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E1 and E2 Elimination Reactions

Elimination reactions are types of organic reactions where by two substituents are taken away from a molecule. They can be performed in a one or two step mechanism. A substituent is an atom or group of atoms that are substituted in the place of a hydrogen atom on the parent chain of a hydrocarbon. There can be two different results of an elimination reaction, either the unsaturation of the molecules increases, which is common of most organic reactions, or the valence of the atom in the molecule will decrease by two. This is also known as a reductive elimination process. Unsaturation in molecules is a chemical compound that contains carbon-carbon double bonds or triple bonds that are found in alkenes or alkynes.

One of the more important classes of elimination reactions are the ones that involve alkyl halides or alkanes. They have good leaving groups when reacting with a Lewis base, they produce an alkene. If the substrate is asymmetric then the regioselectivity is constituted by Zaitsev’s rule. The more common names that these types of elimination reactions are known as are E2 reaction and E1 reaction.

Sir Christopher Ingold put forth the model to explain a strange type of chemical reaction, the E2 mechanism. E2 stands for bimolecular elimination. The E2 reaction is a one step process of elimination involving a single transition state. It is also viewable with primary alkyl halides when a hindered base is used. The reaction rate is manipulated by the alkyl halides as well as the base which is second order. The results of an E2 mechanism is a pi bond, with the leaving groups, usually hydrogen and halogen must be coplanar. Any method of reaction mechanisms that involve a staggered confrontation most often provides for better E2 reactions.

The E1 is a model that explains a particular type of chemical reactions. The E1 stands for unimolecular elimination. This is a two step process which eliminates the ionization and deprotonation. The reaction rate is influenced by the concentration of the alkyl halide as a result of carbonation formation is the slowest rate determining step. E1 elimination reactions can take place without a base, or with a weak base. There is no deuterium isotope effect and no antiperiplanar requirement.

E1 and E2 reaction rate is motivated by halogen’s reactivity, with iodide and bromide giving preference. There is a level of competition between elimination reactions and nucleophilic substitutions. The substitutions usually predominate and the elimination takes place only when the exact circumstances are realized. More often then not, and elimination reaction is favored when certain circumstances are in place. These include; steric hindrance increases, basicity increases and the temperatures increase. The steric bulk of the base increases and the nucleophilic is generally poor. The E1B elimination reaction is a certain type that involves carbonion. With an addition-elimination reaction the elimination happens after the first addition reaction and in the E1 mechanism all the substituents vanish simultaneously. A couple of examples are the dehydrohalogenation, leaving group a halide. The dehydration reaction has a leaving group of water, while the Cope reaction has an amine oxide leaving group.