AIIMS 2006 Chemistry Nucleophilic Substitution Assertion Reason Question

To analyze the question regarding the assertion and reason provided, let's break down each component systematically.

Assertion:

"The presence of nitro group facilitates nucleophilic substitution reactions in aryl halides."

This statement is true. The nitro group ($-NO_2$) is a strong electron-withdrawing group due to its resonance and inductive effects. When present on an aryl halide, it enhances the electrophilicity of the carbon atom bonded to the halogen (the leaving group). This increased electrophilicity makes the carbon more susceptible to attack by nucleophiles, facilitating nucleophilic substitution reactions.

Reason:

"The intermediate carbanion is stabilised due to the presence of nitro group."

This statement is also true. During nucleophilic substitution reactions, particularly in the case of aryl halides, a carbanion intermediate can be formed. The nitro group stabilizes this carbanion through resonance. When the nucleophile attacks, the negative charge that develops on the carbon can be delocalized onto the nitro group, which stabilizes the negative charge. This stability of the carbanion intermediate is crucial for the reaction to proceed efficiently.

Conclusion on Correct Answer:

Both the assertion and the reason are true, and the reason correctly explains why the assertion is true. Therefore, the correct answer is:

A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Clarification on Incorrect Options:

• B) Both Assertion and Reason are true but Reason is not the correct explanation of Assertion. This option is incorrect because the reason provided does indeed explain why the presence of a nitro group facilitates nucleophilic substitution reactions. The stabilizing effect of the nitro group on the carbanion is directly relevant to the assertion.

• C) Assertion is true but Reason is false. This option is incorrect as both the assertion and reason are true. The stabilizing effect of the nitro group on the carbanion is a valid statement.

• D) Both Assertion and Reason are false. This option is also incorrect, as both statements are true. The nitro group does facilitate nucleophilic substitution, and it does stabilize the carbanion intermediate.

Key Concepts:

1. Nucleophilic Substitution Reactions: These reactions involve the replacement of a leaving group (like a halide) with a nucleophile. In the case of aryl halides, the presence of activating groups (like nitro) can significantly enhance the reaction rate.

2. Carbanion Stability: The stability of carbanions can be assessed based on the ability of adjacent groups to stabilize the negative charge via resonance or inductive effects. The nitro group, being a strong electron-withdrawing group, effectively stabilizes the carbanion formed during the nucleophilic substitution process.

In summary, the presence of the nitro group increases the electrophilicity of the aryl halide and stabilizes the intermediate carbanion, thereby facilitating the nucleophilic substitution reaction.