Problem 209 - A Complete Resource Book in Chemistry for JEE Main

Question

2-Hexyne gives trans \(-2\) - Hexene on treatment with (a) \(\mathrm{Pd} / \mathrm{BaSO}_{4}\) (b) \(\mathrm{Li} / \mathrm{NH}_{3}\) (c) \(\mathrm{Pt} / \mathrm{H}_{2}\) (d) \(\mathrm{LiAlH}_{4}\)

Step-by-Step Solution

Answer

Reaction (b) - \\ Li / NH_3, yields trans-2-hexene from 2-hexyne.

Step 1: Identify the Reaction Target

We need to convert 2-hexyne to trans-2-hexene. Trans-alkenes can be obtained from alkynes through partial hydrogenation reactions that add hydrogen in a controlled manner.

Step 2: Examine Reagent (a) - \\( \mathrm{Pd} / \mathrm{BaSO}_{4} \\)

This is known as Lindlar's catalyst, which partially hydrogenates alkynes to cis-alkenes, not trans. Thus, this reagent will not yield trans-2-hexene.

Step 3: Analyze Reagent (b) - \\( \mathrm{Li} / \mathrm{NH}_{3} \\)

This combination is known for the Birch reduction, which converts alkynes to trans-alkenes by transferring electrons and protons in a step-wise manner. Therefore, it will produce trans-2-hexene from 2-hexyne.

Step 4: Consider Reagent (c) - \\( \mathrm{Pt} / \mathrm{H}_{2} \\)

Platinum with hydrogen typically fully hydrogenates alkynes directly to alkanes, not stopping at the alkene stage. Thus, this option does not provide trans-2-hexene.

Step 5: Evaluate Reagent (d) - \\( \mathrm{LiAlH}_{4} \\)

Lithium aluminum hydride is a strong reducing agent, but it is typically not used to reduce alkynes to alkenes. Instead, it is more commonly used to reduce carbonyl groups to alcohols.

Key Concepts

Birch Reduction

The Birch Reduction is a chemical reaction that specifically targets the transformation of alkynes into trans-alkenes. This reduction method is named after chemist Arthur Birch and it stands out as an effective technique to convert an alkyne to a trans-alkene.
This process involves the use of sodium (or lithium) metal in liquid ammonia. The presence of an alcohol as a proton source is also required. The way it works is quite fascinating:

• Firstly, an electron is transferred from the metal to the alkyne molecule. This electron transfer generates a radical anion.
• Next, the radical anion receives a proton from the alcohol, forming a radical.
• Then, another electron is added to the radical to produce an anion.
• Finally, the anion is protonated by another alcohol molecule to give the trans-alkene.

This sequence results in the anti addition of hydrogen atoms to the alkyne's triple bond, forming a trans-alkene. For the conversion of 2-hexyne to trans-2-hexene, the Birch Reduction method perfectly fits the bill due to its specific mechanism that ensures the formation of the trans isomer.

Partial Hydrogenation

Partial hydrogenation is a chemical process where the goal is to add hydrogen to an alkyne in a controlled manner to stop at an alkene stage rather than proceed to a full alkane. But achieving just partial reduction is not always straightforward.
Some catalysts and conditions can achieve partial hydrogenation:

• Lindlar's catalyst: Employing a palladium on calcium carbonate that's been poisoned with quinoline, Lindlar's catalyst allows for the conversion of alkynes to cis-alkenes rather than trans-alkenes. Therefore, it is not suitable when a trans product is desired.
• Other conditions that combine specific metal catalysts and ligands have been developed, aiming to provide either the cis or in some cases—the trans alkene. However, the Birch Reduction remains the go-to for achieving trans-alkenes specifically.

The key to successful partial hydrogenation lies in stopping the addition of hydrogen at the desired stage, which typically requires specialized catalysts to avoid over-reduction.

Trans-Alkene Formation

To specifically form trans-alkenes from alkynes, certain conditions and reagents need to be applied. Trans-alkenes are characterized by the opposite positioning of substituents across a carbon-carbon double bond.
The Birch Reduction method is ideal for this kind of transformation. It ensures the hydrogens added are on opposite sides of the molecule, leading to the trans configuration:

• Trans-alkene formation via Birch Reduction involves electron transfer and stepwise proton addition, ensuring that the bulky substituents do not crowd each other.
• This transformation is often preferred in synthesis when the spatial configuration impacts the compounds' chemical properties or biological activity.

A less commonly used but alternative method could involve the controlled use of specialized catalytic systems, though choices can be limited and may not guarantee the trans geometry. The trans-alkene formation is generally marked by its stability and lower energy compared to its cis counterpart, making it a valuable form in organic chemistry and synthesis.