Chapter 5: Structure and Preparation of Alkenes. Elimination Reactions

E2 mechanism

E2 indicates an elimination, bimolecular reaction, where rate = k [B][R-LG].
This implies that the rate determining step involves an interaction between these two species, the base B, and the organic substrate, R-LG

This pathway is a concerted process with the following characteristics:

Simultaneous removal of the proton, H⁺, by the base, loss of the leaving group, LG, and formation of the π-bond

Let's look at how the various components of the reaction influence the reaction pathway:

Effects of R-
Reactivity order :   (CH₃)₃C-  >  (CH₃)₂CH-   >  CH₃CH₂-  >  CH₃-

In an E2 reaction, the reaction transforms 2 sp³ C atoms into sp² C atoms. This moves the substituents further apart decreasing any steric interactions. So more highly substituted systems undergo E2 eliminations more rapidly. This is the same reactivity trend as seen in E1 reactions.

-LG
The C-LG bond is broken during the rate determining step, so the rate does depend on the nature of the leaving group.
However, if a leaving group is too good, then an E1 reaction may result.

B
Since the base is involved in the rate determining step, the nature of the base is very important in an E2 reaction.
More reactive bases will favour an E2 reaction.

Stereochemistry
E2 reactions occur most rapidly when the H-C bond and C-LG bonds involved are co-planar, most often at 180^o or antiperiplanar. This sets up the s bonds that are broken in the correct alignment to become the p bond. More details ?

The E2 pathway is most common with:

• high concentration of a strong base
• poorer leaving groups
• R-LG that would not lead to stable carbocations  (when the E1 mechanism will occur).

© Dr. Ian Hunt, Department of Chemistry