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The ${\mathbf{S}}_{\mathbf{N}}\mathbf{1}$ reaction is a nucleophilic substitution reaction where carbon-halogen bond is broken which results in positively charged carbon and then nucleophile attacks the carbocation and forms a new bond. Only one reacting species is involved in the rate determining step of the reaction. The term${\mathrm{S}}_{\mathrm{N}}1$ stands for substitution nucleophilic unimolecular. The hydrolysis of tert-butyl bromide with aqueous NaOH solution is an example of ${\mathrm{S}}_{\mathrm{N}}1$reaction.

The ${\mathrm{S}}_{\mathrm{N}}2$reaction is a nucleophilic substitution reaction where bond is broken and another is formed simultaneously. Two reacting species are involved in the rate determining step of the reaction. The term${\mathrm{S}}_{\mathrm{N}}2$stands for substitution nucleophilic bimolecular.

${\mathrm{S}}_{\mathrm{N}}2$reaction

(a)

This reaction probably occurs by an ${\mathrm{S}}_{\mathrm{N}}1$mechanism. HCl converts the poor –OH leaving group into an excellent $-{\mathrm{OH}}_2$leaving group, and the polar solvent stabilizes the carbocation intermediate.

(b)

This reaction takes place with a negatively charged nucleophile ina polar, aprotic solvent. It is very likely that the reaction occurs by an${\mathrm{S}}_{\mathrm{N}}2$ mechanism.