N-butyl bromide reacts faster than t-butyl bromide by ${\mathrm{S}}_{\mathrm{N}}2$ mechanism. Explain why?

Write the mechanism of the following reaction.

${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{OH} \stackrel{\mathrm{HBr}}{\to } {\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{Br} +{\mathrm{H}}_2\mathrm{O}$

Complete the following reactions.

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