SN/E Competition (overall)
SN1 / E1 Competition - Introduction
The fundamental difference between the SN1/E1 competition and the SN2/E2 competition is in the first mechanistic step, or in the formation of the carbenium ion, which is completely identical in both the E1 elimination and the SN1 reaction. Thus, in the first mechanistic step E1 and SN1 reactions obviously do not compete. The competition arises from the mechanistic alternatives following the first step. Much like in the SN2/E2 competition, the nucleophile (base) in the SN1/E1 competition is basically bifunctional - that is, it can either abstract the β proton from the carbenium ion in an E1 elimination, or it can directly attack the α carbon of the carbenium ion in an SN1 reaction.
In contrast to the SN2/E2 competition, the dependence of the reaction rate of E1 and SN1 reactions on the substrate structure is not contrary, but of the same kind. SN1 reactions, as well as E1 eliminations, proceed very slowly with primary heteroalkyl compounds, so that they are usually completely exceeded by the corresponding bimolecular mechanisms (SN2/E2). In contrast to this, in connection with tertiary heteroalkyl compounds, the unimolecular reactions (SN1/E1) run more rapid than the bimolecular reactions (SN2/E2).
The identical rate-determining step of E1 and SN1 reactions, the formation of the carbenium ion, is responsible for the similarly directed dependence of their reaction rates on the substrate structure. Due to the stabilization of the positive charge of the carbenium ion by alkyl substituents, the activation energy is lower in the formation of tertiary carbenium ions than it is for secondary, or, in particular, for primary carbenium ions. Therefore, the reaction rate of E1 and SN1 reactions is much higher with tertiary heteroalkyl compounds than with secondary or primary heteroalkyl compounds.
The question whether an E1 elimination or an SN1 reaction follows the formation of the carbenium ion is mainly determined by the two parameters: The chemical character of the nucleophile (base) and the reaction temperature.