$DDT$ is prepared by reacting chlorobenzene with

Which of the following compounds has the fastest $S_N2$ reaction rate?

$CH_3-CH_2-CH(Br)-CH_3 \xrightarrow{Alc. KOH} X$ (major)
$CH_3-CH_2-CH(NMe_3^+) - CH_3 \xrightarrow{EtONa, \Delta} Y$ (major)
Product $(X)$ and $(Y)$ respectively are:

The reactivity of $2-$bromo$-2-$methylbutane $(I)$,$1-$bromopentane $(II)$ and $2-$bromopentane $(III)$ towards $S_N2$ displacement is such that :

Arrange the following in order of ease of dehydrohalogenation:
$(i)$ Bromocyclohexane
(ii) $3-$Bromocyclohexene
(iii) $3-$Bromocyclohexa$-1,4-$diene
(iv) Bromobenzene

Which of the following is most reactive towards $S_{N}1$ reaction?