SN/E Competition (overall)
SN / E Competition - Introduction
In describing the mechanisms of nucleophilic substitutions and β eliminations, great importance is usually attached to the differentiation between the unimolecular and bimolecular mechanism of the rate-determining step. This pertains to the differentiation between the SN1 and SN2 mechanism on the one hand, and the E1 and E2 mechanism on the other hand. In this connection, the parameters that influence the reaction order are usually described in detail.
Why are these explanations important?
In principle, both the unimolecular and bimolecular alternatives are always possible in nucleophilic substitutions and β eliminations. However, the mechanism that actually occurs considerably influences the structure of the product. Depending on the mechanistic course of the rate-determining step, the stereochemical and regiochemical results of the reactions can differ greatly. In order to obtain a well-defined product, it is, therefore, important to control the mechanism. The parameters that determine the mechanistic course of nucleophilic substitutions and β eliminations can often be varied easily. Thus, the structure of the products of such reactions may usually be controlled.
For a better understanding, substitution and elimination reactions are usually imparted on separately in the initial phase. Due to this, it first appears as though competition is prevalent only between the SN1 and SN2 mechanism on the one hand, and between the E1 and E2 mechanism on the other hand. If the parameters for obtaining a SN2 mechanism under these conditions were to be choosen, it could come as a surprise that elimination the main reaction that proceeds as is the case. Actually, there is also always competition between substitution and elimination, as the starting products of substitutions are, in addition, usually suitable for the mechanistic course of eliminiations and vice versa. However, substitutions and eliminations yield totally different products. Therefore, not only must the reaction order be controlled, but the practical differentiation between substitution and elimination must also be accounted for in order to achieve the desired reaction results.
In substitution as well as elimination reactions, same parameters basically control the reaction order in the same way. This would include, for example, the type of solvent or the structure of the starting product, as well as the stability of potential, intermediate carbenium ions. Therefore, only the competitions between the SN1 and E1 mechanism on the one hand, as well as the SN2 and E2 mechanism on the other hand, are explained below, as the reaction order is largely predetermined by such parameters.