Before we start our discussion of the SN1 and SN2 reactions, let's get introduced to some of the terms. Both reactions are nucleophilic substitution reactions. What that means is that a nucleophile will replace (substitute) something in the starting material. What does it replace? A leaving group. The molecule with the leaving group is called a substrate (starting material). SN1 versus SN2 Summary Chart The chart above summarizes both reactions and major differences between