Problem 95 - A Complete Resource Book in Chemistry for JEE Main

Question

Phenyl diazonium chloride forms an azo dye with 1\. aniline 2\. phenol 3\. \(\mathrm{N}, \mathrm{N}\)-dimethyl aniline 4\. anisole (or methoxybenzene) (a) 1,2 and 3 are correct (b) 2,3 and 4 are correct (c) 1,3 and 4 are correct (d) 1,2 and 4 are correct

Step-by-Step Solution

Answer

The correct choice is (b) 2, 3, and 4 are correct.

Step 1: Understanding Azo Coupling Reactions

Azo dyes are formed by azo coupling reactions, which typically occur between a diazonium salt and an activated aromatic compound with electron-donating groups. The electron-donating groups enhance the reactivity of the aromatic ring towards electrophiles like the diazonium ion.

Step 2: Analyzing Aniline

Aniline has an amino group (-NH) that is a strong electron-donating group due to resonance and hyperconjugation effects. This makes aniline highly reactive to electrophilic aromatic substitutions, thus it can form azo dyes with phenyl diazonium chloride.

Step 3: Analyzing Phenol

Phenol contains a hydroxyl group (-OH), which is another strong electron-donating group. This enhances the activation of the benzene ring towards the electrophilic attack of the diazonium ion, enabling azo dye formation.

Step 4: Analyzing N,N-Dimethyl Aniline

N,N-Dimethyl aniline has a dimethylamino group (-(CH)N-), known for its strong electron-donating ability due to resonance and hyperconjugation. This results in increased reactivity of the benzene ring towards diazonium ions for azo dye formation.

Step 5: Analyzing Anisole

Anisole contains a methoxy group (-OCH) that releases electrons by resonance and is somewhat electron-donating, thus the benzene ring is activated to a moderate extent. This can lead to azo dye formation similar to phenol and aniline.

Step 6: Conclusion and Correct Choice

Phenyl diazonium chloride can form azo dyes with all the listed compounds because they all contain electron-donating groups that activate aromatic rings for coupling reactions. Thus, phenol, N,N-dimethyl aniline, and anisole can all undergo azo coupling. Therefore, option (b) 2, 3, and 4 are correct.

Key Concepts

Phenyl Diazonium Chloride

Phenyl diazonium chloride is a fascinating molecule utilized in organic chemistry for crafting vibrant azo dyes. This compound is a type of diazonium salt, recognized by its distinctive structure containing the diazonium group

• The diazonium group has the formula \( R-N_{2}^{+}Cl^{-} \), where \( R \) represents the aromatic group.


• This positively charged N-N bond is essential for diazonium salts to undergo nucleophilic substitution reactions.


• The presence of the chlorine anion helps stabilize the diazonium cation.

Phenyl diazonium chloride serves as a potent electrophile, or electron acceptor. It readily reacts with aromatic compounds possessing electron-donating groups, enabling the formation of azo dyes in a process known as azo coupling. Understanding the nature and reactivity of phenyl diazonium chloride helps in grasping how it serves as a building block in the synthesis of these colored compounds.

Electron-Donating Groups

Electron-donating groups play a crucial role in azo coupling reactions. These groups enhance the reactivity of aromatic compounds by providing additional electrons to the aromatic ring. The increase in electron density makes the aromatic ring more susceptible to attack from electrophiles.

• Common electron-donating groups include the amino group \(-NH_{2}\), hydroxyl group \(-OH\), and methoxy group \(-OCH_{3}\).


• They typically operate through resonance and inductive effects to donate electrons into the aromatic ring, increasing its electron density.


• These groups are pivotal in the formation of azo dyes, as they activate the ring for electrophilic aromatic substitution.

The effectiveness of azo dye formation is strongly influenced by the strength of the electron-donating group, determining how readily the aromatic compound will participate in coupling with the diazonium salt to create vibrant azo dyes.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a fundamental reaction in organic chemistry, particularly with regards to azo dye synthesis. It allows the introduction of substituents onto an aromatic ring in a controlled manner.

• In this process, an electrophile, such as the diazonium ion, attacks the electron-rich aromatic ring.


• The aromatic ring, activated by electron-donating groups, stabilizes the intermediate cation formed during the substitution.


• The result is the substitution of a hydrogen atom on the ring with the electrophile, creating a new aromatic compound.

This reaction is essential for forming the characteristic azo linkage \( -N=N- \) of azo dyes. The specifics of electrophilic aromatic substitution make it a cornerstone for further understanding more complex synthetics like azo dyes.

Azo Dyes Formation

Azo dyes are a distinct and colorful group of compounds whose formation relies on azo coupling reactions. These dyes are characterized by the azo group \( -N=N- \) linking two aromatic rings.

• During the reaction, the diazonium ion from phenyl diazonium chloride couples with an aromatic compound, leading to the formation of the azo bond.


• This process often results in vibrant colors, attributed to extensive conjugation between the linked aromatic rings.


• Azo dyes are utilized in various applications, including textiles, food additives, and as pigments, due to their intense and diverse color range.

The synthesis of azo dyes is a testament to the utility of electrophilic aromatic substitution reactions and the influence of electron-donating groups. Mastery of these fundamental concepts opens the door to appreciating the full spectrum of azo dye chemistry.