The Sn1 reaction results in similar products as the Sn2 reaction, but the mechanism is entirely unique. It involves both a substrate, such as t-butyl bromide, and a nucleophile. T-butyl bromide, while being a terrible substrate for the Sn2 reaction reacts well in the Sn1 reaction With the Sn1 reaction, t-butyl bromide reacts quite quickly with a nucleophile to form the substitution product.

In the Sn1 reaction, the concentration of the substrate effects the reaction rate, but changing the concentration of the nucleophile has no effect on the rate. The Sn1 reaction follows the following rate equation:
Rate = kr[substrate].

This means that only the substrate is involved in the rate determining step.

The Rate Determining Step--Formation of the carbocation:

Followed by attack of the nucleophile on the carbocation:

Last, the removal of a proton by the bromine:

Since the nucleophile is not involved in the rate determining step, the concentration or the nucleophilicity of the nucleophile has no effect on the reaction rate. This means that poor nucleophiles such as water and alcohols can react to form alcohols and ethers.

Information provided by: http://www.chemhelper.com