E2 Elimination & Mechanism Reaction: Definition & Examples

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  • 0:04 The Meaning of E2
  • 0:36 Examples of E2 Reactions
  • 1:06 The E2 Mechanism
  • 1:53 Stereo &…
  • 3:13 Lesson Summary
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Lesson Transcript
Instructor: Nancy Devino
This lesson focuses on one of the more important reactions in organic chemistry: E2. We'll look at some examples, zoom in for a closer look at the mechanism, then explore some of the stereochemical features of the reaction.

The Meaning of E2

E2 is one of four major reaction mechanisms you'll encounter early in your study of organic chemistry. The E stands for elimination. Two atoms on adjacent carbons are removed, or eliminated, in order to obtain the product. The end result is an alkene with a new C-C pi bond. The 2 in the mechanism name stands for bimolecular. This means that the rate of the reaction depends on the concentration of the substrate, the organic molecule undergoing elimination, as well as the base that is required for the reaction.

Examples of E2 Reactions

All E2 reactions have two things in common: a good leaving group and a hydrogen atom on a carbon adjacent to the one with the leaving group. Alkyl halides and alcohols are the most common reactants in an E2 reaction. Here are some examples of E2 reactions.

Alkyl halides undergo elimination to produce alkenes.
E2 elimination from RX

Alcohols undergo elimination to produce alkenes.

Notice that E2 elimination of an alcohol uses acid, not base. As we'll see when we look at the mechanism, the acid is used to make the -OH group into a better leaving group, which is water.

The E2 Mechanism

Here we see the mechanism of E2 elimination from an alkyl halide, RX.

Bimolecular elimination occurs in a single, concerted step.
E2 mechanism

This reaction works best when the halide is primary or secondary. As noted earlier, a base takes a proton (H+) from a carbon adjacent to the one with the X. The electrons that had formed the C-H bond are now used to make a new C-C pi bond. When the new bond forms, the leaving group leaves with its electron pair.

Elimination of water from a primary alcohol is possible, but difficult. However, if it occurs it will use the E2 mechanism. Unimolecular elimination reactions, also called E1, use a different mechanism that involves a carbocation. Since primary carbocations rarely form, elimination of a primary alcohol will occur via E2 as you can see.

Bimolecular elimination of a primary alcohol requires acid to convert the -OH group to a good leaving group.
E2 alcohol

Stereochemistry & Regiochemistry of E2 Reactions

Let's take another look at the E2 mechanism involving an alkyl halide. Notice that the hydrogen atom removed by the base is anti to the leaving group, or the halide ion, X. This is an important stereochemical feature of the reaction mechanism, and it determines the stereochemistry of the product alkene when the alkyl halide is secondary.

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