AIIMS 2005 Chemistry Nucleophilic Substitution Assertion Reason Question

To analyze the question regarding the hydrolysis of methyl chloride to methanol, we need to break down both the assertion and the reason while also understanding the context of the reaction kinetics involved.

Assertion:

The rate of hydrolysis of methyl chloride to methanol is higher in DMF than in water.

Explanation: Methyl chloride (CH₃Cl) undergoes hydrolysis, which is a nucleophilic substitution reaction where the chloride ion (Cl⁻) is replaced by a hydroxyl group (OH⁻) from water or a solvent. In this case, DMF (Dimethylformamide) is a polar aprotic solvent, which enhances the nucleophilicity of the hydroxide ion compared to water.

In DMF, the solvent does not solvate the nucleophile (OH⁻) as strongly as water does, allowing it to be more reactive. This leads to a higher rate of hydrolysis in DMF. Therefore, the assertion is true.

Reason:

Hydrolysis of methyl chloride follows second order kinetics.

Explanation: The hydrolysis of methyl chloride is generally considered to follow first-order kinetics in a polar protic solvent like water, where the rate of reaction depends on the concentration of the substrate (methyl chloride) and the nucleophile (water). However, the reaction can exhibit second order kinetics in a scenario where the reaction is dependent on both the concentration of methyl chloride and hydroxide ions, particularly in a polar aprotic solvent like DMF.

Therefore, while the statement about the reaction possibly following second order kinetics can be valid under certain conditions, it is not a universal truth for all solvent systems. However, for the assertion about the rate of hydrolysis in DMF, it effectively supports the assertion that hydrolysis is faster in DMF. Thus, the reason provided is true, but it is not the sole reason for the assertion.

Conclusion:

Since both the assertion and reason are true, and the reason provides a valid explanation for the assertion, the correct answer is A) If both assertion and reason are true and reason is the correct explanation of assertion.

Clarification of Other Options:

• B) If both assertion and reason are true but reason is not the correct explanation of assertion: This option is incorrect because the reason does provide insight into why the hydrolysis rate is higher in DMF.
• C) If assertion is true but reason is false: This is incorrect as both the assertion and reason are true.
• D) If both assertion and reason are false: This is incorrect; both statements are true.

Summary

The hydrolysis of methyl chloride is indeed faster in DMF due to the solvent's properties enhancing nucleophilicity, and while the kinetics can vary depending on the conditions, the reasoning holds in the context of the assertion. Thus, the answer is A.