The halide which will undergo $SN^2$ reaction easily
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Ethyl Chloride.
$SN^2$ reactions occur through a single step involving backside attack. For this, the carbon attached to the halogen must be easily accessible. Less steric hindrance increases the rate of $SN^2$ reaction.
Ethyl chloride is a primary alkyl halide. The carbon attached to chlorine is not crowded, so the nucleophile can attack easily. Hence, $SN^2$ reaction occurs readily.
OTHER OPTIONS
Tertiary butyl chloride
The carbon attached to chlorine is tertiary and highly crowded. Backside attack is not possible, so $SN^2$ does not occur.
Benzyl chloride
Although benzyl chloride can undergo $SN^2$ reaction, it also strongly favours $SN^1$ reaction due to resonance stabilisation of the benzyl carbocation. Because of this competing pathway, it is not considered the best answer when the question asks for $SN^2$ reaction easily.
Allyl chloride
Allyl chloride behaves similarly to benzyl chloride. It can undergo both $SN^1$ and $SN^2$ reactions, so it is not the most straightforward case of $SN^2$.
FINALLY
Ethyl chloride undergoes $SN^2$ reaction most easily among the given options because it is a simple primary alkyl halide with minimal steric hindrance.