elimination reaction - E2 mechanism, E1 mechanism, E2 and E1 elimination final notes, Specific elimination reactions

A chemical reaction characterized by the removal of part of a molecule to leave a smaller one. In the following example, bromine is eliminated from dibromoethane to give ethylene:

CH₂Br–CH₂Br + Zn ? CH₂=CH₂ + ZnBr₂.

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 reactions 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.

E2 mechanism

In the 1920s, Sir Christopher Ingold proposed a model to explain a peculiar type of chemical reaction: the E2 mechanism. The reaction rate both influenced by the alkyl halide and the base is second order. The reaction mechanism involving staggered conformation is more favourable for E2 reactions. This reaction type has similarities with the SN2 reaction mechanism.

The reaction fundamental elements are

Breaking of the carbon-hydrogen and carbon-halogen bonds in one step.

An example of this type of reaction in scheme 1 is the reaction of isopropylbromide with potassium ethoxide in ethanol.

E1 mechanism

E1 is a model to explain a peculiar type of chemical elimination reaction. The reaction rate is influenced only by the concentration of the alkyl halide because carbocation formation is the slowest, rate-determining step. Reaction mostly occurs in complete absence of base or presence of only weak base. accompanied by carbocationic rearrangement reactions

An example in scheme 2 is the reaction of tert-butylbromide with potassium ethoxide in ethanol.

E2 and E1 elimination final notes

The reaction rate is influenced by halogens reactivity; 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 SN2 possible) on the other hand has a KIE of 0.85 consistent with a SN2 reaction because in this reaction type the C-H bonds tighten in the transition state.

Specific elimination reactions

The E1cB elimination reaction is a special type of elimination reaction involving carbanions.

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