Overview:

The general form of the S_N1 mechanism is as follows:

Because the mechanism goes through a carbocation, the leaving group must be attached to either a tertiary or secondary carbon to stabilize the intermediate. A methyl or primary leaving group will not form a carbocation. Since it goes through a carbocation intermediate, there are possibilities for alkyl and hydrogen rearrangements (HINT: In mechanism problems if you see a change in the carbon skeleton between the reactant and the product, automatically suspect a carbocation intermediate (ie, E1, Sn1) stabilized by an alkyl or hydrogen rearangement). .

An example ofthe Sn1 Mechanism

Base Strength: Base strength is unimportant, since the base is not involved in the rate determining step (the formation of the carbocation). .

Leaving groups: A good leaving group is required, such as a halide or a tosylate, since the leaving group is involved in the rate-determining step.

Notes: Be wary of rearangements that can occur with the S_N1 reaction. Because it goes through a carbocation intermediate, both hydrogen shifts and alkyl shifts can occur!