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 ^[1]. 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 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.

The reaction fundamental elements are

• Breaking of the carbon-hydrogen and carbon-halogen bonds in one step.
• Formation of a C=C 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 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.
  • 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 a weak base.
• E1 reactions are in competition with S_N1 reactions because they share a common carbocationic intermediate.
• A deuterium isotope effect is absent.
• No antiperiplanar requirement. An example is the pyrolysis of a certain sulfonate ester of menthol:

Only reaction product A results from antiperiplanar elimination, the presence of product B is an indicator for a E1 mechanism ^[2].

• 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.

If S_N1 and E1 pathways are competing, the E1 pathway can be favored by increasing the heat.

[edit] 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 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 ^[3] 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 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:

• 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
• the Takai olefination with two bulky chromium groups.

[edit] See also

• Important publications in organic chemistry

[edit] References

1. ^ March, Jerry (1985). Advanced Organic Chemistry, Reactions, Mechanisms and Structure, third Edition, John Wiley & Sons. ISBN 0-471-85472-7. 
2. ^ 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
3. ^ 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. 

v • d • e Basic reaction mechanisms Nucleophilic substitution Unimolecular nucleophilic substitution (SN1) · Bimolecular nucleophilic substitution (SN2) · Nucleophilic aromatic substitution (SNAr) · Nucleophilic internal substitution (SNi) Elimination reaction Unimolecular elimination · E1cB elimination reaction · Bimolecular elimination Related topics Elementary reaction · Molecularity · Stereochemistry · Catalysis · Collision theory · Solvent effects Chemical kinetics Rate equation · Rate-determining step

v • d • e Topics in Organic Reactions Addition reaction - Elimination reaction - Polymerization - Reagents - Rearrangement reaction - Redox reaction - Regioselectivity - Stereoselectivity - Substitution reaction - List of organic reactions