World Library  
Flag as Inappropriate
Email this Article

Elimination reaction

Article Id: WHEBN0000238527
Reproduction Date:

Title: Elimination reaction  
Author: World Heritage Encyclopedia
Language: English
Subject: E1cB-elimination reaction, Addition reaction, Hydrazone iodination, Arrow pushing, Organic reaction
Collection: Elimination Reactions, Reaction Mechanisms
Publisher: World Heritage Encyclopedia

Elimination reaction

Elimination reaction of cyclohexanol to cyclohexene with sulfuric acid and heat[1]

An elimination reaction is a type of substituents are removed from a molecule in either a one or two-step mechanism.[2] The one-step mechanism is known as the E2 reaction, and the two-step mechanism is known as the E1 reaction. The numbers do not have to do with the number of steps in the mechanism, but rather the kinetics of the reaction, bimolecular and unimolecular respectively. In rare cases, for molecules possessing particularly poor leaving groups, a third type of reaction, E1CB exists.

In most organic elimination reactions, at least one hydrogen is lost to form the double bond: the alkyl halides, with good leaving groups, reacting with a Lewis base to form an alkene. Elimination may be considered the reverse of an addition reaction. When the substrate is asymmetric, regioselectivity is determined by Zaitsev's rule or through Hofmann elimination if the Carbon with the most substituted Hydrogen is inaccessible.


  • E2 mechanism 1
  • E1 mechanism 2
  • E2 and E1 elimination final notes 3
  • See Also 4
  • References 5

E2 mechanism

During the 1920s, Sir Christopher Ingold proposed a model to explain a peculiar type of chemical reaction; the E2 mechanism. E2 stands for bimolecular elimination. The reaction involves a one-step mechanism in which carbon-hydrogen and carbon-halogen bonds break to form a double bond (C=C Pi bond).

The specifics of the reaction are as follows:

  • E2 is a single step elimination, with a single transition state.
  • Typically undergone by primary substituted alkyl halides, but is possible with some secondary alkyl halides and other compounds.
  • The reaction rate is second order, because it's influenced by both the alkyl halide and the base (bimolecular).
  • Because E2 mechanism results in formation of a pi bond, the two leaving groups (often a hydrogen and a halogen) need to be antiperiplanar. An antiperiplanar transition state has staggered conformation with lower energy than a synperiplanar transition state which is in eclipsed conformation with higher energy. The reaction mechanism involving staggered conformation is more favorable for E2 reactions (unlike E1 reactions).
  • E2 typically uses a strong base, it must be strong enough to remove a weakly acidic hydrogen.
  • In order for the pi bond to be created, the hybridization of carbons need to be lowered from sp3 to sp2.
  • The C-H bond is weakened in the rate determining step and therefore a primary deuterium isotope effect much larger than 1 (commonly 2-6) is observed.
  • E2 competes with the SN2 reaction mechanism if the base can also act as a nucleophile (true for many common bases).
Scheme 1. E2 reaction mechanism

An example of this type of reaction in scheme 1 is the reaction of isobutylbromide with potassium ethoxide in ethanol. The reaction products are isobutylene, ethanol and potassium bromide.

E1 mechanism

E1 is a model to explain a particular type of chemical elimination reaction. E1 stands for unimolecular elimination and has the following specificities.

  • It is a two-step process of elimination: ionization and deprotonation.
  • E1 typically takes place with tertiary alkyl halides, but is possible with some secondary alkyl halides.
  • The reaction rate is influenced only by the concentration of the alkyl halide because carbocation formation is the slowest step aka rate-determining step. Therefore first-order kinetics apply (unimolecular).
  • Reaction usually occurs in complete absence of base or presence of only a weak base (acidic conditions and high temperature).
  • E1 reactions are in competition with SN1 reactions because they share a common carbocationic intermediate.
  • A secondary deuterium isotope effect of slightly larger than 1 (commonly 1 - 1.5) is observed.
  • No antiperiplanar requirement. An example is the pyrolysis of a certain sulfonate ester of menthol:
E1 elimination Nash 2008, antiperiplanar relationship in blue
Only reaction product A results from antiperiplanar elimination, the presence of product B is an indication that an E1 mechanism is occurring.[3]
Scheme 2. E1 reaction mechanism

An example in scheme 2 is the reaction of tert-butylbromide with potassium ethoxide in ethanol.

E1 eliminations happen with highly substituted alkyl halides due to 2 main reasons.

  • Highly substituted alkyl halides are bulky, limiting the room for the E2 one-step mechanism; therefore, the two-step E1 mechanism is favored.
  • Highly substituted carbocations are more stable than methyl or primary substituted cations. Such stability gives time for the two-step E1 mechanism to occur.
  • If SN1 and E1 pathways are competing, the E1 pathway can be favored by increasing the heat.

Specific features : 1 . Rearrangement possible 2 . Independent of concentration and basicity of base

E2 and E1 elimination final notes

The reaction rate is influenced by halogen's reactivity, iodide and bromide being favored. Fluoride is not a good leaving group, so eliminations with fluoride as the leaving group have slower rates than other halogens. There is a certain level of competition between elimination reaction and nucleophilic substitution. More precisely, there are competitions between E2 and SN2 and also between E1 and SN1. Substitution generally predominates and elimination occurs only during precise circumstances. Generally, elimination is favored over substitution when

In one study [4] the kinetic isotope effect (KIE) was determined for the gas phase reaction of several alkyl halides with the chlorate ion. In accordance with an E2 elimination the reaction with t-butyl chloride results in a KIE of 2.3. The methyl chloride reaction (only SN2 possible) on the other hand has a KIE of 0.85 consistent with a SN2 reaction because in this reaction type the C-H bonds tighten in the transition state. The KIE's for the ethyl (0.99) and isopropyl (1.72) analogues suggest competition between the two reaction modes.

See Also

E1cB-elimination reaction


  1. ^ Coleman, G. H.; Johnstone, H. F. (1925). "Cyclohexene". Organic Syntheses 5: 33.  
  2. ^  
  3. ^ Nash, J. J.; Leininger, M. A.; Keyes, K. (April 2008). "Pyrolysis of Aryl Sulfonate Esters in the Absence of Solvent: E1 or E2? A Puzzle for the Organic Laboratory".  
  4. ^ Stephanie M. Villano, Shuji Kato, and Veronica M. Bierbaum (2006). "Deuterium Kinetic Isotope Effects in Gas-Phase SN2 and E2 Reactions: Comparison of Experiment and Theory".  
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.