Problem 101 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Azo dyes are prepared from: (a) benzene (b) aniline (c) benzaldehyde (d) benzoic acid

Step-by-Step Solution

Answer

Aniline (option b) is used to prepare azo dyes.

Step 1: Understand Azo Dye Formation

Azo dyes are synthetic colors composed of azo compounds, which have the characteristic -N=N- azo group. They are typically produced through a chemical reaction known as diazotization, followed by azo coupling.

Step 2: Identify Typical Starting Materials

The formation of azo dyes usually starts with aromatic amines. During the diazotization process, aniline is often used because it is a primary aromatic amine (C6H5NH2), which is essential for the formation of the diazonium salt intermediate.

Step 3: Match with Given Options

Review the options: benzene, aniline, benzaldehyde, and benzoic acid. Among these, only aniline matches the requirement for a primary aromatic amine, making it the correct choice for the preparation of azo dyes.

Key Concepts

Diazotization

Diazotization is a vital chemical process used in the preparation of azo dyes. It involves converting aromatic amines into diazonium salts. These salts are key intermediates in many synthesis reactions. During diazotization, an aromatic amine, such as aniline, reacts with nitrous acid (HNO2), which is typically generated in-situ from sodium nitrite (NaNO2) and a strong acid like hydrochloric acid (HCl). This reaction is quite important as diazonium salts are highly reactive and unstable, meaning they must be used quickly once formed.
The key reaction can be represented as follows:

• Aniline (C6H5NH2) + Nitrous Acid (HNO2) → Diazonium Salt (C6H5N2⁺Cl⁻) + Water (H2O)

Understanding this reaction is crucial because it sets the stage for the subsequent azo coupling step. It is worth noting that the diazonium salt can only be formed under cold conditions, which helps in stabilizing the salt before it decomposes or reacts further.

Aromatic Amines

Aromatic amines are compounds that consist of an amino group attached to an aromatic ring. They are fundamental in the formation of azo dyes. One of the most common aromatic amines used in this process is aniline (C6H5NH2). The aromatic ring structure provides stability to the amine, allowing it to undergo the necessary chemical transformations.
The central importance of aromatic amines lies in their ability to form diazonium salts via the diazotization process.

• Anilines are primary aromatic amines, meaning the nitrogen atom is directly attached to the carbon ring.
• They are key to a variety of synthetic applications, including dye and pharmaceutical production.

Not all aromatic compounds possess this amine group, so only those with it, like aniline, are suitable for azo dye production. This defines why aniline is chosen among potential precursors such as benzene or benzoic acid for making azo dyes.

Azo Coupling

Azo coupling is a chemical reaction where the diazonium ion, created during the diazotization process, reacts with an aromatic compound. This step culminates in the formation of the azo dye, which is characterized by the azo group (-N=N-) linking two aromatic rings. The process of azo coupling usually takes place under alkaline or neutral conditions.
The reaction can be illustrated as follows:

• Diazonium Compound (Ar-N2⁺) + Aromatic Compound (Ar'-H) → Azo Dye (Ar-N=N-Ar') + Acid

The choice of coupling partner, often another aromatic amine or phenol, affects the color and properties of the final azo dye. Azo dyes are notable for their vivid colors, wide palette options, and application in textiles and coloring agents. The process of azo coupling is highly versatile, allowing for the synthesis of a vast array of dye types by altering the structures of the starting materials.