Explain the factors affecting $S_{N}1$ reaction.

(N/A) In an $S_{N}1$ reaction,the carbocation is an intermediate product. Thus,the greater the stability of the carbocation,the faster the reaction rate.
$1$. Nature of Substrate: In the case of alkyl halides,$3^{\circ}$ alkyl halides undergo $S_{N}1$ reaction very fast because they form highly stable $3^{\circ}$ carbocations. Allylic and benzylic halides also show high reactivity towards $S_{N}1$ as the resulting carbocations are stabilized by resonance.
$2$. Solvent Effect: The presence of polar protic solvents such as $H_{2}O$,$CH_{3}OH$,and $CH_{3}COOH$ favours the ionisation of the $C-X$ bond and thus promotes the $S_{N}1$ reaction.
$3$. Nucleophile: $S_{N}1$ reactions are favoured by weak nucleophiles,as the rate-determining step does not involve the nucleophile.
$4$. Electronic Effects: The presence of any $+M$ (mesomeric) group favours the $S_{N}1$ reaction because it stabilizes the carbocation intermediate.