Problem 9 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reacts with \(\mathrm{NaNO}_{2}+\mathrm{HCl}\) at \(273-278 \mathrm{~K}\) to give alcohol/phenol? (a) \(\mathrm{CH}_{3} \mathrm{NH}_{2}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{NHCH}_{3}\) (d) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\)

Step-by-Step Solution

Answer

(b) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \) aniline reacts to give phenol.

Step 1: Understanding the Reaction Conditions

The reaction involves sodium nitrite (\( \mathrm{NaNO}_2 \)) and hydrochloric acid (\( \mathrm{HCl} \)) at a temperature of 273-278 K. This is a typical setup for a diazotization reaction, where primary aromatic amines are converted into diazonium salts.

Step 2: Identify the Type of Amines

We need to identify which of the given compounds can undergo a diazotization reaction:- (a) \( \mathrm{CH}_{3} \mathrm{NH}_{2} \) is a primary aliphatic amine.- (b) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \) is a primary aromatic amine, also known as aniline.- (c) \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{NHCH}_{3} \) is a secondary amine.- (d) \( \left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH} \) is a secondary amine.

Step 3: Determine Eligibility for Diazotization

For diazotization, the compound must be a primary amine and aromatic to form stable diazonium salts. Only anilines (primary aromatic amines) undergo this reaction smoothly and can further undergo hydrolysis to form phenols.

Step 4: Identify the Correct Compound for Reaction

Only compound (b), \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2} \) (aniline), is a primary aromatic amine. Aniline reacts with \( \mathrm{NaNO}_2 \) and \( \mathrm{HCl} \) to form a diazonium salt, which upon hydrolysis yields phenol.

Step 5: Conclusion

The given reaction conditions specifically facilitate the diazotization of aniline to form phenol, as no other type of amine in the given options fits this reaction scheme.

Key Concepts

Primary Aromatic Amines

Primary aromatic amines are an essential class of organic compounds that have a fundamental role in diazotization reactions. These compounds are characterized by having an amino group (\(\text{NH}_2\)) directly attached to an aromatic ring, like benzene. A common example of a primary aromatic amine is aniline, which is represented as \(\text{C}_6\text{H}_5\text{NH}_2\).
The key aspect of these amines is their ability to participate in reactions that are uniquely suited to the chemical behavior of aromatic rings. They are distinct from primary aliphatic amines, where the amino group is attached to an aliphatic (non-aromatic) carbon chain.

• The aromatic ring in primary aromatic amines can stabilize certain intermediates through resonance.
• This stabilization makes them the ideal candidates for specific reactions, like diazotization.

When reacting with sodium nitrite (\(\text{NaNO}_2\)) and hydrochloric acid (\(\text{HCl}\)) under cold conditions (273-278 K), aniline can be transformed into a diazonium salt. This step is critical for the overall reaction pathway leading to phenol formation.

Diazonium Salts

Diazonium salts are pivotal intermediates in many organic synthesis processes, particularly in transforming primary aromatic amines into other functional groups.
These salts are created when a primary aromatic amine reacts with \(\text{NaNO}_2\) and \(\text{HCl}\). The reaction proceeds through a diazotization mechanism, forming a diazonium ion, which can be represented generically as \(\text{ArN}_2^+\text{Cl}^-\).
The formation of diazonium salts is temperature-sensitive, and typically carried out at cool temperatures (273-278 K) to ensure the stability of the salt.

• Diazonium salts are known for their versatility and are used to generate a wide range of aromatic compounds.
• One of the most common uses is in converting diazonium salts to phenols via hydrolysis.

The diazonium group (\(\text{N}_2^+\)) is quite reactive, and its presence in the aromatic structure makes it possible to introduce other substituents through further reactions, enabling extensive diversity in chemical synthesis.

Phenol Formation

Phenol formation from diazonium salts is a critical step in organic chemistry transformations, primarily involving the hydrolysis of diazonium salts. When a diazonium salt is formed, it can be converted into a phenol by the addition of water (a process known as hydrolysis).
In this specific context, the phenol formation from aniline goes through the intermediate stage of forming a diazonium salt. Upon warming or in the presence of water, the diazonium group (\(\text{N}_2^+\)) leaves, and a hydroxyl group (\(\text{OH}\)) takes its place on the aromatic ring, resulting in phenol.

• The reaction is represented as \(\text{ArN}_2^+\text{Cl}^- + \text{H}_2\text{O} \rightarrow \text{ArOH}\).
• Phenols have unique chemical properties, mainly due to the hydroxyl group's electron-donating nature.

These properties of phenol make it an important compound in the production of various materials, including dyes, plastics, and pharmaceuticals. Understanding the formation process provides insight into how fundamental chemistry processes can be used to create a wide variety of chemical products.