Question

Arrange the compounds of each set in order of reactivity towards S_N2 displacement:

1. 2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane

2. 1-Bromo-3-methylbutane, 2-Bromo-2-methylbutane, 3-Bromo-2-methylbutane

3. 1-Bromobutane, 1-Bromo-2, 2-dimethylpropane, 1-Bromo-2-methylbutane, 1-Bromo-3-methylbutane.

Answer 1

1.

An S_N2 reaction involves the approaching of the nucleophile to the carbon atom to which the leaving group is attached. When the nucleophile is sterically hindered, then the reactivity towards S_N2 displacement decreases. Due to the presence of substituents, hindrance tp the approaching nucleophile increases in the following order.
1- Bromopentane < 2-bromopentane < 2 – Bromo-2-methyl butane

Hence, the increasing order of reactivity towards S_N2 displacement is

2- Bromo – 2- methyl butane < 2-bromopentane < 1-Bromopentane

2. 

3.

The steric hindrance to the nucleophile in the S_N2 mechanism increases with a decrease in the distance of the substituents form the atom containing the leaving group. Further, the steric hindrance increases in the increases with increase in the number of substituents. Therefore, the increasing order of steric hindrances in the given compounds is as below:
1- Bromobutane < 1-Bromo-3-methylbutane < 1 -Bromo-2-methylbutane < 1 -Bromo-2, 2- demethylbutane.

Hence, the increasing order of reactivity of the given compound towards S_N2 mechanism is

1-Bromo-2, 2-dimethylbutane < 1 -Bromo-2-methyl butane < 1-Bromo-3-methylbutane < 1-Bromobutane.