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

Question

What is the end product \(\mathrm{C}\) in this series? \(\mathrm{CH}_{3} \mathrm{CN} \stackrel{\mathrm{Na} / \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}}{\longrightarrow}\) \(\mathrm{A} \stackrel{\mathrm{HNO}_{2}}{\longrightarrow} \mathrm{B}\) \({ }^{\mathrm{Cu} / 573 \mathrm{~K}}{\longrightarrow} \mathrm{C}\) (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 \(\mathrm{CH}_{3}\mathrm{CHO}\) (acetaldehyde), option (d).

Step 1: Reduction of Acetonitrile

The first reaction involves the reduction of acetonitrile (\(\mathrm{CH}_{3}\mathrm{CN}\)) in the presence of sodium (\(\mathrm{Na}\)) and ethanol (\(\mathrm{C}_{2}\mathrm{H}_{5} \mathrm{OH}\)). This reduction leads to the conversion of the nitrile group (\(\mathrm{CN}\)) into a primary amine, yielding ethylamine, \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2}\) (\(\mathrm{A}\)). Thus, \(\mathrm{A}\) is \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2}\).

Step 2: Diazotization Process

The primary amine \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2}\) produced in Step 1 undergoes diazotization when treated with nitrous acid (\(\mathrm{HNO}_{2}\)). This generates a diazonium salt intermediate, which decomposes to form ethanol (\(\mathrm{B}\) being \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\)).

Step 3: Oxidation of Ethanol

The ethanol (\(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\)) is then oxidized in the presence of copper at 573 K, converting it into acetaldehyde (\(\mathrm{CH}_{3}\mathrm{CHO}\)). Therefore, \(\mathrm{C}\) is \(\mathrm{CH}_{3}\mathrm{CHO}\).

Key Concepts

Reduction of Nitriles

Reduction of nitriles is an essential reaction in organic chemistry. It involves the conversion of nitriles, which are compounds containing the \( \mathrm{CN} \) group, into amines. This is achieved by using a reducing agent, typically in the presence of a metal catalyst and an alcohol solvent.
In this specific scenario, acetonitrile \( (\mathrm{CH}_{3}\mathrm{CN}) \) is reduced using sodium \( (\mathrm{Na}) \) and ethanol \( (\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}) \). This leads to the formation of ethylamine \( (\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2}) \), where the nitrile group is converted into a primary amine.

• The sodium acts as a reducing agent, donating electrons needed for the reduction.
• Ethanol serves as a solvent, helping facilitate the reaction without participating directly.

Overall, this reaction transforms the triple bond in the nitrile group into single bonds connecting to the freshly added hydrogen atoms, thus converting it into an amine.

Diazotization Process

The diazotization process is a unique reaction commonly applied to primary amines. This procedure involves converting a primary amine into a diazonium salt using nitrous acid \( (\mathrm{HNO}_{2}) \).
During the process, the primary amine \( (\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{NH}_{2}) \), which was formed in the reduction step, reacts with nitrous acid. This creates a temporary diazonium ion intermediate. However, not all diazotization reactions end at the formation of stable diazonium salts.

• In some cases, as it happens here, the diazonium ion spontaneously decomposes to produce a different compound.
• The spontaneity often leads to a substitution reaction where the diazonium group is replaced by another molecule or ion, generating alcohol, such as ethanol \( (\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}) \) in this case.

Diazotization is a delicate process, as the stability of the diazonium salts determines the final products that can be obtained.

Oxidation of Alcohols

Oxidation of alcohols is an important transformation in organic synthesis. It typically involves the conversion of alcohols into aldehydes or ketones, depending on the type of alcohol and the strength of the oxidizing agent.
In this context, ethanol \( (\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}) \), which was obtained from the earlier diazotization process, is oxidized into acetaldehyde \( (\mathrm{CH}_{3}\mathrm{CHO}) \).
The oxidation is facilitated by copper \( (\mathrm{Cu}) \) metal at high temperatures (573 K).

• Copper acts as a catalyst, speeding up the reaction rate without being consumed.
• High temperatures ensure that the ethanol molecules have sufficient energy to undergo the oxidation process.

This conversion of alcohols to aldehydes highlights the nuanced ability to alter simple alcohols into valuable carbonyl compounds.