Elimination reaction

From Academic Kids

An elimination reaction is a type of organic chemical reaction in which two groups 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 (this is known as reductive elimination). Elimination reactions are often an alkyl halide reacting with a Lewis base to form an alkene. The one and two-step mechanisms are named and known as "E1" and "E2".


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 elimination bimolecular and has the following specificities.

  • It is a one-step process of elimination.
  • Typical of primary or secondary substituted alkyl halides.
  • The reaction rate is influenced by both the alkyl halide and the base.
  • Reaction often present with strong base.

eg. H-CH2-CH2-Br + OH- → CH2=CH2 + H2O + Br-

The reaction fundamental elements are

  • Breakings of the Carbon-Hydrogen and Carbon-Halogen bonds.
  • Formation of Carbon=Carbon Pi bond.

The E2 mechanism (elimination bimolecular) is a dual process where Carbon-Hydrogen and Carbon-Halogen bonds break in the same elementary step. In order for the Pi bond to be created; the hybridization of carbons need to be lowered from sp3 to sp2.

Because E2 mechanism results in the formaiton of a Pi bond, the two leaving groups (often a Hydrogen and a halogen) need to be coplanar. Anti-coplanar transition state has staggered conformation (lower energy) and syn-coplanar transition state is in eclipsed conformation (higher energy). Reaction mechanism involving staggared conformation is more favourable for E2 reactions.

E1 mechanism

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

  • It is a two-step process of elimination ionization and deprotonation.
  • 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.
  • Reaction mostly present in absence of base or weak ones.

Step 1: (CH3)3C-Br → (CH3)3C+ + Br-

Step 2: (CH3)3C+ → (CH3)2C=CH2 + H+

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.

E2 and E1 elimination final notes

The reaction rate is influenced by halogens reactivity; iodide and bromide being favored. Fluoride is too unreactive and its level of basicity too high. As a last note, there is a certain level of competition between elimination reaction and nucleophilic substitution. More precisely, there are competitions between E2 and SN2 and also between E1 and SN1. Substitution generally predominates and elimination occurs only during precise circumstances. Generally, when steric hindrance, basicity or temperature increases elimination is favored over substitution.

Elimination polymerization

Polymerization used to be principally classified via reaction type, either addition or elimination\condensation reaction. This is now no longer as much in favour, since it obscures a far more important aspect of polymerization, namely the kinetics. These days polymerization is usually categorised as being either step growth or chain growth, with side excursions into living/controlled polymerization.

An elimination reaction (in polymer chemistry) results in the spitting out of a small molecule during the reaction, hence the name. The formation of nylon is called an elimination reaction, since it occurs via the reaction between a carboxylic acid and an amine, generating the amide bond and eliminating water, and the cross linking reaction that occurs during the thermosetting of a phenol formaldehyde resin is also considered to be an example of an elimination reaction. Within classical organic chemistry however, these reactions could be classed more formally as substitution reactions. This doesn't work very well either however; there isn't really, as such, the exchange of one functional group for another. The whole point of classifying a reaction by its type is that it should tell you something about how the reaction proceeds and the conditions under which it would work best. And here we have reached the point at which the definitions obscure, rather than illuminate, the underlying principles in which we are interested.de:Eliminierung ja:脱離反応


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