NEET 2025 · ChemistryPrevious Year Question
Given below are two statements: one labelled Assertion (A) and the other Reason (R). Assertion (A): $\ce{CH3CH2CH2I}$ (n-propyl iodide) undergoes $\ce{S_N2}$ reaction faster than $\ce{CH3CH2CH2Cl}$ (n-propyl chloride). Reason (R): Iodine is a better leaving group because of its large size. In the light of the above statements, choose the correct answer from the options given below.
- A.(A) A is true but R is false
- B.(B) A is false but R is true
- C.(C) Both A and R are true and R is the correct explanation of A✓
- D.(D) Both A and R are true but R is not the correct explanation of A
Correct Answer
(C) (C) Both A and R are true and R is the correct explanation of A
Solution & Explanation
Both statements are true and R correctly explains A. Iodide is a better leaving group than chloride: the larger, more polarisable iodide ion carries its negative charge more diffusely and the weaker $\ce{C-I}$ bond breaks more easily. A better leaving group lowers the $\ce{S_N2}$ transition-state energy, so $\ce{CH3CH2CH2I}$ reacts faster than $\ce{CH3CH2CH2Cl}$.
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