AIIMS 2003 Chemistry Nucleophilic Substitution Reactions Assertion Reason Question

To analyze the given question regarding the reaction of benzyl bromide in acetone-water and whether it follows an $S_N2$ mechanism, let's break down both the assertion and the reason.

Assertion:

Benzyl bromide when kept in acetone water produces benzyl alcohol.

Explanation: Benzyl bromide (C₆H₅CH₂Br) is a primary alkyl halide. When benzyl bromide is treated with a nucleophile in a polar aprotic solvent like acetone, a nucleophilic substitution reaction occurs. In this case, water acts as a nucleophile.

The reaction can be represented as follows:

1. The hydroxide ion (from water) attacks the carbon atom bonded to the bromine atom.
2. This leads to the displacement of the bromide ion and the formation of benzyl alcohol (C₆H₅CH₂OH).

The overall reaction can be summarized as:

$\text{C}_6\text{H}_5\text{CH}_2\text{Br} + \text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_5\text{CH}_2\text{OH} + \text{Br}^-$

Reason:

The reaction follows $S_N2$ mechanism.

Explanation: The $S_N2$ mechanism is characterized by a single concerted step where the nucleophile attacks the carbon atom from the opposite side of the leaving group (bromine in this case), resulting in a transition state where both the nucleophile and the leaving group are partially bonded to the carbon atom.

For benzyl bromide, the $S_N2$ mechanism is plausible because:

• It is a primary halide, allowing for steric accessibility, which is essential for nucleophilic attack.
• The polar aprotic solvent (acetone) stabilizes the transition state, facilitating the reaction.

Conclusion:

Since both the assertion and the reason are correct, the correct answer is A. The reaction does produce benzyl alcohol, and it indeed follows the $S_N2$ mechanism.

Why Other Options Are Incorrect:

• Option B (Assertion True, Reason False): This would imply that the assertion is correct, but the reason does not support it, which is not the case here. The reason correctly identifies the mechanism.
• Option C (Assertion False, Reason True): This would suggest the reaction does not produce benzyl alcohol, which is incorrect as shown in the assertion.
• Option D (Both False): This would claim both the assertion and the reason are false, which is also incorrect since both statements are true.

Summary

• Assertion: True - Benzyl bromide does yield benzyl alcohol in acetone-water.
• Reason: True - The substitution reaction follows the $S_N2$ mechanism due to the nature of the substrate and solvent.

Thus, the correct answer is A.