes:

• It consists of palladium (Pd), calcium carbonate (CaCO3), and lead (Pb).
• This mix ensures the hydrogenation stops at the alkene stage instead of fully reducing to alkanes.
• The reaction ensures syn addition, where hydrogen atoms add on the same side, creating cis-alkenes.

For our specific purpose of creating trans-alkenes, Lindlar's catalyst is not suitable due to its majorly producing cis products. However, knowing its selective nature is vital for applications requiring such stereochemistry.

Clemmensen reduction

The Clemmensen reduction is utilized for reducing carbonyl compounds like aldehydes and ketones into corresponding alkanes. It involves using zinc amalgam (Zn/Hg) in hydrochloric acid (HCl):

• This reduction is highly effective for converting carbonyls without affecting other functional groups.
• However, this is not applicable for hydrogenating alkynes and altering their stereochemistry.

For converting 3-hexyne into trans-3-hexene, this method doesn't apply but it's essential to recognize its value in organic synthesis for different transformations.