Question

The correct structures of A and B formed in the following reactions are : 

 

• A. A
• B. B
• C. C
• D. D

Hint:

In molecules with both amino (–NH\(_2\)) and hydroxyl (–OH) groups, the amino group is generally a stronger nucleophile and will react preferentially in acylation reactions. This selectivity is a key principle in organic synthesis.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We need to identify the products A and B in a two-step reaction sequence starting from p-nitrophenol. The first step is a reduction, and the second step is a selective acylation.
Step 2: Detailed Explanation:
Reaction 1: Formation of A
The starting material is p-nitrophenol. It is treated with H\(_2\)/Pd in ethanol. This is a standard catalytic hydrogenation reaction. This reagent is excellent for reducing a nitro group (–NO\(_2\)) to a primary amino group (–NH\(_2\)) without affecting other functional groups like the phenolic hydroxyl (–OH) or the benzene ring.
\[ \text{p-Nitrophenol} \xrightarrow{\text{H}_2/\text{Pd, C}_2\text{H}_5\text{OH}} \text{p-Aminophenol} \] So, product A is p-aminophenol.
Reaction 2: Formation of B
Product A (p-aminophenol) is treated with 1.0 equivalent of acetic anhydride ((CH\(_3\)CO)\(_2\)O). Acetic anhydride is an acylating agent that introduces an acetyl group (CH\(_3\)CO–).
p-Aminophenol has two nucleophilic sites: the nitrogen of the amino group and the oxygen of the hydroxyl group. We need to determine which is more reactive.
The lone pair on the amino nitrogen is more available and more basic/nucleophilic than the lone pair on the phenolic oxygen (which is delocalized into the benzene ring). Therefore, the amino group will react preferentially with the acylating agent.
Since only one equivalent of acetic anhydride is used, it will selectively acylate the more nucleophilic amino group.
\[ \text{p-Aminophenol} + 1 \text{ eq. Acetic Anhydride} \rightarrow \text{N-(4-hydroxyphenyl)acetamide} \] This is an N-acylation reaction. Product B is N-(4-hydroxyphenyl)acetamide (also known as Paracetamol).
Step 3: Matching with Options:
We look for the option where A is p-aminophenol and B is N-(4-hydroxyphenyl)acetamide. This corresponds to the structures shown in option (C).
Step 4: Final Answer:
The correct structures for A and B are given in option (C).