Which of the following compounds will undergo $E_2$ elimination reaction to give a single product in the presence of a strong base?

$A$ halide $C_5H_{11}Br$ on treatment with alc. $KOH$ gives $2$-pentene only. The halide will be

The correct order of the rate of reaction of the following reactants with a nucleophile by $S_{N}1$ mechanism is:
(Given: Structure $I$ and $II$ are rigid)

In the following pairs of halogen compounds,which compound undergoes faster $S_N 1$ reaction?

Identify the product '$B$' in the following reaction sequence:
$CH_3CH_2CH(OH)CH_3 \xrightarrow{PBr_3} A \xrightarrow{Mg, \text{dry ether}} B$

Arrange the compounds of each set in order of reactivity towards $S_{N}2$ displacement:
$(i)$ $2-Bromo-2-methylbutane$,$1-Bromopentane$,$2-Bromopentane$
$(ii)$ $1-Bromo-3-methylbutane$,$2-Bromo-2-methylbutane$,$2-Bromo-3-methylbutane$
$(iii)$ $1-Bromobutane$,$1-Bromo-2,2-dimethylpropane$,$1-Bromo-2-methylbutane$,$1-Bromo-3-methylbutane$.