Problem 73 - A Complete Resource Book in Chemistry for JEE Main

Question

Compound A on reduction gives B, which on further reaction with \(\mathrm{CHCl}_{3}\) and alcoholic \(\mathrm{KOH}\) gives compounds C, which on further hydrolysis gives aniline. The compound \(\mathrm{A}\) is (a) nitrosobenzene (b) methylamine (c) nitromethane (d) nitrobenzene

Step-by-Step Solution

Answer

Compound A is nitrobenzene (d).

Step 1: Identify the Process of Reduction

Compound A is reduced to form compound B. In organic chemistry, reduction of aromatic nitro compounds, such as nitrobenzene, often results in the formation of aniline or derivatives thereof.

Step 2: Recognize Known Organic Reactions

When compound B reacts with \(\mathrm{CHCl}_{3}\) and alcoholic \(\mathrm{KOH}\), an intermediate compound known as isocyanate can be formed. Isoyanates can hydrolyze to form aniline.

Step 3: Analyze the Reaction Pathway

Nitrobenzene (option d) on reduction forms aniline, which is the product B. Then aniline can form benzyl isocyanate in the Hoffman rearrangement, which finally hydrolyzes to yield aniline again.

Step 4: Compare With Given Options

Identify which compound among the provided options can follow the reduction and subsequent reactions to eventually lead to the production of aniline. Nitrobenzene is capable of these transformations, as illustrated in the prior steps.

Key Concepts

Reduction of Nitro Compounds

In organic chemistry, the reduction of nitro compounds is a fundamental reaction that transforms nitro groups (O_2) into amino groups (H_2), transforming compounds like nitrobenzene into aniline. This process is typically achieved by using reducing agents such as hydrogen gas under high pressure, or chemical reducing agents like iron with hydrochloric acid.

During the reduction of nitrobenzene, the molecule undergoes a series of steps: the nitro group is firstly reduced to a nitroso group (O), then to a hydroxylamine (H_2OH), and finally to an amino group (H_2). The overall reaction simplifies as follows:\[ \text{C}_6\text{H}_5\text{NO}_2 \xrightarrow{\text{reduction}} \text{C}_6\text{H}_5\text{NH}_2 \]
These reactions are crucial as they provide pathways to produce amines, which serve as fundamental building blocks in synthetic chemistry, pharmaceuticals, dyes, and agricultural chemicals.

Hoffman Rearrangement

The Hoffman rearrangement, also known as the Hoffman degradation, is a reaction that transforms primary amides into primary amines with the formal loss of one carbon atom. It’s an important reaction for amine synthesis, particularly when adjusting the carbon chain length is required.
In the context of the given exercise, after reduction of nitrobenzene to aniline, aniline can react with chloroform ( Cl_3) in the presence of alcoholic potassium hydroxide (KOH). This unique reaction transforms an amine into an isocyanate intermediate through a method that briefly involves the formation of a carbonic imido compound.

The reaction mechanism can be summarized as follows:

• Formation of a chloroformate intermediate
• Rearrangement to an isocyanate
• Hydrolysis of isocyanate to produce the corresponding amine

Hoffman rearrangement is especially valuable in the synthesis of amines without extending the carbon chain, thereby allowing for selective synthesis of desired amine derivatives.

Aniline Synthesis

Aniline synthesis is a key process in the production of aniline, an important precursor in the manufacture of various industrial chemicals and polymers. Aniline is often synthesized from nitrobenzene via reduction processes.
Aniline itself is fascinating due to its simple structure and significant utility. After the reduction of nitrobenzene, through the methods described earlier, aniline may undergo several further transformations to yield valuable products.
Many industrial processes rely entirely on aniline for producing dyes, rubber processing chemicals, and pharmaceuticals.

• Aniline can be used in dyes like aniline yellow and mauveine.
• It forms urethane and other polymers as various chain intermediates are altered and created.
• The synthesis method based on reduction and rearrangement pathways results in high purity aniline, essential for specialized chemical reactions.

Aniline synthesis is crucial in chemistry due to its wide-ranging applications, making understanding of its production methods essential for students in organic chemistry.