Elimination Reaction

Elimination reactions are endothermic reactions that occur at high temperatures (>50℃). Saturated alkyl halides and alcohols give unsaturated alkenes by elimination reactions. In elimination reactions, competition occurs between nucleophilic substitution and elimination. Elimination reactions are used in the preparation of alkenes by alkyl halides and alcohols. 

Before understanding the elimination reactions and their mechanisms, you must have an idea of what are α and β carbons and hydrogens. So, let’s start with a brief introduction of α and β carbons and then we will move on to a detailed explanation of elimination reactions and their various mechanisms. 

What are 𝛼-Carbon and 𝛼-Hydrogen? 

α-carbon refers to the 1st carbon atom attached to a functional group in an organic molecule. Carbon atom directly attached to a halogen group, aromatic group, or double or triple bond is also called α-carbon. All the hydrogen atoms attached to the α-carbon are called α-hydrogens. Examples are given below:

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What are 𝛽-Carbon and 𝛽-Hydrogen?

The carbon atom present next to α-carbon is called  𝛽-carbon and the hydrogen atoms attached to β-carbons are called β-hydrogens. Examples are shown above in the above image. 

What is Elimination Reaction? 

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. Examples are given below:

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Important Methods of Elimination Reaction

Generally, the differentiation of the elimination reaction is based on the kind of atoms or groups of atoms that leave the molecules. On the basis of this, there are two major distinguished types of the reaction:

• Dehydration reactions

• Dehydrohalogenation

When there is an elimination of water molecules mostly from the compound such as alcohol, the reaction method is known as the dehydration method. Sometimes dehydration method is also referred to as a beta elimination reaction where the living group and hydrogen atoms are placed at neighbour carbon atoms whereas in the hydrohalogenation reaction, there is a removal of a hydrogen atom as well as a halogen atom. α-elimination and γ- and δ-elimination are some of the other types of elimination reactions. 

Types of Elimination Reactions 

Elimination reactions can be of the following three types on the basis of mechanisms:

E1 Reaction 

E2 Reaction 

E1 CB (Conjugate Base) Reaction 

All three types of reactions differ in their mechanisms although all are endothermic reactions and occur at high temperature. In all three reactions, the product is always an alkene.

E1 Reaction

E1 reactions are those elimination reactions which take place by two steps and are unimolecular. In E1 reactions, E stands for elimination while 1 stands for its unimolecular nature. Unimolecular means its rate of reaction depends on one molecule or its order of reaction is 1. Few specific points about E1 reactions are listed below –

It is a two-step process of elimination: Formation of carbocation(ionisation) and deprotonation. 

The rate of reaction depends on only one molecule or reactant, so it is of 1st order kinetics. 

The reaction is endothermic and occurs at high temperature.

The reaction occurs in the presence of weak bases only. 

In this reaction, polar protic solvent such as H₂O is used.

In E1 reaction of alkyl halide, rate of reaction will be proportional to ability of leaving group to leave the group as then only carbocation will form. For example, as we know Iodine is a better leaving group than Cl and Br, so alkyl halide having iodine as a leaving group will give reaction at a higher rate. 

The step which involves formation of carbocation is the slow step and the rate determining step. It depends on the stability of carbocation. 

E1 reactions take place with tertiary alkyl halides, secondary alkyl halides and alcohols. Primary alkyl halides do not give E1 reactions, they follow SN1 reaction mechanisms as their carbocations are least stable. Order of stability of carbocations –

Tertiary > Secondary > Primary

A tertiary carbocation is the most stable carbocation while primary is the least. 

E1 reactions compete with SN1 reactions because they share a common carbocation intermediate. 

In some E1 reactions, rearrangement of carbocation also takes place. 

It follows the Saytzeff rule when small base attacks. In this, more stable alkene becomes the major product. According to the Saytzeff rule in dehydrohalogenation reactions, the more substituted alkene will be the major product.

General representation of E1 reaction –

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Mechanism of E1 or Elimination-1 Reaction 

E1 reaction mechanism takes place by following two steps –

1. Formation of carbocation 

2. Deprotonation 

Formation of carbocation: 

In presence of a polar protic solvent, the leaving group leaves the α-carbon as anion and carbocation is formed. It is a slow step and the rate-determining step. The rate of reaction depends upon the ability of the leaving group to leave and the stability of carbocation. The reaction is given below –

Deprotonation:

In this step, carbocation reacts with a weak base and deprotonation takes place at β-carbon. This results in the formation of alkene. The reaction is given below –

It is a 1st order reaction as the rate of reaction depends on the concentration of one reactant.

E1 Reaction with Alkyl Halide 

We are explaining the E1 reaction of alkyl halide by taking 2-bromo-2-methylpropane as an example. 2-bromo-2-methylpropane undergoes elimination-1 reaction in presence of H₂O. Two step reaction is given below –

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1st step – formation of carbocation is a slow step and rate determining step. 

E1 Reaction with Alcohols 

E1 reactions of alcohols take place by three steps – formation of protonated alcohol, formation of carbocation, and formation of alkene (Deprotonation). It is also called dehydration of alcohol. It is explained by taking an example of 2-methyl propanol which reacts with con. Sulfuric acid. Sulfuric acid behaves as a weak base with alcohol. All three steps are given below –

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E2 Reaction 

E2 reactions are those elimination reactions which take place by single step and are bimolecular. In E2 reactions, E stands for elimination while 2 stands for its bimolecular nature. Bimolecular means its rate of reaction depends on two reactants (or two molecules) or its order of reaction is 2. Few specific points about E2 reactions are listed below –

It is a single step process of elimination: Ionisation and deprotonation takes place simultaneously. 

The rate of reaction depends on only two molecules or reactants. So, it is of 2nd order kinetics. 

The reaction is endothermic and occurs at high temperature like E1 reactions.

The reaction occurs in the presence of a strong base only. 

In this reaction, the polar aprotic solvent is used.

E2 reactions take place by the formation of a transition state. 

As the reaction involves only one step, so is the rate-determining step. 

E2 reactions take place mainly with primary substituted alkyl halides and secondary alkyl halides. Primary alkyl halides do not give E1 reactions. 

In the E2 reaction, the formation of the pi bond takes place, so the two leaving groups need to be antiperiplanar. That’s why it is also called anti-elimination. 

E2 reactions compete with SN2 reactions if the base used in the reaction can also act as a nucleophile. 

In E2 reactions, the base must be strong enough to remove a weakly acidic hydrogen.

In the reaction for the formation of the pi bond, the hybridization of carbons needs to be lowered from sp3 to sp2. 

It also follows the Saytzeff rule. In this, a more crowded alkene becomes the major product. According to the Saytzeff rule in dehydrohalogenation reactions, the more substituted alkene will be the major product.

General representation of E2 reaction –

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E2 Reaction Mechanism 

It was proposed by Sir Christopher Ingold in the 1920s. In E2 elimination, a strong base attacks the substrate and forms a transitional state in which C-X and C-H bonds start breaking while pi bond and H-B- (B- = strong base) bond starts forming. This transition state changes into an alkene. General representation of the reaction mechanism is given below –

E1cB (Conjugate Base) Reaction 

E1cB reactions are those elimination reactions that occur under basic solution and the hydrogen to be removed is relatively acidic while the leaving group is relatively a poor or weak one. It is a two-step process. The first base generates a stabilised anion then lone pairs of anions moves to the neighbouring atom and expels the leaving group, thus forming the double or triple bond. E1cB stands for Elimination Unimolecular conjugate Base. An example of E1cB reaction is given below –

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This is all about Elimination Reaction.