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

An elimination reaction is a type of organic reaction in which two 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 refer not to the number of steps in the mechanism, but rather to the kinetics of the reaction: E2 is bimolecular (second-order) while E1 is unimolecular (first-order). In cases where the molecule is able to stabilize an anion but possesses a poor leaving group, a third type of reaction, E1CB, exists. Finally, the pyrolysis of xanthate and acetate esters proceed through an "internal" elimination mechanism, the Ei mechanism.

E2 mechanism

The E2 mechanism, where E2 stands for bimolecular elimination, 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:

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 isobutene, 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 specifications

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 for two main reasons.

Specific features :

  1. Rearrangement possible
  2. Independent of concentration and basicity of base

Competition among mechanisms

The reaction rate is influenced by the reactivity of halogens, 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 the 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

An example of Elimination predominating substitution is visible from a special case of Williamson synthesis. When 3° haloalkane reacts with an alkoxide, due to strong basic character of the alkoxide and less reactivity of 3° group towards SN2, an alkene will be formed instead of the expected ether with a 3° group.

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.

Elimination reactions other than β-elimination

β-Elimination, with loss of electrofuge and nucleofuge on vicinal carbons, is by far the most common type of elimination. The ability to form a stable product containing a C=C or C=X bond, as well as orbital alignment considerations, strongly favors β-elimination over other elimination processes.[5] However, other types are known, generally for systems where β-elimination cannot occur.


The next most common type of elimination reaction is α-elimination. For a carbon center, the result of α-elimination is the formation of a carbene, which includes "stable carbenes" such as carbon monoxide or isocyanides. For instance, α-elimination the elements of HCl from chloroform (CHCl3) in the presence of strong base is a classic approach for the generation of dichlorocarbene, :CCl2, as a reactive intermediate. On the other hand, formic acid undergoes α-elimination to afford the stable products water and carbon monoxide under acidic conditions. α-Elimination may also occur on a metal center, one particularly common result of which is lowering of both the metal oxidation state and coordination number by 2 units in a process known as reductive elimination. (Confusingly, in organometallic terminology, the terms α-elimination and α-abstraction refer to processes that result in formation of a metal-carbene complex.[6] In these reactions, it is the carbon adjacent to the metal that undergoes α-elimination.)

In certain special cases, γ- and higher eliminations to form three-membered or larger rings is also possible in both organic and organometallic processes. For instance, certain Pt(II) complexes undergo γ- and δ-elimination to give metallocycles.[7] More recently, γ-silyl elimination of a silylcyclobutyl tosylate has been used to prepare strained bicyclic systems.[8]


Many of the concepts and terminology related to elimination reactions were proposed by Christopher Kelk Ingold in the 1920s.

See also


  1. ^ Coleman, G. H.; Johnstone, H. F. (1925). "Cyclohexene". Organic Syntheses. 5: 33. doi:10.15227/orgsyn.005.0033.
  2. ^ March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.), New York: Wiley, ISBN 0-471-85472-7
  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". Journal of Chemical Education. 85 (4): 552. Bibcode:2008JChEd..85..552N. doi:10.1021/ed085p552.
  4. ^ Stephanie M. Villano; Shuji Kato; Veronica M. Bierbaum (2006). "Deuterium Kinetic Isotope Effects in Gas-Phase SN2 and E2 Reactions: Comparison of Experiment and Theory". J. Am. Chem. Soc. 128 (3): 736–737. doi:10.1021/ja057491d. PMID 16417360.
  5. ^ Anslyn, Eric V. (2006). Modern physical organic chemistry. Dougherty, Dennis A., 1952-. Sausalito, CA: University Science. ISBN 1891389319. OCLC 55600610.
  6. ^ Crabtree, Robert H. (2009). The organometallic chemistry of the transition metals (5th ed.). Hoboken, N.J.: Wiley. ISBN 9780470257623. OCLC 268790870.
  7. ^ Moore, Stephen S.; DiCosimo, Robert; Sowinski, Allan F.; Whitesides, George M. (1981-02-01). "Ring strain in bis(triethylphosphine)-3,3-dimethylplatinacyclobutane is small". Journal of the American Chemical Society. 103 (4): 948–949. doi:10.1021/ja00394a043. ISSN 0002-7863.
  8. ^ Kelly, Christopher B.; Colthart, Allison M.; Constant, Brad D.; Corning, Sean R.; Dubois, Lily N. E.; Genovese, Jacqueline T.; Radziewicz, Julie L.; Sletten, Ellen M.; Whitaker, Katherine R. (2011-04-01). "Enabling the Synthesis of Perfluoroalkyl Bicyclobutanes via 1,3 γ-Silyl Elimination". Organic Letters. 13 (7): 1646–1649. doi:10.1021/ol200121f. ISSN 1523-7060. PMID 21366262.