Problem 65 - A Complete Resource Book in Chemistry for JEE Main

Question

If a compound reacts with \(\mathrm{HNO}_{2}\) but does not form a dye with \(\beta-\) naphthol. The compound may contain (a) \(\mathrm{NH}_{2}\) group as the main functional group (b) \(\mathrm{NH}_{2}\) group in the side chain (c) The compound may have a phenolic group (d) Both (a) and (b)

Step-by-Step Solution

Answer

The compound may contain NH_{2} group in the side chain (b).

Step 1: Understanding the Problem

The exercise is about determining the presence of specific functional groups in a compound based on its reactivity with nitrous acid (HNO_2) and its ability to form a dye with β-naphthol.

Step 2: Chemical Reactivity Analysis

If a compound reacts with nitrous acid (HNO_2), it likely contains an NH_{2} group (a primary amine or akin group), as primary amines form diazonium salts with nitrous acid.

Step 3: Dye Formation Test

The compound does not form a dye with β-naphthol, which suggests that the compound does not create an azo dye. Typically, azo dyes are formed when a diazonium salt reacts with a phenol or an activated aromatic compound.

Step 4: Concluding Functional Groups Analyzed

Given the reactivity details, the compound must contain an NH_{2} group to react with nitrous acid but not in a configuration that allows dye formation with β-naphthol. Thus, it may not contain a phenol group nor an aromatic system needed for dye formation.

Step 5: Determining Correct Answer

Since the compound reacts with HNO_{2} (requiring NH_{2}) but doesn't form a dye, it seems reasonable to conclude that the NH_{2} group is present, but not in a position interacting to form an azo dye. This matches option (b) best: NH_{2} group in the side chain.

Key Concepts

Functional Group Identification

Identifying functional groups in organic chemistry is essential to understanding how a compound will react. Functional groups are specific groups of atoms within a molecule that determine its chemical behavior. In the compound described, identifying an \(\text{NH}_2\) group is crucial. Compounds with amine groups, like \(\text{NH}_2\), are highly reactive with certain reagents, such as nitrous acid \(\text{HNO}_2\).
When a compound reacts with \(\text{HNO}_2\), it often indicates the presence of a primary amine functional group, which can form a diazonium salt under such conditions. This is a critical first step in narrowing down which groups are present.
If the functional group doesn't lead to accomplish dye formation, it further implies that while the \(\text{NH}_2\) group exists, other necessary components such as activated aromatic rings that facilitate dye formation may be missing or arranged in a non-reactive format.

Amines and Diazonium Salts

Amines, particularly primary amines, are key players in organic chemistry reactions due to their ability to form diazonium salts. When a primary amine \((\text{NH}_2)\) reacts with nitrous acid \( (\text{HNO}_2) \), a transformation occurs, forming a diazonium salt. This reaction is an integral part of many synthetic pathways for creating dyes and other complex organic compounds.
Diazonium salts are unique because they are both reactive and unstable. They provide a versatile intermediate that can be used in further chemical reactions, especially in coupling reactions to form azo compounds.
However, the stability of diazonium salts depends on the surrounding conditions, such as temperature and pH, which need to be carefully controlled. If mismanaged, these salts can decompose, leading to less desirable side products.

Azo Dye Formation

Azo dye formation is an important process in the field of organic chemistry, particularly in the dye and textile industries. Azo dyes are characterized by the presence of the azo group \((-N=N-)\), which is responsible for their vivid colors. These dyes are generally formed through a coupling reaction between a diazonium salt and a phenol or an activated aromatic compound.
In the context of the original exercise, the formation of an azo dye needed both the presence of a diazonium salt and an appropriate aromatic partner like \(\beta\)-naphthol to proceed.
The failure to form such a dye helps pinpoint the position and nature of the \(\text{NH}_2\) group and the absence of an aromatic system capable of undergoing this further reaction. This observation not only helps in determining the compound's structure but also in understanding the underlying requirements for azo dye formation.