Suggest a possible reason for the following observation: Neo-pentyl chloride $({\mathrm{CH}}_3{)}_3 \mathrm{C}—{\mathrm{CH}}_2\mathrm{Cl}$ does not follow ${\mathrm{S}}_{\mathrm{N}}2$mechanism.

$\mathrm{P},\mathrm{Q},\mathrm{R}$ and $\mathrm{S}$undergoes ${\mathrm{S}}_{\mathrm{N}}2$ reaction with $\mathrm{KI}$in acetone. What is the order of the rate of reaction?

Give the IUPAC name of the compound.