Problem 7 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is used for the conversion of 2-hexyne into trans-2-hexane? (a) \(\mathrm{H}_{2} / \mathrm{Pd} / \mathrm{BaSO}_{4}\) (b) \(\mathrm{H}_{2}, \mathrm{PtO}_{2}\) (c) \(\mathrm{NaBH}_{4}\) (d) \(\mathrm{Li}-\mathrm{NH}_{3} / \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\)

Step-by-Step Solution

Answer

The correct reagent is (d) \(\mathrm{Li}-\mathrm{NH}_{3} / \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\).

Step 1: Analyze the Target Product

We want to convert 2-hexyne into trans-2-hexane. This means we're performing a partial reduction of the alkyne to an alkene and specifically forming the trans-alkene.

Step 2: Determine the Required Reaction Type

To achieve a trans-alkene (E-alkene), we need to use a reagent that selectively reduces alkynes to trans-alkenes rather than fully reducing them to alkanes or forming cis-alkenes.

Step 3: Evaluate the Given Options

Let's evaluate each option:- (a) \(\mathrm{H}_{2} / \mathrm{Pd} / \mathrm{BaSO}_{4}\) is known as Lindlar's catalyst which is used to convert alkynes to cis-alkenes.- (b) \(\mathrm{H}_{2}, \mathrm{PtO}_{2}\) usually reduces to alkanes completely.- (c) \(\mathrm{NaBH}_{4}\) is typically used to reduce carbonyl groups, not alkynes.- (d) \(\mathrm{Li}-\mathrm{NH}_{3} / \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) is a system that reduces alkynes to trans-alkenes.

Step 4: Select the Correct Option

Based on our analysis, option (d) \(\mathrm{Li}-\mathrm{NH}_{3} / \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) is the reagent that selectively reduces alkynes to trans-alkenes.

Key Concepts

Partial Reduction of Alkynes

In organic chemistry, partial reduction of alkynes is a crucial process used to transform an alkyne into an alkene. This process is termed 'partial' because it stops at the alkene stage rather than going all the way to an alkane, which is a fully hydrogenated form. This is important when the precise molecular geometry of an alkene is needed. During partial reduction, one of the two carbon-carbon triple bonds in the alkyne is broken, and hydrogen atoms are added to form a double bond.
This process requires specific catalysts or chemical environments to achieve the desired outcome. Various reduction agents can be chosen to control the nature of the alkyne to alkene conversion, targeting either cis or trans configurations of the resulting alkene.

Trans-alkene Formation

Trans-alkenes are characterized by having their substituents on opposite sides of the double bond. In chemical structures, this trans-configuration minimizes repulsion between bulky groups, often making trans-alkenes more stable compared to their cis counterparts. To prepare trans-alkenes from alkynes, certain reduction techniques must be employed.

• The use of the \(\mathrm{Li}-\mathrm{NH}_3 / \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}\) reagent allows for selective reduction, providing a trans-alkene.
• This procedure operates through a radical mechanism, involving single-electron transfers that specifically facilitate the formation of trans-alkenes.

For example, in converting 2-hexyne to trans-2-hexene, the reagent selectively interacts with the triple bond's electrons, ensuring that the addition of hydrogens results in the trans-configuration. This technique is highly valued for its ability to fine-tune the stereochemistry of organic compounds.

Selective Reduction Agents

Choosing the right reduction agents is key when aiming for specific chemical product formations in organic reactions. Selective reduction agents are tailored to achieve the desired changes without altering other parts of the molecule.
Let's look at some commonly used reduction agents:

• Lindlar's catalyst, composed of \(\mathrm{H_2 / Pd / BaSO_4}\), targets alkynes but steers them toward forming cis-alkenes.
• On the other hand, the use of liquid ammonia and lithium, \(\mathrm{Li}-\mathrm{NH}_3\), is effective for forming trans-alkenes from alkynes.

These agents work under specific conditions that facilitate partial and selective reductions, steering the conversion process within desired geometric orientations. Understanding the range and specificity of these reduction agents enables chemists to finely tune reactions to achieve the targeted molecular frameworks, crucial in producing specific organic products efficiently.