AIIMS 2005 Chemistry Electrophilic Substitution Reactions Assertion Reason Question

To analyze the question effectively, we will evaluate both the Assertion and the Reason provided, focusing on the principles of aromatic chemistry and the Friedel-Crafts acylation reaction.

Assertion:

p-O₂N – C₆H₅COCH₃ is prepared by Friedel Crafts acylation of nitrobenzene.

Reason:

Nitrobenzene easily undergoes electrophilic substitution reaction.

Correct Answer: D (Both Assertion and Reason are false)

Explanation:

1. Why the Assertion is False:

   • Friedel-Crafts acylation is a type of electrophilic aromatic substitution (EAS) reaction where an acyl group is introduced to an aromatic ring. However, nitrobenzene (C₆H₅NO₂) is a very poor substrate for Friedel-Crafts reactions. This is primarily due to the strong electron-withdrawing nature of the nitro group (-NO₂), which significantly deactivates the aromatic ring towards electrophilic attacks.
   • The presence of the nitro group decreases the electron density of the aromatic system, making it less nucleophilic and, therefore, less reactive toward electrophiles. As a result, the acylation of nitrobenzene to form p-O₂N – C₆H₅COCH₃ is not feasible under standard Friedel-Crafts conditions.

2. Why the Reason is False:

   • The statement that "Nitrobenzene easily undergoes electrophilic substitution reaction" contradicts the fundamental behavior of nitrobenzene. Due to the electron-withdrawing effects of the nitro group, nitrobenzene is actually a poor candidate for electrophilic substitution. Instead of facilitating substitution, the nitro group hinders it, making the aromatic ring much less reactive compared to benzene or other more electron-rich aromatics.
   • Nitrobenzene can undergo some electrophilic substitution reactions, but these reactions require much harsher conditions or specific catalysts, which is not characteristic of "easily" undergoing such reactions.

Summary of Each Option:

• Option A: Incorrect, as both the Assertion and Reason are false.
• Option B: Incorrect, as both the Assertion and Reason are false.
• Option C: Incorrect, as both the Assertion and Reason are false.
• Option D: Correct, as both the Assertion and the Reason are indeed false.

Key Concepts:

• Friedel-Crafts Acylation: This is an electrophilic aromatic substitution that introduces an acyl group (–CO–R) into an aromatic compound. The reaction typically requires a strong electrophile and a Lewis acid catalyst (like AlCl₃).

• Electrophilic Aromatic Substitution (EAS): In EAS, an electrophile replaces a hydrogen atom on the aromatic ring, generally requiring an electron-rich aromatic substrate.

To summarize, because nitrobenzene is deactivated towards electrophilic substitution due to the presence of the -NO₂ group, it cannot be used effectively in Friedel-Crafts acylation, making both the Assertion and the Reason false. Thus, the correct answer is D.