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JEE Main levelSingle correct optionDifficulty 2 of 3o3129

Assertion (A): The SN2 reaction of $CH X_{3} CH X_{2} CH X_{2} - Br$ with $NaCN$ is faster in DMSO than in methanol. Reason (R): DMSO has a higher boiling point than methanol.

AA is false but R is true
BBoth A and R are true, and R is the correct explanation of A
CBoth A and R are true, but R is not the correct explanation of A
DA is true but R is false

Answer: C

The assertion is true: methanol hydrogen-bonds to

CN X^{-}

and stabilises it, whereas DMSO leaves it poorly solvated and more reactive. The reason is also a true fact (DMSO boils at about 189

^{\circ}

C, methanol at 65

^{\circ}

C), but boiling point has no bearing on the rate at a fixed temperature, so it is not the explanation.

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