Problem 36 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following compounds on treatment with \(\mathrm{NaNO}_{2} / \mathrm{HCl}\) and then coupled with phenol, produces p-hydroxyazobenzene? (a) phenol (b) aniline (c) azobenzene (d) nitrobenzene

Step-by-Step Solution

Answer

Aniline (b)

Step 1: Understanding the Reaction Requirements

In order to produce p-hydroxyazobenzene, a compound must first form a diazonium salt when treated with NaNO2 and HCl. This occurs at low temperatures in the range of 0-5 °C.

Step 2: Identifying Candidate Compounds

We need a compound that can be converted into a diazonium ion. Aromatic amines like aniline can undergo diazotization to form aryl diazonium salts, which can further proceed to couple with phenol.

Step 3: Analyzing Each Option

(a) Phenol does not form a diazonium compound. (b) Aniline forms a diazonium ion when treated with NaNO2 and HCl. (c) Azobenzene cannot form a diazonium salt directly as it lacks a NH2 group. (d) Nitrobenzene cannot form a diazonium salt as the nitro group is not reducible to the diazonium group without converting back to an amine.

Step 4: Determining Feasibility of Coupling with Phenol

Upon forming a diazonium ion, aniline can couple with phenol to produce the azo compound p-hydroxyazobenzene, whereas the other compounds cannot form diazonium salts needed for this reaction.

Step 5: Concluding with the Correct Compound

The compound that successfully undergoes these reactions is aniline (option (b)).

Key Concepts

Diazotization

Diazotization is a chemical reaction where an aromatic amine is converted into a diazonium salt. When you treat these amines with sodium nitrite (\( \mathrm{NaNO}_{2} \)) and hydrochloric acid (\( \mathrm{HCl} \)), diazonium salts form at low temperatures, typically 0-5 °C.
This transformation is crucial for creating a variety of azo compounds, which are a group of colorful organic compounds used in dyes and pigments.Some key points of diazotization:

• Only primary aromatic amines can form diazonium salts.
• The reaction is highly temperature-sensitive since higher temperatures can lead to unwanted byproducts.
• The resulting diazonium salts can easily react with phenols and other compounds to form azo dyes.

Understanding diazotization is fundamental in organic chemistry, especially in synthesizing complex organic structures like azo compounds.

Aniline

Aniline is a primary aromatic amine and plays a pivotal role in the diazotization reaction. As a starting point for synthesizing diazonium salts, aniline is comprised of an amino group (\( \mathrm{NH}_2 \)) attached directly to the benzene ring.
When subjected to sodium nitrite and hydrochloric acid, it forms the corresponding diazonium salt.Important aspects of aniline:

• Acts as a precursor for many dyes and pigments, including azo dyes.
• Its chemical structure, with the amino group, is crucial for diazotization.
• Due to its reactive nature, it's a key intermediate in many synthetic pathways.

Aniline's ability to form a diazonium ion makes it uniquely qualified to participate successfully in azo coupling reactions.

p-Hydroxyazobenzene

p-Hydroxyazobenzene is a type of azo compound that results from the coupling between a diazonium salt and phenol, specifically at the para position. The term "azo" refers to the functional group \( \mathrm{-N=N-} \), which is characterized by vibrant colors, making them useful as dyes, known as azo dyes.
During formation, the diazonium salt derived from aniline couples with phenol under alkaline conditions.Features of p-Hydroxyazobenzene:

• Exhibits strong, vivid color due to the azo linkage.
• Frequently utilized in creating synthetic colors and dyes.
• Serves as a fundamental structure in colorant chemistry.

This compound exemplifies the efficient and versatile nature of azo coupling reactions in generating colorful organic molecules.