Question

Name the reagent used in the conversion of benzene to nitrobenzene.

• A. Concentrated \(HNO_3\) only
• B. Concentrated \(H_2SO_4\) only
• C. Mixture of concentrated \(HNO_3\) and \(H_2SO_4\)
• D. Dilute \(HNO_3\)

Hint:

In nitration of benzene, concentrated \(H_2SO_4\) helps generate the electrophile \(NO_2^+\) from \(HNO_3\), which then substitutes a hydrogen atom in the benzene ring.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks for the specific reagent required to carry out the nitration of benzene, which is an electrophilic aromatic substitution reaction.
Step 2: Detailed Explanation:
The conversion of benzene (\(C_6H_6\)) to nitrobenzene (\(C_6H_5NO_2\)) requires an electrophile to attack the electron-rich benzene ring. The required electrophile is the nitronium ion (\(NO_2^+\)).

Nitric acid (\(HNO_3\)) alone, especially when dilute, is not a strong enough electrophile to react with the stable benzene ring.
To generate the powerful nitronium ion electrophile, concentrated nitric acid is treated with an even stronger acid, which is concentrated sulfuric acid (\(H_2SO_4\)).
The sulfuric acid protonates the nitric acid, which then loses a molecule of water to form \(NO_2^+\).
The overall reaction for the generation of the electrophile is: \[ \text{HNO}_3 + 2\text{H}_2\text{SO}_4 \rightleftharpoons \text{NO}_2^+ + \text{H}_3\text{O}^+ + 2\text{HSO}_4^- \] This mixture of concentrated nitric acid and concentrated sulfuric acid is known as the "nitrating mixture".
Step 3: Final Answer:
The correct reagent is a mixture of concentrated \(HNO_3\) and \(H_2SO_4\).