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JEE Main levelSingle correct optionDifficulty 2 of 3o1025

For the reaction $HO X^{-} + CH X_{3} Br CH X_{3} OH + Br X^{-}$ carried out in ethanol-water mixtures, increasing the proportion of water produces

Ano change, because no ionisation occurs
Ba small decrease in rate, because the charge is more dispersed in the transition state than on the localised $OH X^{-}$
Ca large increase in rate, because charge separation develops in the transition state
Da small increase in rate, because water is the better nucleophile

Answer: B

The anionic nucleophile has its charge concentrated on a small oxygen, while in the transition state the charge is spread over

O \cdot \cdot \cdot C \cdot \cdot \cdot Br

. A more polar protic solvent therefore stabilises the reactants slightly more than the transition state, so the rate falls modestly.

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