Explain the factors affecting `S_(N)1` reaction.

Explanation

In a `S_(N)1` reaction, the carbocation is an intermediate product. Thus, greater is the stability of carbocation, faster is the reaction rate. In case of alkyl halides, `3^(@)` alkyl halides undergo `S_(N)1` reaction very fast because it will form high stable `3^(@)` carbocations.

For the same reasons, allylic and benzylic halides show high reactivity towards the `S_(N)1` as they are stabilised by resonance.

The presence of polar protic solvents such as `H_(2)O, CH_(3)OH, CH_(3)COOH` etc. favours the ionisation of C - X bond and thus favours the `S_(N)1` reaction.
`S_(N)1` reaction is favoured by a weak nucleophile.
The presence of any +M group favours the `S_(N)1` reaction because it will stabilise the carbocation.