Consider the reactions (i) $\left(\mathrm{CH}_3\right)_2 \mathrm{CH}-\mathrm{CH}_2 \mathrm{Br}…

Consider the reactions
(i) $\left(\mathrm{CH}_3\right)_2 \mathrm{CH}-\mathrm{CH}_2 \mathrm{Br} \stackrel{\mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}}{\longrightarrow}\left(\mathrm{CH}_3\right)_2 \mathrm{CH} -\mathrm{CH}_2 \mathrm{OC}_2 \mathrm{H}_5+\mathrm{HBr}$
(ii) $\left(\mathrm{CH}_3\right)_2 \mathrm{CH}-\mathrm{CH}_2 \mathrm{Br} \stackrel{\mathrm{C}_2 \mathrm{H}_5 \mathrm{O}^{-}}{\longrightarrow}\left(\mathrm{CH}_3\right)_2 \mathrm{CH} -\mathrm{CH}_2 \mathrm{OC}_2 \mathrm{H}_5+\mathrm{Br}^{-}$
The mechanisms of reactions (i) and (ii) are respectively
  1. $\mathrm{S}_{\mathrm{N}} 1$ and $\mathrm{S}_{\mathrm{N}} 2$
  2. $\mathrm{S}_{\mathrm{N}} 1$ and $\mathrm{S}_{\mathrm{N}} 1$
  3. $\mathrm{S}_{\mathrm{N}} 2$ and $\mathrm{S}_{\mathrm{N}} 2$
  4. $\mathrm{S}_{\mathrm{N}} 2$ and $\mathrm{S}_{\mathrm{N}} 1$

Solution

$\mathrm{C}_2 \mathrm{H}_5 \mathrm{OH}$ being a weaker nucleophile, when used as a solvent in case of hindered $1^{\circ}$ halide, favours $\mathrm{S}_{\mathrm{N}} 1$ mechanism while $\mathrm{C}_2 \mathrm{H}_5 \mathrm{O}^{-}$being a strong nucleophile in this reaction favours $\mathrm{S}_{\mathrm{N}} 2$ mechanism.

Asked in: NEET 2011 (Mains)

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