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

Question

Compound \(\mathrm{A}\) on reduction gives \(\mathrm{B}\), which on further reaction with \(\mathrm{CHCl}_{3}\) and alcoholic KOH gives compounds \(\mathrm{C}\), 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 \( \mathrm{A} \) is (d) nitrobenzene.

Step 1: Understanding the Reaction Pathway

We begin by analyzing the reaction pathway described in the problem. The compound \( \mathrm{A} \) is reduced to compound \( \mathrm{B} \). Then, \( \mathrm{B} \) undergoes a reaction with chloroform (\( \mathrm{CHCl_3} \)) and alcoholic potassium hydroxide (KOH) to form compounds \( \mathrm{C} \). Finally, \( \mathrm{C} \) is hydrolyzed to give aniline (\( \mathrm{C_6H_5NH_2} \)). This sequence suggests that compound \( \mathrm{B} \) is likely an intermediate capable of furnishing an aryl amine upon further reaction steps.

Step 2: Identifying the Final Product

The ultimate goal is to obtain aniline. Aniline is an aromatic amine, structurally \( \mathrm{C_6H_5NH_2} \). Thus, compound \( \mathrm{C} \) must be capable of yielding this aromatic amine upon hydrolysis.

Step 3: Recognizing the Role of \( \mathrm{CHCl_3} \) and Alcoholic KOH

\( \mathrm{CHCl_3} \) and alcoholic KOH are known to participate in the Reimer-Tiemann reaction, which introduces a \( \mathrm{CHO} \) group to phenolic compounds. However, if an amine intermediate is involved, it must form an isocyanide, a characteristic reaction with primary amines and chloroform under basic conditions.

Step 4: Determining the Structure of Compound \( \mathrm{B} \)

For \( \mathrm{B} \) to react with \( \mathrm{CHCl_3} \) and KOH to yield aniline, \( \mathrm{B} \) should be a primary amine. Considering reduction reactions, compound \( \mathrm{A} \) could be \( \mathrm{C_6H_5NO_2} \) (nitrobenzene), which upon reduction, gives aniline (\( \mathrm{C_6H_5NH_2} \)).

Step 5: Verifying Compound \( \mathrm{A} \)

Since the reduction of \( \mathrm{A} \) results in aniline directly, compound \( \mathrm{A} \) must be nitrobenzene, making option (d) the correct choice. The Reimer-Tiemann pathway aligns with the transformation of a nitro group to an amine, reaffirming this choice.

Key Concepts

Organic Chemistry Reactions

Organic chemistry involves countless reactions, each with its unique mechanisms and transformations. These reactions are fundamental to creating new compounds and understanding the structure and reactivity of organic molecules.

Organic chemistry reactions often include processes like substitution, addition, elimination, and rearrangement. These reactions can involve different functional groups, which are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Understanding how and why these reactions occur allows chemists to predict product formation and design synthesis pathways.

What makes organic reactions fascinating is their variety and versatility:

• **Substitution Reactions**: An atom or group in a molecule is replaced by another atom or group.
• **Addition Reactions**: Atoms are added to a molecule, typically seen in alkenes and alkynes.
• **Elimination Reactions**: Molecules lose atoms or groups to form double or triple bonds.
• **Rearrangement Reactions**: The structure of the molecule changes by reconfiguring atoms.

Understanding these reactions' mechanisms is crucial for problem-solving in exams like JEE Main, where students are expected to know how different chemical reactions contribute to compound transformation.

Aniline Synthesis

Aniline, or phenylamine, is an aromatic amine with the formula \( \mathrm{C_6H_5NH_2} \). It is a significant compound in the chemical industry used for making dyes, rubber processing chemicals, and pharmaceuticals. The synthesis of aniline is an important reaction in organic chemistry, often involving the reduction of nitrobenzene.

The process typically begins with nitrobenzene \( \mathrm{C_6H_5NO_2} \), which is reduced to aniline using a suitable reducing agent, such as tin and hydrochloric acid, or iron filings with HCl. This reduction transforms the nitro group \( \mathrm{-NO_2} \) into an amino group \( \mathrm{-NH_2} \).

The reduction can be summarized as:\[ \mathrm{C_6H_5NO_2 + 6[H] \rightarrow C_6H_5NH_2 + 2H_2O} \]This process showcases the conversion of a compound containing a nitro group to one with an amino group, exemplifying a classic transformation in organic synthesis.

Reimer-Tiemann Reaction

The Reimer-Tiemann reaction is a fascinating reaction primarily used to introduce a formyl group \( \mathrm{(-CHO)} \) into the ortho position of phenols. It employs chloroform \( \mathrm{CHCl_3} \) and a strong base, such as potassium hydroxide \( \mathrm{KOH} \). However, in our context, the focus is on its application with amines.

Involving amines under similar conditions can lead to the formation of isocyanides. This is due to the reaction of primary amines with \( \mathrm{CHCl_3} \) in the presence of a base. Importantly:

• The reaction conditions—strong base and chloroform—are conducive to forming intermediates like isocyanides, particularly with primary aromatic amines.
• When taking an initial compound like nitrobenzene, reducing it first to an aniline allows subsequent reactions under Reimer-Tiemann conditions for further transformations or intermediate formation.

This process highlights the versatility of the Reimer-Tiemann reaction beyond phenols, particularly relevant for students preparing for competitive exams who need to understand these reaction conditions more broadly.

Reduction Reactions in Organic Chemistry

Reduction reactions in organic chemistry are processes where a molecule gains electrons, often by the addition of hydrogen. This process is typically utilized in transforming nitro groups to amino groups, a common need in preparing aromatic amines like aniline.

A typical scenario in organic chemistry is the reduction of nitrobenzene to aniline. This reaction proceeds as follows:

• Nitrobenzene, with its nitro group, is reduced using specific agents like iron and hydrochloric acid.
• During this reduction, the \( \mathrm{NO_2} \) group is effectively transformed into the \( \mathrm{NH_2} \) group, producing aniline.

This reduction process showcases the transformation of complex functional groups and serves as a pillar for numerous industrial synthesis processes. The insights gained here can be pivotal for students, as understanding reduction reactions are essential, especially when dealing with transformations leading to important industrial and laboratory chemicals.