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SN/E Competition (overall)

SN2 / E2 Competition - Introduction

In SN2/E2 competition, the issue of which mechanism outdoes the other is dependent upon the rates of the competing reactions. The product ratio (SN2/E2) is dependent on the quotient of the reaction rates of the two mechanistic alternatives. When a reaction is to be controlled in order for a specific mechanism to be followed, the quotient must be as large or as small as possible, respectively. Due to this, the energy of the desired mechanism's transition state should be minimized as much as possible without lowering the energy of the transition state of the competing mechanism, as well. This is because the reaction rate is directly correlated with the energy of the transition state.

The energies of the transition states of SN2 and E2 reactions are influenced by several parameters. The most significant of these in the SN2/E2 competition are the type of base that is applied and the structure of the substrate.

Fig.1
SN2/E2 competition.

The starting products of an E2 reaction are a heteroatom-substituted alkyl compound with a hydrogen atom in β position and a base. The hydrogen in β position is abstracted as a proton by the base and the heteroatom leaves the molecule as an anion. Characteristic of the base is a lone electron pair, which may accept a proton. However, due to this lone electron pair, the base is, in addition, a nucleophile. Instead of the abstraction of the hydrogen in β position, in principle a nucleophilic attack by the base on the heteroatom-substituted carbon may therefore occur. This is the basic prerequisite of the existence of SN/E competition.

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