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 \(\mathrm{N}, \mathrm{N}\)-dimethylaniline is (a) methyl orange (b) phenolphthalene (c) indigo (d) phenanthroline

Step-by-Step Solution

Answer

(a) methyl orange

Step 1: Identify the Coupling Reaction

The reaction involved is a coupling of the diazonium salt with an aromatic compound. In this case, it's the diazonium salt of sulphanilic acid with \( \mathrm{N}, \mathrm{N} \)-dimethylaniline.

Step 2: Understand the Reaction Products

The product formed from this coupling reaction is an azo dye. Coupling of diazonium salts with aniline derivatives typically forms azo compounds that serve as dyes.

Step 3: Recognize the Specific Dye Formed

The specific azo dye formed from the coupling of the diazonium salt of sulphanilic acid with \( \mathrm{N}, \mathrm{N} \)-dimethylaniline is methyl orange.

Step 4: Match with Options

From the provided options, identify which one matches the chemical reaction described: (a) methyl orange, (b) phenolphthalene, (c) indigo, (d) phenanthroline. As analyzed, the coupling forms methyl orange.

Key Concepts

Diazonium Salt

Diazonium salts are a group of organic compounds that play a key role in various chemical reactions, especially in dye synthesis. These compounds are highly reactive due to the presence of a diazonium group \(-N_{2}^{+}\), which is typically bonded to an aromatic ring. The presence of the positively charged nitrogen makes them very useful in transferring nitrogen-containing groups to other molecules.

The formation of diazonium salts usually involves the reaction between an aromatic amine and nitrous acid. This acid is often generated in situ by reacting sodium nitrite with a strong acid, like hydrochloric acid.

Diazonium salts have some intriguing properties that make them suitable for further chemical transformations:

• They can undergo substitution reactions, where the diazonium group is replaced by another group like halogens, -OH, or -CN.
• They are intermediates in the synthesis of azo dyes, a large class of synthetic dyes.

In the given exercise, the diazonium salt of sulphanilic acid reacts with N,N-dimethylaniline in a coupling reaction to form an azo dye known as methyl orange.

Coupling Reaction

A coupling reaction is a fundamental process in organic chemistry used to join two organic molecules, often resulting in the formation of dyes and pigments. In the context of azo dye formation, it involves the reaction between a diazonium salt and an aromatic amine or phenol.

The coupling transpires as the diazonium compound reacts with an electron-rich aromatic compound, like N,N-dimethylaniline in our exercise. This reaction is crucial because the azo linkage \(-N=N-\) that is formed is responsible for the vibrant colors associated with azo dyes.

Key Characteristics of a Coupling Reaction:

• It typically occurs at a position ortho or para to existing electron-donating groups on the aromatic ring.
• The reaction usually happens under mildly acidic to neutral pH conditions to promote stability and reactivity of the diazonium ion.
• It results in the formation of an azo compound, characterized by the distinct -N=N- double bond.

When the diazonium salt of sulphanilic acid couples with N,N-dimethylaniline, methyl orange, a well-known azo dye and pH indicator, is synthesized.

Methyl Orange Indicator

Methyl orange is one of the most prominent azo dyes, known mostly for its function as a pH indicator. It is characterized by its transition between colors at different pH levels.

When in acidic solutions, methyl orange appears red. As the pH increases, it shifts color to a yellowish hue. This clear color change around the pH range of 3.1 to 4.4 makes it particularly useful in titration procedures where a strong acid is titrated against a strong base.

Why Methyl Orange is Useful:

• It has a sharp and distinct color change, which can be easily observed.
• The color transition contrasts sharply in the relevant pH range, aiding in accurate assessments of acidity or alkalinity.

The synthesis of methyl orange involves the coupling reaction of diazonium salts with N,N-dimethylaniline. This process highlights the significance of chemical transformations in producing useful compounds for scientific and industrial applications.