The S_N1 reaction is a type of substitution reaction. In this, the rate equation is often shown as having first-order dependence on electrophile and zero-order dependence on nucleophile.

In the S_N1 reaction, a planar carbenium ion is formed first, which then reacts further with the nucleophile. Since the nucleophile is free to attack from either side, this reaction is associated with racemization.