Problem 69 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify C in this series. \(\mathrm{CH}_{3} \mathrm{CN} \stackrel{\mathrm{Na} / \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{OH}}{\longrightarrow} \mathrm{A} \stackrel{\mathrm{HNO}_{2}}{\longrightarrow} \mathrm{B} \stackrel{[\mathrm{O}]}{\longrightarrow} \mathrm{C}\) (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NHOH}\) (b) \(\mathrm{CH}_{3} \mathrm{COOH}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CONH}_{2}\) (d) \(\mathrm{CH}_{3} \mathrm{CHO}\)

Step-by-Step Solution

Answer

The series results in C being acetic acid, which matches option (b) \( \mathrm{CH}_3\mathrm{COOH} \).

Step 1: Identify Reaction Type for A

The first reaction involves the conversion of acetonitrile ( CH_3CN ) with sodium in ethanol (C_2H_3OH ). This is a typical reduction reaction that converts a nitrile to a primary amine. Therefore, the compound A is ethylamine ( CH_3CH_2NH_2 ).

Step 2: Reaction of A to Form B

The reaction of ethylamine with nitrous acid ( HNO_2 ) leads to the formation of a diazonium salt, which immediately decomposes to form ethanol ( CH_3CH_2OH ). Hence, the compound B is ethanol.

Step 3: Oxidize B to Identify C

The oxidation of ethanol ( CH_3CH_2OH ) leads to the formation of acetic acid ( CH_3COOH ). Oxidizing ethanol first produces acetaldehyde which further oxidizes to form acetic acid. Therefore, compound C is acetic acid.

Key Concepts

Reduction Reaction

In chemistry, a reduction reaction is a key part of many processes and involves the gain of electrons by an atom, ion, or molecule. This process decreases the oxidation state of the involved substance. It's important to remember that in every redox reaction, there is a pair of complementary processes: reduction and oxidation. So, in simpler terms, when something gains electrons, that's reduction.

In the original exercise, acetonitrile (CH_3CN) is reduced using sodium (Na) in ethanol (C_2H_3OH) as a solvent. This results in the reduction of nitrile to a primary amine, ethylamine (CH_3CH_2NH_2). Sodium donates electrons during this process, making it a reducing agent.

Common signs of a reduction reaction include:

• Gain of hydrogen
• Loss of oxygen
• Decrease in oxidation number

Recognizing these signs can help you identify reduction reactions in chemical equations.

Oxidation Reaction

Oxidation reactions are the opposite of reduction reactions and involve the loss of electrons, leading to an increase in the oxidation state of a substance. In a redox reaction, oxidation and reduction occur simultaneously.

In the context of the given exercise, the oxidation process occurs when ethanol (CH_3CH_2OH) is oxidized to acetic acid (CH_3COOH). Alcohols like ethanol can undergo oxidation in several steps. Initially, the primary alcohol forms an aldehyde and, if the conditions are conducive, further oxidizes into an acid.

Characteristics of an oxidation process include:

• Loss of hydrogen
• Gain of oxygen
• Increase in oxidation number

Learning to spot these characteristics will help you easily identify oxidation reactions in complex chemical scenarios.

Ethanol Oxidation

Ethanol oxidation is a classic example of an oxidation reaction where a primary alcohol like ethanol is transformed through different stages. Initially, ethanol (CH_3CH_2OH) is converted into acetaldehyde (CH_3CHO), and with sufficient oxidizing conditions, it further oxidizes to acetic acid (CH_3COOH).

This process is crucial in organic synthesis as well as biological systems. Ethanol oxidation is often carried out using oxidizing agents like chromic acid (H_2CrO_4) or potassium permanganate (KMnO_4). Understanding this multi-step process is vital for grasping how primary alcohols behave in oxidation reactions.

Key points to remember about ethanol oxidation:

• It involves two main steps: formation of an aldehyde, followed by the formation of an acid.
• Proper oxidizing conditions are essential for complete oxidation to acetic acid.
• Ethanol oxidation products are widely used in food, beverages, and industrial applications.

Recognizing these steps and conditions can greatly aid in mastering oxidation reactions involving alcohols.