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Assertion (A): 1-iodobutane undergoes $S_N2$ reaction faster than 1-chlorobutane.
Reason (R): Iodine is a better leaving group because of its large size.
A
Both A and R are true and R is the correct explanation of A.
B
Both A and R are true but R is not the correct explanation of A.
C
A is true but R is false.
D
A is false but R is true.
Explanation
Both A and R are true, and Reason explains Assertion because iodide is a superior leaving group due to its large size and lower basicity.
Detailed Solution
In $S_N2$ nucleophilic substitution, the rate-determining step involves simultaneous displacement of the leaving group. The rate follows the order: $\text{R}-\text{I} > \text{R}-\text{Br} > \text{R}-\text{Cl} > \text{R}-\text{F}$. Iodide ion ($\text{I}^-$) has a larger ionic radius, lower charge density, and weaker $\text{C}-\text{I}$ bond strength, making it a much better leaving group (weaker conjugate base of strong acid HI) than chloride ($\text{Cl}^-$). Thus, Assertion and Reason are true and Reason is the correct explanation of Assertion.