The S_N1 mechanism, analogous to the S_N2 mechanism, requires good leaving groups.

TsO^- > I^- > Br^- > H₂O > Cl^-

Water does not react with 2-Fluoropropane since fluorine is a bad leaving group, but it does with 2-Bromopropane.

[1] Reaction does not take place (Fluorine bad leaving group)

The leaving group influences the rate of S_N1, since the slow step in the mechanism is dissociation from the substrate. The better the leaving group, the faster the reaction.

[3] Quick reaction

[4] Slow reaction (chlorine worse leaving group than fluorine)

tert-Butyl bromide ionizes faster than tert-Butyl chloride, because bromine is a better leaving group than chlorine. As the dissociation of the substrate is the slow step of the mechanism, the first reaction is faster than the second.