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

Aniline (\(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{2}\)) reacts with \(\mathrm{NaNO}_{2}\) and \(\mathrm{HCl}\) to give phenol.

Step 1: Understand the Reaction

The reaction of an amine with NaNO2 and HCl at low temperatures (273-278 K) usually results in diazotization. Primary aromatic amines like aniline undergo diazotization to form diazonium salts. These diazonium salts can be hydrolyzed to yield phenols.

Step 2: Identify Eligible Amines

We need to identify which of the compounds is an aromatic amine that can form a diazonium ion. Look at the options: - (a) \(\mathrm{CH}_{3}\mathrm{NH}_{2}\) is a primary aliphatic amine, which does not form stable diazonium salts. - (b) \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{2}\) is aniline, a primary aromatic amine. - (c) \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CH}_{2}\mathrm{NHCH}_{3}\) is a secondary amine, not eligible. - (d) \((\mathrm{CH}_{3})_{2} \mathrm{NH}\) is a secondary amine, not eligible.

Step 3: Determine the Reacting Amine

Among the given options, only \(\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{NH}_{2}\), aniline, is a primary aromatic amine capable of undergoing diazotization to form a diazonium salt, which can further react to produce a phenol.

Key Concepts

Primary Aromatic Amine

In the field of organic chemistry, primary aromatic amines are fascinating compounds. They consist of an amine group (\(\mathrm{NH}_2\)) attached directly to an aromatic ring. In the context of our exercise, we look specifically at aniline, which is represented as \(\mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2\). This simple yet crucial molecule plays a key role in many reactions due to its structure.

• Aromatic Ring: The benzene ring—depicted by \(\mathrm{C}_6\mathrm{H}_5\)—provides stability and unique reactivity.
• Amine Group: The presence of the \(\mathrm{NH}_2\) group allows aniline to participate in a variety of chemical reactions, like diazotization.

Primary aromatic amines are known for undergoing diazotization, a process that involves reacting with nitrous acid (formed in situ from \(\mathrm{NaNO}_2\) and \(\mathrm{HCl}\)) to produce diazonium salts under cold conditions.
The ability of these amines to form stable diazonium salts is unique compared to aliphatic amines, which tend to decompose. This makes them valuable in synthetic chemistry applications.

Diazonium Salt

Diazonium salts are key intermediates in organic synthesis, especially when formed from primary aromatic amines like aniline. When aniline reacts with \(\mathrm{NaNO}_2\) and \(\mathrm{HCl}\) at low temperatures, it undergoes diazotization. This reaction produces benzene diazonium chloride, a type of diazonium salt.

• Stability: These salts are stable only at cool temperatures, making the reaction conditions crucial.
• Structure: They feature a \(\mathrm{N}\equiv\mathrm{N}^+\) group connected to the aromatic ring, which retains the aromatic nature of the compound.

Due to their reactivity, diazonium salts are versatile intermediates:

• Substitution Reactions: They can be used to introduce various functional groups onto the aromatic ring.
• Decomposition: Under certain conditions, they can undergo decomposition to yield phenols.

These multiple pathways illustrate how diazonium salts expand the toolkit of synthetic chemists and highlight their importance in the conversion processes in organic syntheses.

Phenol Formation

Phenol formation from diazonium salts is a classic transformation used in organic chemistry. This conversion is particularly important because it allows for the introduction of the hydroxy group into an aromatic ring.
Starting from a diazonium salt like benzene diazonium chloride, the hydrolysis process can be initiated. This involves substituting the diazonium group with a hydroxyl group \(\mathrm{OH}\).

• Hydrolysis: The reaction with water, usually in slightly warmed conditions, aids in this transformation from diazonium salt to phenol.
• Product: The resulting product is phenol (\(\mathrm{C}_6\mathrm{H}_5\mathrm{OH}\)), characterized by its distinctive aromatic ring attached to a hydroxyl group.

Phenol itself holds substantial importance due to its properties and use in a wide range of applications—from plastics and pharmaceuticals to dyes and resins.
Thus, understanding the diazotization reaction and subsequent phenol formation not only uncovers the reactive potential of diazonium salts but also provides insight into the synthesis of valuable aromatic compounds.