Elimination reaction often competes with substitution reactions. In this reaction, a substrate (typically an alkyl halide) eliminates one equivalent (unit) of acid to form an alkene. Two possible mechanisms are available for this elimination reaction – E1 and E2 mechanisms.
Elimination reaction is a type of reaction is mainly used to transform saturated compounds (organic compounds which contain single carbon-carbon bonds) to unsaturated compounds (compounds which feature double or triple carbon-carbon bonds). Besides, it is an important method for the preparation of alkenes.
An elimination reaction is a type of a chemical reaction where several atoms either in pairs or groups are removed from a molecule. The removal usually takes place due to the action of acids and bases or action of metals. It can also happen through the process of heating at high temperatures.
Important Methods of Elimination Reaction
Normally, elimination reactions are distinguished by the kind of atoms or groups of atoms that leave the molecule. Due to this, there are two main methods involved in this type of reaction;
In the dehydration method, there is the elimination of a water molecule mostly from compounds such as alcohol. Sometimes, this method is also called Beta elimination reaction where the leaving group and H are placed at neighbour carbon atoms. On the other hand, in dehydrohalogenation, there is a removal of a hydrogen atom and a halogen atom.
Some of the other common types of elimination reactions are α-elimination and γ- and δ-elimination.
Mechanism Of Elimination Reaction
The elimination reaction consists of three fundamental events and they are;
- Proton removal.
- C-C pi bond is formed.
- There is a breakage in the bond of the leaving group.
Depending on the reaction kinetics, elimination reactions can occur mostly by two mechanisms namely E1 or E2 where E is referred to as elimination and the number represent the molecularity.
- In the E1 mechanism which is also known as unimolecular elimination, there are usually two steps involved – ionization and deprotonation.
- During ionization, there is a formation of carbocation as an intermediate. In deprotonation, a proton is lost by the carbocation.
- This happens in the presence of a base which further leads to the formation of a pi-bond in the molecule.
- In E1, the reaction rate is also proportional to the concentration of the substance to be transformed.
- It exhibits first-order kinetics.
E 1 mechanism shares the features of the SN1 reaction. The initial step is the formation of a carbocation intermediate through the loss of the leaving group. This slow step becomes the rate-determining step for the whole reaction.
- In an E2 mechanism which refers to bimolecular elimination is basically a one-step mechanism.
- Here, the carbon-hydrogen and carbon-halogen bonds mostly break off to form a new double bond.
- However, in the E2 mechanism, a base is part of the rate determining step and it has a huge influence on the mechanism.
- The reaction rate is mostly proportional to the concentrations of both the eliminating agent and the substrate.
- It exhibits second-order kinetics.
The E2 mechanism can generally be represented as below. In the below-mentioned representation, B stands for base and X stands for the halogen.
The rate of the E2 reaction is
Rate = k[RX][Base]
So the reaction rate depends on both the substrate (RX) and the base involved. In the elimination reaction, the major product formed is the most stable alkene.
Elimination Reaction Example
One of the common examples of elimination reaction is the dehydration of alcohol. Here the process takes place in the presence of a base such as an ethoxide ion (C2H5O–). It can be represented as;
“E2 and E1 reactions differ significantly in the nature of the transition states that determine the regiochemistry of the product”. The E2 pathway involves a transition state leading from starting material directly to the product. The product forming step of an E1 reaction is more exothermic than that of an E2 reaction. Thus the E1 reaction has a relatively early transition state, closely resembling the carbocation formed in the rate determining step.
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