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) \( \mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2 \) reacts to form phenol.

Step 1: Understanding the Reaction Conditions

The reaction involves using sodium nitrite \( \mathrm{NaNO}_2 \) and hydrochloric acid \( \mathrm{HCl} \) at 273-278 K. This reaction typically leads to diazotization, where a primary aromatic amine is converted into a diazonium salt, which can further undergo replacement by water to form phenol.

Step 2: Identify the Nature of the Compounds

To undergo diazotization, the compound reacting with \( \mathrm{NaNO}_2 + \mathrm{HCl} \) should be a primary aryl amine. Let's analyze each option: (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.(c) \( \mathrm{C}_6 \mathrm{H}_5 \mathrm{CH}_2 \mathrm{NHCH}_3 \) is a secondary amine linked to an aromatic ring.(d) \( \left(\mathrm{CH}_3\right)_2 \mathrm{NH} \) is a secondary aliphatic amine.

Step 3: Determine the Reactivity

Primary aromatic amines like \( \mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2 \) can be diazotized to form diazonium salts because they have both the necessary amine group and the aromatic ring to stabilize the diazonium ion. These can further react with water, producing phenol.

Step 4: Conclusion of the Compound Forming Phenol

Among the given options, only option (b) \( \mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2 \) has the necessary structure to undergo diazotization in the given reaction conditions. The diazonium salt formed from this reaction can subsequently be hydrolyzed to yield phenol.

Key Concepts

Primary Aromatic Amines

Primary aromatic amines are a crucial class of compounds in organic chemistry. They contain an amine group \((\mathrm{NH}_2)\) directly attached to an aromatic ring, such as benzene. This structural arrangement is key for their reactivity.
These amines typically have the general formula \(\mathrm{ArNH}_2\), where \(\mathrm{Ar}\) stands for the aromatic group.
A common example is aniline, \(\mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2\), which plays a significant role in various chemical reactions.

Primary aromatic amines can undergo diazotization, a unique process that distinguishes them from aliphatic amines.
Diazotization involves the conversion of the \(\mathrm{NH}_2\) group into a diazonium group \((\mathrm{N}_2^+Cl^-)\) in the presence of nitrous acid, \(\mathrm{HNO}_2\).
This reaction typically requires cold temperatures around 273-278 K to proceed efficiently.

• Aromatic structure allows stabilization of the intermediate diazonium ion.
• The presence of the \(\mathrm{NH}_2\) group on the aromatic ring is essential for forming diazonium salts.

Recognizing primary aromatic amines is an essential step in predicting their reactivity and the possible transformations they can undergo.

Diazonium Salt Formation

Diazonium salt formation is a critical process in organic chemistry, often used to synthesize various compounds like phenols and aromatic halides.
When primary aromatic amines react with sodium nitrite \(\mathrm{NaNO}_2\) and hydrochloric acid \(\mathrm{HCl}\) at low temperatures, they form diazonium salts. This reaction is known as diazotization.
The overall process primarily occurs at temperatures between 273 and 278 K to ensure stability of the diazonium ion.

• The amine reacts with nitrous acid \(\mathrm{HNO}_2\), generated in situ by the reaction between \(\mathrm{NaNO}_2\) and \(\mathrm{HCl}\).
• This forms an unstable intermediate, which rearranges to create the diazonium ion, \(\mathrm{N}_2^+Cl^-\).
• The diazonium salt is generally stable at cold temperatures and decomposes at higher temperatures, which is why controlling the reaction temperature is crucial.

Diazonium salts are versatile intermediates in organic synthesis, allowing for further chemical transformations into a multitude of useful compounds.

Conversion to Phenol

Once we have synthesized diazonium salts from primary aromatic amines, we can proceed to convert them into phenols. This transformation is quite important in organic chemistry, as phenols themselves are valuable compounds in the production of various chemicals and pharmaceuticals.
The conversion to phenol primarily involves the hydrolysis of the diazonium salt.

To perform this conversion, the diazonium salt is treated with water, often under warm conditions or with a catalyst.
This processes results in the replacement of the diazonium group \(\mathrm{N}_2^+\) with a hydroxyl group \(\mathrm{OH^-}\), leading to the formation of phenol (\(\mathrm{ArOH}\)).

• The hydrolysis of diazonium salts is usually straightforward, given the instability of the diazonium group at higher temperatures.
• This conversion is distinguished by the release of nitrogen gas \(\mathrm{N}_2\), which is not only a driving force for the reaction but also a good indicator of its progress.

This conversion is an economical method of phenol synthesis, offering a simple route to access these essential compounds from primary aromatic amines.