Skip to main content
Chemistry LibreTexts

6.11 E2 Elimination

E2, bimolecular elimination, was proposed in the 1920s by British chemist Christopher Kelk Ingold. Unlike E1 reactions, E2 reactions remove two subsituents with the addition of a strong base, resulting in an alkene.

Introduction

E2 reactions are typically seen with secondary and tertiary alkyl halides, but a hindered base is necessary with a primary halide. The mechanism by which it occurs is a single step concerted reaction with one transition state. The rate at which this mechanism occurs is second order kinetics, and depends on both the base and alkyl halide. A good leaving group is required because it is involved in the rate determining step. The leaving groups must be coplanar in order to form a pi bond; carbons go from sp3 to sp2 hybridization states.

General Reaction

General Reaction1.bmp

In this reaction Ba represents the base and X representents a leaving group, typically a halogen. There is one transition state that shows the concerted reaction for the base attracting the hydrogen and the halogen taking the electrons from the bond. The product be both eclipse and staggered depending on the transition states. Eclipsed products have a synperiplanar transition states, while staggered products have antiperiplanar transition states. Staggered conformation is usually the major product because of its lower energy confirmation. 

An E2 reaction has certain requirements to procede:

  • A strong base is necessary especially necessary for primary alkyl halides. Secondary and tertirary primary halides will procede with E2 in the precesence of a base (OH-, RO-, R2N-)
  • Both leaving groups should be on the same plane, this allows the double bond to form in the reaction. In the reaction above you can see both leaving groups are in the plane of the carbons.
  • Follows Zaitsev's rule, the most substituted alkene is usually the major product.
  • Hoffman Rule, if a strically hindered base will result in the least substituted product.

Reaction Coordinate

Reaction Coordinate.bmp

References

  1. Vollhardt, K. Peter C., and Neil E. Schore. Organic Chemistry Structure and Function. New York: W. H. Freeman, 2007.
  2. Organic chemistryBy Marye Anne Fox, James K. Whitesell

Problems

1. 

 E2-1.bmp

2. What is the major product and why?

 

3. What is the major prodcut when 2-bromo-2-methylbutane reacts with with sodium ethoxide?

 

4. 

 E2-4.bmp

 5. 

E2-5.bmp

 

Answers are attached

Further Reading

Michigan State Virtual Textbook of Organic Chemistry

            E2 Elimination

MasterOrganicChemistry

            The E2 Mechanism

          Elimination Reactions And Cyclohexane Rings

Carey 4th Edition On-Line Activity

        E2 mechanism

Khan Academy

            Mechanism

            Regioselectivity

            Stereoselectivity

            Stereospecificity

            Substituted Cyclohexanes

Leah4Sci

            E2 Reaction Rate & Mechanism (vid 1 of 4) Bimolecular Beta-Elimination        by Leah Fisch

            E2 Reaction (vid 2 of 4) Using Newman Projections To Predict Products by      Leah4sci

            E2 reaction (vid 3 of 4) using chair conformations for anti coplanar reactions

            E2 Reaction (vid 4 of 4) Big Bulky Base for Anti-Zaitsev Product

Cliffs Notes

Slide Presentations

Web Pages

        Sparknotes on SN2 and E2

          basics of e2 elmination

Videos

          E2 elimination video

          E2 elimination and examples