Problem 3 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The most appropriate reagent for conversion of \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{CN}\) into \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) is :(a) \(\mathrm{NaBH}_{4}\) (b) \(\mathrm{CaH}_{2}\)(c) \(\mathrm{LiAlH}_{4}\) (d) \(\mathrm{Na}(\mathrm{CN}) \mathrm{BH}_{3}\)

Step-by-Step Solution

Answer

The most appropriate reagent is \(\mathrm{LiAlH}_{4}\).

Step 1: Analyze the Reaction Type

The reaction involves the conversion of an aliphatic nitrile \(\mathrm{C}_{2} \mathrm{H}_{5}\mathrm{CN}\) to a primary amine \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\). This is a reduction reaction where the nitrile group (\(-\mathrm{CN}\)) is reduced to an amine group (\(-\mathrm{NH}_{2}\)).

Step 2: Identify Possible Reducing Agents

We need to choose a reducing agent that can effectively reduce the nitrile group to an amine. Common reducing agents for such transformations are metal hydrides like \(\mathrm{LiAlH}_{4}\) and \(\mathrm{NaBH}_{4}\).

Step 3: Evaluate Effectiveness of Reducing Agents

\(\mathrm{NaBH}_{4}\) is a mild reducing agent and is typically used to reduce carbonyl groups (aldehydes and ketones), but not nitriles. \(\mathrm{LiAlH}_{4}\) is a strong reducing agent capable of reducing nitriles to primary amines. \(\mathrm{CaH}_{2}\) is mainly used for dehydrating alcohols rather than reducing nitriles. \(\mathrm{Na}( ext{CN}) \mathrm{BH}_{3}\) is another mild reducing agent used primarily for aldehydes and ketones as well. Thus, \(\mathrm{LiAlH}_{4}\) is the appropriate choice.

Step 4: Select the Most Appropriate Reagent

Based on the analysis of the effectiveness of each reagent, \(\mathrm{LiAlH}_{4}\) is the most suitable reducing agent to convert \(\mathrm{C}_2\mathrm{H}_5\mathrm{CN}\) into \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{NH}_2\) as it can perform the reduction of nitriles to primary amines.

Key Concepts

Reducing Agents

Reducing agents play a crucial role in chemical reactions, especially in converting more complex structures into simpler ones. They donate electrons to other molecules, prompting reduction reactions. There are various reducing agents available for different chemical reactions:

• Metal Hydrides: Like \(\text{LiAlH}_4\) and \(\text{NaBH}_4\), these are commonly used in organic synthesis to reduce diverse functional groups.
• Hydrogen Gas: Often used in industrial reactions, combined with specific catalysts to reduce compounds.

Choosing the correct reducing agent is key to the success of a chemical transformation. It depends on both the functional group to be reduced and the desired product after reduction. Understanding the properties of each reducing agent helps in planning efficient and targeted reactions.

Nitrile to Amine Conversion

The conversion of nitriles to amines is an essential process in organic chemistry. This reaction involves converting the nitrile group (\(-\text{CN}\)) into a primary amine (\(-\text{NH}_2\)). It is important for forming amines, which are vital intermediates or end products in various chemical syntheses, including pharmaceuticals.
In practice, the nitrile to amine transformation is a reduction reaction. The choice of reducing agent greatly influences the efficiency and yield of the reaction. Strong reducing agents are often necessary to effectively break the triple bond in nitriles and replace it with a single bond, attaching hydrogen atoms to produce the amine.
This conversion underscores the significance of understanding the reaction conditions and the specific role of reducing agents in controlling the chemical pathway to achieve desired outcomes.

LiAlH4 as Reducing Agent

\(\text{LiAlH}_4\), or lithium aluminum hydride, is one of the most powerful reducing agents known. It is particularly famous for its ability to reduce nitriles to primary amines efficiently. When discussing the reduction of \(\text{C}_2\text{H}_5\text{CN}\) to \(\text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2\), \(\text{LiAlH}_4\) proves to be the ideal candidate.

• It is highly reactive and demonstrates the capacity to break the strong \(\text{C}-\text{N}\) triple bond in nitriles.
• It delivers hydride ions (\(\text{H}^-\)), which nonpolar, favorably interact with the polar nitrile group.

Safety and handling are pivotal while using \(\text{LiAlH}_4\), due to its high reactivity, especially with water, which can lead to violent reactions. Nevertheless, its effectiveness in nitrile to amine conversions makes it invaluable in synthetic organic chemistry.