Haloalkanes And Haloarenes

Arrange the following in the order of increasing reactivity towards \(\mathrm{SN}_{\mathrm{2}}\) reactions.

$$\begin{array}{ll} \left(\mathrm{CH}_3\right)_3 \mathrm{CCH}_2 \mathrm{Br}(\mathrm{P}) ; & \\\mathrm{CH}_3\left(\mathrm{CH}_2\right)_3 \mathrm{Br}(\mathrm{Q}) ; &\\ \mathrm{CH}_3 \mathrm{CH}_2-\mathrm{CH}\left(\mathrm{CH}_3\right)-\mathrm{CH}_2 \mathrm{Br}(\mathrm{R}) ; & \\\left(\mathrm{CH}_3\right)_2 \mathrm{CHCH}_2 \mathrm{CH}_2 \mathrm{Br}(\mathrm{S}) \end{array}$$