(i)
The nucleophile approaches the carbon atom to which the leaving group is linked in an SN2 reaction. The reactivity for SN2 displacement reduces when the nucleophile is sterically inhibited. The following arrangement increases the impediment to the approaching nucleophile due to the presence of substituents.
1-Bromopentane < 2-bromopentane < 2-Bromo-2-methylbutane
As a result, the following is the increasing order of reactivity toward SN2 displacement.
2-Bromo-2-methylbutane < 2-Bromopentane < 1-Bromopentane
(ii)
Because the increasing order of steric hindrance in alkyl halides is 1° < 2° < 3°,the increasing order of reactivity towards SN2 displacement is 3° < 2° < 1°..
As a result, the following compounds can be grouped in ascending order of reactivity toward SN2 displacement:
2-Bromo-2-methylbutane < 2-Bromo-3-methylbutane < 1-Bromo-3-methylbutane
(iii)
In the SN2 process, the steric barrier to the nucleophile increases as the distance between the substituents and the atom containing the leaving group decreases. Furthermore, when the number of substituents rises, the steric hindrance increases. As a result, the following is the increasing order of steric hindrances in the provided compounds:
1-Bromobutane < 1-Bromo-3-methylbutane < 1-Bromo-2-methylbutane < 1-Bromo-2, 2-dimethylpropane
Hence, the increasing order of reactivity of the given compounds towards SN2 displacement is:
1-Bromo-2, 2-dimethylpropane < 1-Bromo-2-methylbutane < 1-Bromo-3- methylbutane < 1-Bromobutane