Elimination Reactions: E1 and E2

Table of Contents

When a nucleophile attacks hydrogen rather than carbon, elimination reactions occur. A carbon-carbon double bond into a molecule containing only a single bond must necessarily involve the elimination of atoms or groups from two adjacent carbons.

elimination reaction

1,2-elimination of HX: Dehydrohalogenation

Dehydrohalogenation involves the elimination of a halogen atom as well as a hydrogen atom from a carbon adjacent with the one losing the halogen. A base is necessary, and its role is to abstract the hydrogen as a proton.


The base :B can be neutral or negatively charged: for example, H2O or OH.

Difference Between E1 and E2 Reaction Mechanism

E1 reaction mechanismE2 reaction mechanism
E1 reaction is known as elimination unimolecular reaction.E2 reaction is known as elimination bimolecular reaction.
It involves two steps.It involves a single step.
E1 reaction follow first-order kinetics.E2 reaction follows second-order kinetics.
Reactivity of alkyl halide in E1 is
3o >2o >1o
Reactivity of alkyl halide in E2 is
3o >2o >1o
Elimination by E1 shows strong saytzeff’s orientation.show a large hydrogen isotope effect
E1 mechanisms are not accompanied by a primary hydrogen isotopes effectE2 reactions are regioselective and favor the formation of Zaitsev products
Rate: K [RX] Rate: K [RX] [OH]
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  • J. March, Advanced Organic Chemistry, Wiley Eastern Ltd., 1986.
  • Wang, Z., Comprehensive Organic Name Reactions and Reagents, John Wiley & Sons, Inc.,2010
  • Morrison, R. T., & Boyd, R. N., Organic Chemistry, Allyn and Bacon, Inc., 1987.
elimination reactions

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