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.

Explanation

The reaction follows `S_(N)2` path if the substrate offers minimum steric hinderance to the nucleophile. Thus order of reactivity of alkyl halides by `S_(N)2` path is :
Methyl halide `gt 1^(@)` halide `gt 2^(@)` halide `gt gt gt 3^(@)` halide.
(i) `underset((1^(@)))("1-Bromopentane")gt underset((2^(@)))("2-Bromopentane")gt underset((3^(@)))("2-Bromo-2-methylbutane")`
(ii) `underset((1^(@)))("1-Bromo-3-methylbutane")gt underset((2^(@)))("2-Bromo-3-methylutane")gt underset((3^(@)))("2-bromo-2-methylbutane")`
(iii) 1-bromobutane `gt` 1-Bromo-3-methylbutane `gt` 1-Bromo-2-methylbutane `gt` 1-Bromo-2, 2 - dimethyl propane
In (iii), all the halides are primary `(1^(@))`. However, the presence of bulky group such as `-CH_(3)` (methyl) on `beta` - carbon decreases the reactivity towards `S_(N)2` due to steric hindrance. Thus, 1-Bromo-2, 2-dimethyl propane is least reactive.