Question

Consider the reactions
$\left(i\right)(C{H}_3{)}_{2}CH-C{H}_{2}Br\stackrel{C_2{H}_{5}OH}{⟶}(C{H}_3{)}_{2}CH-C{H}_{2}O{C}_2{H}_5+HBr\left(ii\right)(C{H}_3{)}_{2}CH-C{H}_{2}Br\stackrel{C_2{H}_5{O}^{-}}{⟶}(C{H}_3{)}_{2}CH-C{H}_{2}O{C}_2{H}_5+B{r}^{-}$
The mechanisms of reaction (i) and (ii) are respectively:

• A. $S_{N}1$ and $S_{N}2$
• B. $S_{N}1$ and $S_{N}1$
• C. $S_{N}2$ and $S_{N}2$
• D. $S_{N}1$ and $S_{N}1$

Answer 1

The correct option is A $S_{N}1$ and $S_{N}2$

The nucleophile is not involved in the RDS in $S_{N}1$ reaction. thus it does not affect the rate of $S_{N}1$ reaction.
However, a stronger nucleophile may lead to side reactions. So, weak & moderate nucleophiles are preferred.
$C_2{H}_{5}OH$ being a weaker nucleopbile, when used as a solvent in case of hindered $1^{\circ }$ halide, favours $S_{N}1$ mechanism while $C_2{H}_5{O}^{-}$ being a strong nucleophile in this reaction favours $S_{N}2$ mechanism. Hence correct option is a.