An SN1 reaction is a nucleophilic substitution reaction in which the rate-determining step involves one component. The reaction name derives from S standing for 'substitition', N for 'nucleophilic' and the 1 denoting the kinetic order of the reaction - or simply the number of reaction components involved in the rate-determining step.
SN1 reactions are two-step, unimolecular reactions and proceed via an intermediate carbocation. The first carbocation-forming step is the slower of the two and therefore determines the rate of the reaction. The second step involves the rapid attack of the nucleophile to the newly-formed carbocation. The reaction has two transition states, one before and one after the intermediate. Lowering the energy of these transition states will lower the activation energy of the reaction.
Figure 1: General SN1 reaction mechanism. L is the leaving group. Nu is the nucleophile. The carbocation intermediate is an electrophile.
Unlike the SN2 reaction - in which we see inversion of stereochemistry in the product - the SN1 reaction provides a racemic product. Racemization of the stereochemistry takes place as the nucleophile is able to approach the planar carbocation intermediate from either side, providing both product stereoisomers.
Factors that affect the SN1 reaction:
Solvent Polar, protic solvents usually speed up the rate of an SN1 reaction, as their large dipole moment helps to stabilize the intermediate carbocation. Polar, aprotic solvents are not used in SN1 reactions because some of them can react with the carbocation intermediate and provide unwanted products.
Nucleophile Since the nucleophile is not involved in the rate-determining step on an SN1 reaction, the strength of the nucleophile does not affect the reaction. However, if you have more than one nucleophile competing to bond with the carbocation, you could end up with a mixture of products. (Sometimes a solvent can act as a nucleophile too!)
Leaving group The better the leaving group, the faster the SN1 reaction. This is because the leaving group is involved in the rate-determining carbocation formation step.