Explain the factors favouring $S_{N}2$ reaction.

(N/A) The $S_{N}2$ reaction involves the backside attack of a nucleophile on the carbon atom bonded to the leaving group. The following factors favour this reaction:
$1$. Steric Hindrance: The reaction is highly sensitive to steric hindrance. As the number of alkyl groups attached to the carbon bearing the leaving group increases,the approach of the nucleophile is hindered. Thus,the order of reactivity is: $\text{Methyl halide} > 1^{\circ} > 2^{\circ} > 3^{\circ}$.
$2$. Nucleophile Strength: $A$ strong,negatively charged nucleophile increases the rate of the $S_{N}2$ reaction.
$3$. Solvent: Polar aprotic solvents (e.g.,acetone,$DMSO$,$DMF$) are preferred because they do not solvate the nucleophile strongly,keeping it reactive.
$4$. Leaving Group: $A$ good leaving group (e.g.,$I^- > Br^- > Cl^-$) facilitates the reaction by departing easily.