Elimination reaction

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.

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.

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 .

• 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.

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 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.

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.

See also

• Important publications in organic chemistry

References