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Chapter 5:
Structure and Preparation of Alkenes.
Elimination Reactions
Ch 5 contents

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,

This pathway is a concerted process with the following characteristics:

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

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.

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.

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.

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

antiperiplanar arrangement in E2 reaction
The outcome of E2 reactions is controlled by the stereochemical requirements described above. Where there is a choice, the more stable alkene will be the major product.

The E2 pathway is most common with:

A typical example is the dehydrohalogenation of alkyl halides using KOtBu / tBuOH.
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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary