When the solvent is changed from ethanol to a more polar ethanol-water mixture, the rates of (P) (C2H5)3N + CH3I -> (C2H5)3N+-CH3 I-, (Q) the S_N1 solvolysis of (CH3)3C-Br, and (R) HO- + CH3I -> CH3OH + I- change as follows:

P increases markedly, Q increases markedly, R decreases slightly

P no change, Q increases markedly, R decreases markedly

P increases markedly, Q increases markedly, R decreases slightly

P increases markedly, Q decreases, R no change

P decreases, Q increases markedly, R increases markedly

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JEE Advanced levelSingle correct optionDifficulty 3 of 3o1047

When the solvent is changed from ethanol to a more polar ethanol-water mixture, the rates of (P) $(C X_{2} H X_{5}) X_{3} N + CH X_{3} I (C X_{2} H X_{5}) X_{3} N X^{+} - CH X_{3} I X^{-}$ , (Q) the $S_{N} 1$ solvolysis of $(CH X_{3}) X_{3} C - Br$ , and (R) $HO X^{-} + CH X_{3} I CH X_{3} OH + I X^{-}$ change as follows:

AP no change, Q increases markedly, R decreases markedly
BP increases markedly, Q increases markedly, R decreases slightly
CP increases markedly, Q decreases, R no change
DP decreases, Q increases markedly, R increases markedly

Answer: B

In P the neutral reactants develop charge in the transition state, and in Q a neutral substrate ionises, so a more polar solvent stabilises the transition state and raises the rate. In R the localised charge on

OH X^{-}

is dispersed in the transition state, so the polar protic solvent stabilises the reactants slightly more and the rate falls a little.

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When the solvent is changed from ethanol to a more polar ethanol-water mixture, the rates of (P) (CX2​HX5​)X3​N+CHX3​I​(CX2​HX5​)X3​NX+−CHX3​ IX−, (Q) the SN​1 solvolysis of (CHX3​)X3​C−Br, and (R) HOX−+CHX3​I​CHX3​OH+IX− change as follows:

Answer: B

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