Problem 1 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Aniline dissolved in dilute \(\mathrm{HCl}\) is reacted with sodium nitrate at \(0{ }^{\circ} \mathrm{C}\). This solution was added dropwise to a solution containing equimolar mixture of aniline and phenol in dil. HCl. The structure of the major product is:

Step-by-Step Solution

Answer

The major product is p-hydroxyazobenzene.

Step 1: Recognition of Reagents

First, recognize that aniline is being reacted with sodium nitrate in the presence of dilute \(\text{HCl}\). This reaction involves the formation of a diazonium salt. Aniline reacts with nitrous acid (generated by sodium nitrite and hydrochloric acid) at \(0{ }^{\circ} \text{C}\) to form benzenediazonium chloride.

Step 2: Formation of Benzenediazonium Chloride

Under these conditions, aniline ( C_6H_5NH_2) reacts with nitrous acid to form benzenediazonium chloride ( C_6H_5N_2^+Cl^- ). The chemical reaction is as follows: \( C_6H_5NH_2 + HNO_2 + HCl \rightarrow C_6H_5N_2^+Cl^- \) \( + 2H_2O \)

Step 3: Reaction with Equimolar Mixture

The benzenediazonium chloride is added to a solution containing an equimolar mixture of aniline and phenol in dilute HCl. This step involves an electrophilic substitution reaction where the diazonium ion couples with the phenol in the acidic medium.

Step 4: Formation of the Major Product

In the presence of the acidic medium from the dilute HCl, the diazonium salt undergoes coupling with phenol to form p-hydroxyazobenzene. Phenol is more reactive towards diazonium ion coupling than aniline, which tends to form azo-dyes. Therefore, the major product is p-hydroxyazobenzene.

Key Concepts

Benzenediazonium chloride

Benzenediazonium chloride is an important organic compound, often used in synthetic chemistry. It is formed when aniline reacts with nitrous acid in the presence of hydrochloric acid at a low temperature (0°C). This reaction involves an interesting transformation where the amine group in aniline is converted into a diazonium salt. This process is crucial because benzenediazonium chloride acts as a key intermediate in many subsequent chemical reactions. Please remember that this compound is very unstable and can decompose above 5°C. As a result, reactions involving benzenediazonium chloride are typically performed at or below 0°C to maintain its stability.

Electrophilic substitution

The concept of electrophilic substitution is central to understanding many reactions in organic chemistry, particularly those involving aromatic compounds. In the case of the exercise, benzenediazonium chloride undergoes an electrophilic substitution reaction in the presence of an equimolar mixture of phenol and aniline. The diazonium ion acts as an electrophile, which means it is an atom or a group of atoms that is deficient in electrons and thus inclined to react with electron-rich species. When benzenediazonium chloride is added to the solution, it couples with phenol instead of aniline. This preference is because phenol has a highly activated aromatic ring due to the electron-donating effect of the hydroxyl group, which enhances its reactivity towards the electrophilic diazonium ion. Thus, the major product formed is a result of this selective electrophilic substitution.

Azo compound formation

The formation of azo compounds is a noteworthy process in organic chemistry, especially within the context of dyes. The coupling of benzenediazonium chloride with phenol leads to the formation of an azo compound, specifically p-hydroxyazobenzene, in the given exercise. Azo compounds are characterized by the presence of a nitrogen-nitrogen double bond (N=N), which is key to their colorful properties.

During the reaction, the diazonium ion, being an effective electrophile, couples with the phenol at the para position to the hydroxyl group. This position is preferred because it leads to a more stable transition state. The resulting p-hydroxyazobenzene is an example of an azo compound that has applications in creating dyes and pigments. This reaction showcases how simple aromatic compounds can be transformed into complex, useful materials through coupling reactions.