Problem 22 - A Complete Resource Book in Chemistry for JEE Main

Question

The indicator that is obtained by coupling the diazonium salt of sulphanilic acid with N, N-dimethylaniline is (a) methyl orange (b) phenolphthalene (c) indigo (d) phenanthroline

Step-by-Step Solution

Answer

(a) methyl orange.

Step 1: Identify the Reaction Type

The reaction involves coupling a diazonium salt with an amine, specifically sulphanilic acid with N, N-dimethylaniline. This is known as an azo coupling reaction, a common method to produce azo dyes.

Step 2: Recall Common Azo Dyes

Azo dyes are characterized by the presence of an azo group (-N=N-) and are often brightly colored. Methyl orange is an example of a known azo dye, often used as an indicator in titrations.

Step 3: Check Component Suitability

Methyl orange is synthesized from sulphanilic acid and an amine. The given pairing, with N,N-dimethylaniline, fits the profile for creating such a dye.

Step 4: Select the Best Match

Given the structure and described process, associate this synthesis with the known structure of methyl orange, which is an azo dye derived from a similar coupling reaction.

Key Concepts

Diazonium Salts

Diazonium salts are fascinating compounds that play a crucial role in the azo coupling reaction. They are typically composed of an aromatic amine group where the nitrogen atom is bonded to a chloride ion. These salts are represented as Ar-N extsuperscript{+}≡N Cl extsuperscript{-}, where Ar refers to an aromatic group. Due to their structure, they are highly reactive intermediates useful in organic synthesis.

One of the most important characteristics of diazonium salts is their ability to participate in coupling reactions, particularly forming azo compounds. These reactions often occur in slightly alkaline conditions.

• They are crucial in forming bright azo dyes.
• Act as intermediates in creating various aromatic compounds.

Diazonium salts are typically synthesized from the reaction of aromatic amines with nitrous acid. The process involves using a cold acidic solution to maintain stability, as these salts decompose at higher temperatures. Despite their reactivity, they are advantageous in industrial and laboratory settings for synthesizing colors and indicators.

Azo Dyes

Azo dyes are a class of synthetic dyes characterized by the presence of one or more azo groups (-N=N-). These dyes are known for their vivid colors and extensive use across various industries. The azo group acts as a chromophore, meaning it is the part of the molecule responsible for its color.

Azo dyes can cover a spectrum of colors, making them a versatile choice for many applications. They can be synthesized through the azo coupling reaction where diazonium salts react with aromatic compounds.

• They are widely used in textile dyeing, providing long-lasting hues.
• Used in biological staining and as pH indicators in lab reactions.

Because of their structural diversity and vibrant colors, azo dyes are crucial in scientific research and industrial applications, including indicators like methyl orange.

Chemical Indicators

Chemical indicators are specially formulated substances that signal the presence, absence, or concentration of various chemicals in a solution through noticeable changes. In most cases, these changes come in the form of color shifts.

Indicators like methyl orange, a type of azo dye, are essential in titration experiments where the endpoint of a reaction is determined by a distinct color change. Methyl orange is commonly used as an acid-base indicator due to its clear color change from red under acidic conditions to yellow under basic conditions.

• The change is noticeable and occurs over a narrow pH range.
• They aid in determining acidity or alkalinity in laboratory experiments.

This property of undergoing a color change makes chemical indicators invaluable in scientific analysis and educational labs.

Synthesis of Methyl Orange

The synthesis of methyl orange exemplifies the typical process involved in creating azo dyes. This particular dye is produced through an azo coupling reaction, involving diazonium salts. Specifically, sulphanilic acid is first converted into its diazonium salt form, which is then coupled with N,N-dimethylaniline.

The process is carefully controlled to ensure the proper reaction conditions:

• Maintain a low temperature to stabilize diazonium salts, preventing premature decomposition.
• Conduct the reaction in an alkaline medium to facilitate coupling.

Methyl orange's structural formula symbolizes its function as a pH indicator. Its synthesis highlights the beauty of organic reactions in creating compounds with specific, useful properties in both industrial applications and scientific inquiry. This dye, with its distinct color changes, showcases the intricate dance of chemistry that leads to the creation of something both useful and beautiful.