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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]. Either the unsaturation of the molecule increases (as in most organic elimination reactions) or the valence of an atom in the molecule decreases by two, a process known as reductive elimination.

An important class of elimination reactions are those involving alkyl halides, or alkanes in general, with good leaving groups, reacting with a Lewis base to form an alkene in the reverse of an addition reaction. When the substrate is asymmetric, regioselectivity is determined by Zaitsev's rule. The one and two-step mechanisms are named and known as E2 reaction and E1 reaction, respectively.

Contents

E2 mechanism

In the 1920s, Sir Christopher Ingold proposed a model to explain a peculiar type of chemical reaction: the E2 mechanism. E2 stands for bimolecular elimination and has the following specificities.

  • It is a one-step process of elimination with a single transition state.
  • Typical of secondary or tertiary substituted alkyl halides. It is also observable with primary alkyl halides if a hindered base is used.
  • The reaction rate, influenced by both the alkyl halide and the base, is second order.
  • Because E2 mechanism results in formation of a pi bond, the two leaving groups (often a hydrogen and a halogen) need to be coplanar. An antiperiplanar transition state has staggered conformation with lower energy and a synperiplanar transition state is in eclipsed conformation with higher energy. The reaction mechanism involving staggered conformation is more favourable for E2 reactions.
  • Reaction often present with strong base.
  • 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 the deuterium isotope effect is larger than 1.
  • This reaction type has similarities with the SN2 reaction mechanism.

The reaction fundamental elements are

  • Breaking of the carbon-hydrogen and carbon-halogen bonds in one step.
  • Formation of a C=C Pi bond.
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.

E1 elimination Nash 2008, antiperiplanar relationship in blue
Only reaction product A results from antiperiplanar elimination, the presence of product B is an indicator for a E1 mechanism [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. 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.

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. 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 a 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.

Specific elimination reactions

The E1cB elimination reaction is a special type of elimination reaction involving carbanions. In an addition-elimination reaction elimination takes place after an initial addition reaction and in the Ei mechanism both substituents leave simultaneously in a syn addition.

In each of these elimination reactions the reactants have specific leaving groups:

See also

References

  1. ^ Organic Syntheses I:185 http://orgsynth.org/orgsyn/pdfs/CV1P0183.pdf
  2. ^ March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.), New York: Wiley, ISBN 0-471-85472-7  
  3. ^ Pyrolysis of Aryl Sulfonate Esters in the Absence of Solvent: E1 or E2? A Puzzle for the Organic Laboratory John J. Nash, Marnie A. Leininger, and Kurt Keyes Journal of Chemical Education • Vol. 85 No. 4 April 2008 552
  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". J. Am. Chem. Soc. 128 (3): 736–737. doi:10.1021/ja057491d.  
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