Nucleophilic Substitution Reactions
Substitution reactions are one of the most important types of reactions available to the synthetic organic chemist. Originally the difference between unimolecular and bimolecular substitution reactions was deduced from kinetic studies on a wide range of reagents. The rate of a unimolecular substitution depends only upon the concentration of the substrate, and not upon the concentration of the nucleophile. Such reactions are called unimolecular nucleophilic substitution reactions, Sn1. The other major type of nucleophilic substitution reaction is bimolecular, and is given the label Sn2. In this case, the rate of the reaction is observed to be dependent upon both the concentration of the species being substituted, that is, the substrate, and also upon the incoming nucleophile. The mechanism for the Sn2 reaction occurs in only one step, in contrast to the two steps involved in the Sn1 reaction.