Elimination reaction

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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. 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 1 E2 mechanism 2 E1 mechanism 3 E2 and E1 elimination final notes 4 Specific elimination reactions 5 See also 6 References

[edit] 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 the 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 S_N2 reaction mechanism.

Saturated (sp³-hyrbridized) carbons will not react as readily with E2 and it will with E1 due to the steric hindrince. If S_N1 and E1 are competing for the reaction, the E2 can be achieved by increasing the heat.

The reaction fundamental elements are

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

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.

[edit] E1 mechanism

E1 is a model to explain a peculiar 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.
  • Ionization, Carbon-halogen breaks to give a carbocation intermediate.
  • Deprotonation of the carbocation.
• Typical of tertiary and some secondary substituted alkyl halides.
• The reaction rate is influenced only by the concentration of the alkyl halide because carbocation formation is the slowest, rate-determining step. Therefore first order kinetics apply.
• Reaction mostly occurs in complete absence of base or presence of only weak base.
• E1 reactions are in competition with S_N1 reactions because they share a common carbocationic intermediate.
• Deuterium isotope effect is absent.
• accompanied by carbocationic rearrangement reactions

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.

[edit] E2 and E1 elimination final notes

The reaction rate is influenced by halogens reactivity; iodide and bromide being favored. Fluoride is too reactive and its level of basicity too high. There is a certain level of competition between elimination reaction and nucleophilic substitution. More precisely, there are competitions between E2 and S_N2 and also between E1 and S_N1. Substitution generally predominates and elimination occurs only during precise circumstances. Generally, elimination is favored over substitution when

• steric hindrance increases
• basicity increases
• temperature increases
• the steric bulk of the base increases for example Potassium tert-butoxide
• the nucleophile is poor

In one study ^[1] 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 S_N2 possible) on the other hand has a KIE of 0.85 consistent with a S_N2 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.

[edit] Specific elimination reactions

The E1cB elimination reaction is a special type of elimination reaction involving carbanions.

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

• the dehydration reaction is one where the leaving group is water.
• the Bamford-Stevens reaction with a tosyl hydrazone leaving group assisted by alkoxide
• the Cope reaction with an amine oxide leaving group
• the Hofmann elimination with quaternary amine leaving group
• the Chugaev reaction with a methyl xanthate leaving group
• the Grieco elimination with a selenoxide leaving group
• the Shapiro reaction with a tosyl hydrazone leaving group assisted by alkyllithium
• Hydrazone iodination with a hydrazone leaving group assisted by iodine
• A Grob fragmentation with degree of unsaturation increasing in one of the leaving groups.
• the Kornblum–DeLaMare rearrangement (elimination over a (H)C-O(OR) bond) with an alcohol leaving group forming a ketone

[edit] See also

• Important publications in organic chemistry

[edit] References

1. ^ Deuterium Kinetic Isotope Effects in Gas-Phase SN2 and E2 Reactions: Comparison of Experiment and Theory Stephanie M. Villano, Shuji Kato, and Veronica M. Bierbaum J. Am. Chem. Soc.; 2006; 128(3) pp 736 - 737; DOI abstract