Formation of $2-$butene from $2-$bromobutane is according to

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

The relative reactivity of the following halides toward $S_{N}2$ reaction follows the order:
$(P)$ $CH_2=CH-Cl$
$(Q)$ $CH_3-CH_2-Cl$
$(R)$ $CH_3-O-CH_2-CH_2-Cl$
$(S)$ $C_6H_5-CH_2-Cl$

The product of the reaction between ethylbenzene and $N$-bromosuccinimide $(NBS)$ is:

Arrange the following in their correct order of reactivity towards ${S_N}^1$ reaction.

The reaction of $CH_3-CH=CH-C_6H_4-OH$ with $HBr$ gives: