The S_N1 reaction

The S_N1 reaction is a two-step reaction in which

1. The leaving group leaves, forming a carbocation. This is the slow step, and so the rate is dependent only on the concentration of the substrate.
2. The nucleophile attacks the carbocation. It can do this from either side, typically in a 50/50 ratio. Therefore about half the product has retained the original configuration, and about half is inverted.
3. Protonated nucleophiles, such as methanol, then lose a proton to the solvent.

Click to see this diagrammed.

Reaction animation:
animated GIF

SN1		SN2		E1		E2