Elimination reaction

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:

• E2 is a single step elimination, with a single transition state.
• It is typically undergone by primary substituted alkyl halides, but is possible with some secondary alkyl halides and other compounds.
• The reaction rate is second order, because it's influenced by both the alkyl halide and the base (bimolecular).
• Because the E2 mechanism results in the 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).
• E2 typically uses a strong base. It must be strong enough to remove a weakly acidic hydrogen.
• In order for the pi bond to be created, the hybridization of carbons needs to be lowered from sp³ to sp².
• 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 S_N2 reaction mechanism if the base can also act as a nucleophile (true for many common bases).

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 is a model to explain a particular type of chemical elimination reaction. E1 stands for unimolecular elimination and has the following specifications

• It is a two-step process of elimination: ionization and deprotonation.
  • 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.
• The reaction rate is influenced only by the concentration of the alkyl halide because carbocation formation is the slowest step, as known as the rate-determining step. Therefore, first-order kinetics apply (unimolecular).
• The reaction usually occurs in the complete absence of a base or the presence of only a weak base (acidic conditions and high temperature).
• E1 reactions are in competition with S_N1 reactions because they share a common carbocationic intermediate.
• A secondary deuterium isotope effect of slightly larger than 1 (commonly 1 - 1.5) is observed.
• There is no antiperiplanar requirement. An example is the pyrolysis of a certain sulfonate ester of menthol:

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]

• It is accompanied by carbocationic rearrangement reactions

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.

• 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 cations. 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.

Specific features :

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

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 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 around the α-carbon increases.
• a stronger base is used.
• temperature increases (increase entropy)
• the base is a poor nucleophile. Bases with steric bulk, (such as in Potassium tert-butoxide), are often poor nucleophiles.

An example of Elimination predominating substitution is visible from a special case of Williamson synthesis. When 3° haloalkane is 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 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.

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.

• E1cB-elimination reaction

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