Nucleophilic Substitution Reactions at the Saturated C Atom

Nucleophiles are usually anions or neutral species but not cations. This chapter focuses on the nucleophilic substitution reactions at the saturated, that is, the sp 3 -hybridized, C atom (abbreviated “S N reactions”). In these reactions, alkyl groups are transferred to the nucleophiles. Organic electrophiles of this type are referred to as alkylating agents. It is possible to distinguish between good and poor nucleophiles when SN reactions are carried out as competition experiments. There the nucleophiles are made available as mixtures to a standard alkylating agent. The nucleophile that reacts to form the main product is then the “better” nucleophile. Within a group of nucleophiles that attack at the electrophile with the same atom, the nucleophilicity decreases with decreasing basicity of the nucleophile. The Hammond postulate implies that a good leaving group is a stabilized species, not a high-energy species. Therefore good leaving groups are usually weak, not strong, bases. In contrast to the S N 2 mechanism, the structure of the transition state of the rate-determining step in the S N 1 mechanism cannot be depicted in a simple way. As an aid, one therefore uses a transition state model.