In a ${\mathrm{S}}_{\mathrm{N}}2$ substitution reaction of the type

$\mathrm{R}-\mathrm{Br}+{\mathrm{Cl}}^{-}\stackrel{\mathrm{DMF}}{⟶}\mathrm{R}-\mathrm{Cl}+{\mathrm{Br}}^{-}$ 

which one of the following has the highest relative rate ? 

The correct order of increasing reactivity of $\mathrm{C}-\mathrm{X}$ bond towards nucleophile substitution in the following compounds is:

Identify $\mathrm{Z}$ in the sequence of reactions:
${\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{CH}={\mathrm{CH}}_2\underset{{\mathrm{H}}_2{\mathrm{O}}_2}{\overset{\mathrm{HBr}}{\to }}\mathrm{Y}\stackrel{{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{ONa}}{\to }\mathrm{Z}$