Elimination reaction.pdf

Elimination reaction • The reaction rate, influenced by both the alkyl halide and the base (bimolecular), 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 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).

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 do not have to do with the number of steps in the mechanism, but rather the kinetics of the reaction, bimolecular and unimolecular respectively.

• E2 typically uses a strong base, it needs a chemical strong enough to pull off 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 .

In most organic elimination reactions, at least one hydrogen is lost to form the double bond: the unsaturation of the molecule increases. It is also possible that a molecule undergoes reductive elimination, by which the valence of an atom in the molecule decreases by two. An important class of elimination reactions are those involving 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.

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• 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.

E2 mechanism

Scheme 1. E2 reaction 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.

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.

The specifics of the reaction are as follows:

2 E1 mechanism

E1 is a model to explain a particular type of chemical • E2 is the first step of elimination with a single elimination reaction. E1 stands for unimolecular elimitransition state. nation and has the following specificities. • Typically undergone by primary substituted alkyl halides, but is possible with some secondary alkyl halides.

• It is a two-step process of elimination: ionization and deprotonation. 1

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5 • Ionization: the carbon-halogen bond breaks to give a carbocation intermediate. • Deprotonation of the carbocation. • E1 typically takes place with tertiary alkyl halides, but is possible with some secondary alkyl halides.

REFERENCES

• 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.

• The reaction rate is influenced only by the concen- Specific features : 1 . Rearrangement possible 2 . Indetration of the alkyl halide because carbocation for- pendent of concentration and basicity of base mation is the slowest step aka rate-determining step. Therefore first-order kinetics apply (unimolecular). • Reaction usually occurs in complete absence of base 3 E2 and E1 elimination final notes or presence of only a weak base (acidic conditions and high temperature). The reaction rate is influenced by halogen's reactivity, iodide and bromide being favored. Fluoride is not a good • E1 reactions are in competition with SN1 reactions leaving group. There is a certain level of competition because they share a common carbocationic interbetween elimination reaction and nucleophilic substimediate. tution. More precisely, there are competitions between • A secondary deuterium isotope effect of slightly E2 and SN2 and also between E1 and SN1. Substitution generally predominates and elimination occurs only larger than 1 (commonly 1 - 1.5) is observed. during precise circumstances. Generally, elimination is • No antiperiplanar requirement. An example is the favored over substitution when pyrolysis of a certain sulfonate ester of menthol: • steric hindrance increases • basicity increases • temperature increases • the steric bulk of the base increases (such as in Potassium tert-butoxide) E1 elimination Nash 2008, antiperiplanar relationship in blue

• the nucleophile is poor

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 • Accompanied by carbocationic rearrangement reac- the other hand has a KIE of 0.85 consistent with a SN2 reaction because in this reaction type the C-H bonds tighten tions in the transition state. The KIE’s for the ethyl (0.99) and isopropyl (1.72) analogues suggest competition between the two reaction modes. Only reaction product A results from antiperiplanar elimination, the presence of product B is an indication that an E1 mechanism is occurring.[3]

4 See Also Scheme 2. E1 reaction mechanism

E1cB-elimination reaction

An example in scheme 2 is the reaction of tertbutylbromide with potassium ethoxide in ethanol.

5 References

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.

[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

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[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, and Veronica M. Bierbaum (2006). “Deuterium Kinetic Isotope Effects in GasPhase SN2 and E2 Reactions: Comparison of Experiment and Theory”. J. Am. Chem. Soc. 128 (3): 736–737. doi:10.1021/ja057491d. PMID 16417360.

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