Which of the following alkyl halides has the highest boiling point?

Which among the following is $NOT$ a feature of $S_{N}2$ mechanism?

Consider the reaction,
$CH_3CH_2CH_2Br + NaCN \to CH_3CH_2CH_2CN + NaBr$
This reaction will be the fastest in

The compound most reactive towards $S_{N}1$ reaction is :

Which alkyl halide from the following pairs would you expect to react more rapidly by an $S_N2$ mechanism? Explain your answer.
$(i)$ $CH_3CH_2CH_2CH_2Br$ or $CH_3CH_2CH(Br)CH_3$
$(ii)$ $CH_3CH_2CH(Br)CH_3$ or $(CH_3)_3CBr$
$(iii)$ $CH_3CH(CH_3)CH_2CH_2Br$ or $CH_3CH_2CH(CH_3)CH_2Br$

What is the order of reactivity of alkyl halides with ammonia?