Rearrange the compounds of each of the following sets in order of reactivity towards S_N2 displacement:

(i) 2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane

(ii) 1 -Bromo-3-methylbutane, 2-Bromo-2-methyl-butane, 2-Bromo-3-methylbutane

(iii) 1-Bromobutane, 1-Bromo-2, 2-dimethylpropane, 1- Bromo-2-methylbutane.

(i) In S_N2 reaction, the formation of transition state is involved in S_N2 reactions. So, more the stability of transition state, more is the reactivity towards S_N2 reactions. Now, if there is steric hindrance due to surrounding groups, less is the stability.

As steric crowding is most in compounds having 3° carbon atoms followed by 2° then 1°, so the order of reactivity towards S_N2 reaction would be reverse.

1-Bromopentane

2-Bromopentane

2-Bromo-2-methylbutane.

1-Bromopentane has a 1° carbon atom, 2-Bromopentane has 2° carbon atom and 2-Bromo-2-methylbutane, has 3° carbon atom.

So, the reaction towards S_N2 reaction is:-

1-Bromopentane>2-Bromopentane>2-Bromo-2-methylbutane.

(ii) Similarly due to steric crowding,

1-bromo-3 methylbutane > 2-bromo-3-methylbutane > 2-bromo-2-methyl butane.

(iii) Similarly due to steric crowding,

1-bromobutane > 1 -bromo-3-methylbutane > 1-bromo-2-, 2-dimethylpropane > 1-bromo-2, 2-dimethyl propane.