Question

Which of the following will undergo $S_{N}2$ reaction ?

• A.
• B.
• C.
• D.

Answer 1

The correct option is C

(a)- In this reaction, the reactant is tertiary alkyl halide. Since tertiary alkyl halide is bulky it does not undergo back attack. So $S_{N}2$ reaction is not possible. This reaction is E1 elimination due to the stable carbocation and bulky and strong basic nature of BuOK.

(b)- In this reaction, the reactant is secondary alkyl halide. So it can undergo all possible reaction. But the base used here is strong so it will undergo E2 or $S_{N}2$ reaction. Due to the solvent nature of alcohol and the temperature effect, the $O{H}^{-}$ will act as a strong base and it will lead to E2 elimination.

(c)- In this reaction, the reactant is primary alkyl halide. Thus, it is suitable to $S_{N}2$ and $E2$ reaction. But the relative rate of reaction for primary alkyl halide toward E2 reaction is less because it forms less substituted alkenes. Also the $O{H}^{-}$ is smaller in size which make it ease to undergo back attack. Therefore, the given reaction undergoes $S_{N}2$ reaction.

(d)- In this reaction, the reactant is secondary alkyl halide. So it can undergo all possible reaction. But the base used here is very weak $H_{2}O$, it does not act as base or a nucleophile, so the reaction will go only by $S_{N}1$ reaction.