Elimination Reaction

The organic reaction in which two or more atoms are removed from a molecule is called an elimination reaction. The removal is done through the action of metals, acids, or bases, or even by increasing heat to a high temperature. In an elimination reaction, the single bonds between carbon atoms are transformed to double or triple bonds. In other words, saturated compounds are transformed into unsaturated compounds. 

Elimination reaction is categorised based on the type of atoms leaving a molecule. For example, the removal of two hydrogen atoms is termed as dehydrogenation. The removal of one hydrogen and two oxygen atoms (one water molecule) is termed dehydration or β-elimination. The removal of one hydrogen and one halogen atom is termed dehydrohalogenation. The removal of two halogens is termed dehalogenation. Depending on reaction kinetics, the elimination reaction is also classified as E1 and E2 reactions.

Mechanism of an Elimination Reaction

An elimination reaction involves the following basic mechanisms:

• Elimination of a proton

• Formation of the pi bond between two carbon atoms (C=C)

• Breaking of the bond linked to a leaving group

E1 Reaction

• A unimolecular elimination reaction is referred to as an E1 reaction. 

• The rate of this reaction is k [R-LG], which implies that the step in which a molecular species is decomposed, determines the rate of the reaction. 

• The first step of the E1 reaction involves the loss of a leaving group (LG) which leads to the formation of a positive charge on a carbon atom (carbocation intermediate). This can be illustrated as follows: 

• The second step involves the loss of an H+ ion to a base (B) which leads to the formation of a pi bond. This can be illustrated as follows: 

• The rate of the reaction is determined by the step where the leaving group is lost by the organic compound to form an intermediate. 

• The reaction may be fast or slow, based on the nature of the leaving group. This is because the rate-determining step is based on the loss of leaving group. 

• The nature of the Base (B) is unimportant since the second step does not affect the rate of the reaction. 

E2 Reaction 

• A bimolecular elimination reaction is referred to as an E2 reaction. 

• The rate of this reaction can be written as k[B][R-LG], which implies that the step in which there is an interaction between the Base (B) and the organic substrate (R-LG) is the rate-determining step of the reaction. 

• In this reaction, there is a simultaneous removal of the leaving group, removal of the proton (H+) by the base, and the formation of the pi-bond. 

• The E2 reaction can be illustrated as follows: 

• The sp3 hybridised carbon atoms change to sp2 hybridised. 

• The nature of the leaving group affects the rate of the reaction as it leaves during the rate-determining step of the reaction. 

• An E2 reaction may change to an E1 reaction if the leaving group is too good. 

• The nature of the base also affects the rate of the reaction as it also leaves during the rate-determining step of the reaction.

• A more reactive base will favour the speed of the E2 reaction. 

E1CB Reaction 

• A unimolecular (E1) conjugate base (CB) reaction is referred to as an E1CB reaction.

• This is a two-step reaction. 

• In the first step, hydrogen is removed by a base (B) which leads to the formation of an alkoxide anion. In other words, the first step involves deprotonation to form an anion. The first step can be illustrated as follows: 

• In the second step, a pi bond is formed when the leaving group (LG) is removed and is displaced by a pair of electrons from the conjugate base (hence the term CB) as follows: 

• A poor leaving group slows down the reaction, hence the second step of the reaction determines its rate.

Conclusion 

The organic reaction in which two or more atoms are removed from a molecule is termed an elimination reaction. The mechanism of an elimination reaction involves the removal of a proton, the formation of a pi bond, and the removal of a leaving group.

There are three types of elimination reactions:  E1, E2, and E1CB. In the E1 reaction, a leaving group is removed, followed by the removal of a hydrogen ion by a base, which leads to the formation of a pi bond. In the E2 reaction, there is a simultaneous removal of the leaving group, removal of the H+ ion by the base, and the formation of the pi-bond. In an E1CB reaction a hydrogen ion is removed by a base, followed by the formation of a pi bond, when the leaving group (LG) is removed and is displaced by a pair of electrons from the conjugate base.