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Halogen atoms are used instead of one or several hydrogen atoms in compounds termed alkyl halides. Some examples of alkyl halides are methyl chloride and 2-chloropropane.

The carbocation intermediate has a primary role in the ${\mathit{S}}_{\mathbf{N}}\mathbf{1}$ reaction. Compounds generating highly stable intermediates are more reactive than those generating less stable ones.

The stability of carbocation influences the rate of an $S_{N}1$reaction

The first compound has $4\mathrm{Ï€}$electrons and is aromatic. It produces an unstable intermediate, and it can be given as:

Carbocation generated from the first compound

The carbocation generated from the second compound can be given as:

Carbocation generated from the second compound

The third compound has $6\mathrm{Ï€}$electrons and is aromatic. It gives rise to a very stable intermediate as:

Carbocation generated from the third compound

The increasing reactivity of the alkyl halides can be given as:

Increasing reactivity of alkyl halides

In the third compound, the aromatic carbocation is delocalized over the whole ring, making it a stable intermediate. Hence it can be easily formed in an $S_{N}1$reaction.