Question

The most important chemical reactions of the haloalkanes are their substitution reactions.
(i) What are $S{N_1}$ reactions?
(ii) Arrange the four isomeric Bromo butanes in the increasing order of their reactivity towards $S{N_1}$ reactions.

Answer 1

We must know that $S{N_1}$ reactions are nucleophilic substitution reactions. Tertiary halides are considerably faster in chemical reactivity than secondary alkyl halides, which are faster in reactivity than primary halides.

Complete answer:
(i) $S{N_1}$ reaction
$S{N_1}$ stands for Nucleophilic substitution unimolecular reaction. The $S{N_1}$ reaction is a nucleophilic Substitution reaction in which the rate determining step involves only one component.
$S{N_1}$ reaction mechanism follows a two-step process. First, the carbocation is formed by the removal of the leaving group (bond breaking mechanism). Then the carbo-cation is attacked by the nucleophile (bond-forming mechanism). The protonated nucleophile then undergoes deprotonation to give the required product. And the first step is the rate-determining step.

(ii)The following four are the structures of isomeric Bromo-butane.
Order of reactivity of isomeric Bromobutane structures towards $S{N_1}$ reactions is as follows:
$C{H_3}C{H_2}C{H_2}C{H_2}Br{\text{ }} < {\text{ }}{\left( {C{H_3}} \right)_2}CHC{H_2}Br{\text{ }} < {\text{ }}C{H_3}C{H_2}CH\left( {Br} \right)C{H_3} < {\text{ }}{\left( {C{H_3}} \right)_3}CBr$
1-Bromo-butane and 1-Bromo-2 methyl -propane are primary bromides .
Out of these primary bromides, 1-Bromo-2 methyl –propane is more reactive than 1-Bromo-butane as the carbo-cation intermediate structure obtained from 1-Bromo-2 methyl –propane is more stable than carbo-cation intermediate structure obtained from 1-Bromo-butane. This is because of the greater electron donating inductive effect of ${\left( {CH3} \right)_2}CH -$ group.
2-Bromo-butane is a secondary bromide and 2-Bromo-2-methylpropane is a tertiary bromide.
We know that tertiary halides are considerably faster than secondary alkyl halides, which are faster in reactivity than primary.
Hence, the Order of reactivity of isomeric Bromobutane structures towards SN1 reactions is as follows:
1-Bromo-butane < 1-Bromo-2 methyl -propane < 2-Bromo-butane < 2-Bromo-2-methylpropane

Note:
We must know that the $S{N_1}$ reactions are important as they describe a mechanism of chemical reactivity and also describe a bond-breaking process with respect to the rate-determining step.
The reactivity in $S{N_2}$ reactions for isomeric Bromo butanes follows the reverse order due to the steric hindrance around the electrophilic carbon increases in respective order.