Sn1 reactions are named so because their rate equation feautures one term.

I.e. d[Prod]/dt = k[Organic]

This is because there is a slow step where the leaving group needs to leave, much slower than the second step.

The reaction mechanism is as follows:

1. Electrons of the leaving group's bond return to the leaving group (usually because more electronegative)

2. A carbocation intermediate is formed.

3. The substituent is able to attach to the carbocation, and sometimes the proton is removed by the solvent itself in solvolysis.

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Factors affecting product isomerism:

1. Remember: Alkyl groups are weakly electron donating

    This implies that carbocations are stabilized by alkyl groups

    And so the more chains a carbon atom has the more stable it is as a carbocation.

    As such, the isomer preferred is the one where the more stable cation is the one left.

    This can also be used to favour SN1 more.