Question

Which would undergo $S{N}^2$ reaction faster in the above pair and Why?

Answer 1

$S{N}^2$ attack occurs if the backside route of attack is not sterically hindered by substituents on the substrate. Therefore, this mechanism usually occurs at an unhindered primary carbon centre. If there is steric crowding on the substrate near the leaving group, such as at a tertiary carbon centre, the substitution will involve an $S{N}^1$ rather than an $S{N}^2$ mechanism. $S{N}^2$ reactions are generally favored in primary alkyl halides or secondary alkyl halides with an aprotic solvent. They occur at a negligible rate in tertiary alkyl halides due to steric hindrance.