Problem 53 - A Complete Resource Book in Chemistry for JEE Main

Question

What is the end product \(\mathrm{C}\) in this series? \(\mathrm{CH}_{3} \mathrm{CN} \stackrel{\mathrm{Na} / \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}}{\longrightarrow} \mathrm{A} \stackrel{\mathrm{HNO}_{2}}{\longrightarrow} \mathrm{B}\) \(\stackrel{\mathrm{Cu} / 573 \mathrm{~K}}{\longrightarrow} \mathrm{C}\) (a) \(\mathrm{CH}_{3} \mathrm{COOH}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NHOH}\) (c) \(\mathrm{CH}_{3} \mathrm{CONH}_{2}\) (d) \(\mathrm{CH}_{3} \mathrm{CHO}\)

Step-by-Step Solution

Answer

The end product \( \mathrm{C} \) is acetaldehyde, \( \mathrm{CH}_3\mathrm{CHO} \), corresponding to option (d).

Step 1: Identify the Reaction Type

The first step involves the conversion of acetonitrile \( \mathrm{CH}_{3}\mathrm{CN} \) to compound \( \mathrm{A} \) using sodium in ethanol. This reaction is known as a reduction of the nitrile group to form an aldehyde. The reaction can be represented as \( \mathrm{CH}_{3}\mathrm{CN} + 4[H] \rightarrow \mathrm{CH}_{3}\mathrm{CH}_2\mathrm{NH}_2 \). Compound \( \mathrm{A} \) is ethylamine \( \mathrm{CH}_{3}\mathrm{CH}_2\mathrm{NH}_2 \).

Step 2: Perform the Diazotization

Next, compound \( \mathrm{A} \), ethylamine, reacts with \( \mathrm{HNO}_{2} \) to form compound \( \mathrm{B} \). This process is called diazotization, which converts primary amines to diazonium salts. Thus, \( \mathrm{CH}_{3}\mathrm{CH}_2\mathrm{NH}_2 + \mathrm{HNO}_{2} \rightarrow \mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{N}_{2}^{+}\mathrm{Cl}^{-} + \mathrm{H}_{2}\mathrm{O} \) results in ethyl diazonium chloride, which temporarily breaks down to form ethane bound with a diazonium group.

Step 3: Catalytic Decomposition

Finally, compound \( \mathrm{B} \) decomposes when treated with copper at \( 573 \mathrm{~K} \), leading to compound \( \mathrm{C} \). The reaction results in the replacement of the diazonium group with hydrogen, forming acetaldehyde \( \mathrm{CH}_{3}\mathrm{CHO} \). This is due to the generation of nitrogen gas and the subsequent decomposition induced by copper.

Step 4: Identify the End Product

From the sequence of reactions, it is determined that the final product (compound \( \mathrm{C} \)) is acetaldehyde, \( \mathrm{CH}_{3}\mathrm{CHO} \), which matches option (d).

Key Concepts

Reduction of Nitriles

In organic chemistry, the reduction of nitriles is an important reaction that transforms a nitrile into different forms such as amines or aldehydes. In our exercise, we focus on transforming acetonitrile (\( \mathrm{CH}_{3}\mathrm{CN} \)) into ethylamine \( \mathrm{CH}_{3}\mathrm{CH}_2\mathrm{NH}_2 \) using sodium in ethanol. This process involves the addition of hydrogen atoms, reducing the nitrile to its corresponding amine.

• The reaction is carried out by using sodium and ethanol, both acting as reducing agents.
• Reduction results in the breaking of the C-N triple bond present in nitriles.
• This chemical process falls under the broader category of reduction reactions, which primarily include gain of hydrogen or loss of oxygen.

By understanding the reduction of nitriles, students can predict other amines or aldehydes formation from various nitrile compounds.

Diazotization

Diazotization is a fascinating chemical reaction where a primary amine is converted into a diazonium salt. This reaction is crucial in organic synthesis, especially when dealing with nitrogen-containing compounds. In this case, ethylamine (\( \mathrm{CH}_{3}\mathrm{CH}_2\mathrm{NH}_2 \)) undergoes diazotization with nitrous acid (\( \mathrm{HNO}_{2} \)) to form ethyl diazonium chloride.

• The diazonium salt is usually unstable at high temperatures or in the presence of strong nucleophiles.
• This instability plays a significant role in further reactions, as it prepares the compound for further transformation or decomposition reactions.
• Diazotization is primarily used for the preparation of aromatic diazonium salts but can also apply to aliphatic amines in elaborate synthesis plans like this one.

Understanding diazotization helps students grasp the intricacies of forming diazonium compounds from amines, providing a deeper insight into complex reaction pathways.

Catalytic Decomposition

The process of catalytic decomposition involves breaking down complex molecules into simpler ones using catalysts and external energy like heat. In the given exercise, ethyl diazonium chloride decomposes in the presence of copper at a high temperature of 573 K.

• Catalysts like copper facilitate easier breaking of chemical bonds in the diazonium compound.
• The decomposition of diazonium compounds generally involves the liberation of nitrogen gas, simplifying the entire molecule.
• The reaction often results in the formation of a more stable molecule; in this context, acetaldehyde (\( \mathrm{CH}_{3}\mathrm{CHO} \)).

Catalytic decomposition highlights the importance of catalysts in accelerated reactions and their role in the efficient conversion of complex reactants to simple products.

Aldehyde Formation

Aldehyde formation is a significant step in organic synthesis, often achieved by reducing nitriles or decomposing diazonium salts. The reaction sequence in the exercise concludes with the formation of acetaldehyde (\( \mathrm{CH}_{3}\mathrm{CHO} \)) as the end product.

• Aldehydes contain the functional group \(-CHO\), characterized by a carbon atom double-bonded to oxygen and single-bonded to hydrogen.
• The synthesis of aldehydes can happen through multiple routes, including reduction using specific catalysts or agents.
• Aldehydes are crucial in organic chemistry due to their reactivity and role as intermediates in further reactions.

By grasping aldehyde formation, students learn how various precursors, through reduction or decomposition, can originate versatile functional groups for further chemical transformations.