Elimination Reactions: E1 and E2 Reaction Mechanisms

Elimination reactions are a type of chemical reactions that usually result in the formation of a cyclic structure or a multiple bond (such a double or triple bond). The formation of a pi-bond necessarily calls for the removal of two atoms or groups from the substrate as a byproduct. This process is known as the Elimination reaction.

To form a double bond, a halogen and a hydrogen atom are removed from adjacent carbon atoms. As a result of this removal, multiple bonds are introduced. This elimination reaction can be classified into two types: E₁ and E₂. Let us learn more about Elimination reaction in this article.

Key Terms: Elimination reaction, E1 reaction, E2 reaction, Substitution reaction, rate of the reaction, carbonation, carbon, haloalkane, dehydrohalogenation

---

Mechanism of Elimination Reaction

[Click Here for Sample Questions]

The following two steps contribute to the mechanism of the elimination reaction: first, the leaving group forms a carbocation intermediate; Second, a base after taking a proton away from a neighboring carbon, forms a double bond.

An alkene is formed when a Haloalkane containing hydrogen on β carbon is treated with an ethanolic solution of potassium hydroxide. In this reaction, a double bond between α and β carbon is formed by releasing a halogen attached to an α carbon and hydrogen to a β carbon of haloalkane. This reaction is called the β elimination reaction or dehydrohalogenation.

Mechanism Of Elimination Reaction

Saytzeff's rule of elimination

Some haloalkanes produce a mixture of olefins in varying proportions. It is explained by Saytzeff ’s Rule, which states that ‘Alkene synthesis (in a dehydrohalogenation reaction) produces multiple products, but the favored result is the alkene with the most alkyl groups attached to the doubly bonded carbon (more substituted double bonds are generated).'

Also Read:

---

Types of Elimination Reaction

[Click Here for Sample Questions]

Chemical reactions known as elimination reactions occur when two atoms or groups are eliminated from a molecule; usually, this leads to the formation of a double or triple bond. Removal of atoms occurs due to the action of the acid or alkali or metal present in the reaction. Furthermore, high temperature also contributes to the elimination reaction.

Alkenes and alkynes are important intermediates in organic synthesis, and their synthesis involves elimination reactions. The primary types of elimination reactions include Unimolecular and Biomolecular elimination reaction.

---

E1 Reaction

[Click Here for Sample Questions]

It is a unimolecular reaction. The rate-determining step consists of the formation of a carbocation intermediate. The stability of the carbocation intermediate determines the reactivity of the E1 reaction. 

• The Order of reactivity for the E1 reaction is 3^o > 2^o > 1^o.
• The (same reactive intermediate) carbocation is used in both the elimination and substitution reactions.
• As a result, both products are produced in similar proportions.
• The entropy of the reaction favors this reaction, hence increasing the temperature favors the E1 reaction.

E1 reaction

Stereochemistry of E1 reaction:

• E1 eliminations generally lead to more stable stereochemistry. 
• The rate of the E1 reaction is solely determined by the substrate, hence the more stable the carbocation, the faster the reaction will be. 
• The production of carbocation is the most time-consuming process. 
• The formation of alkenes doesn’t require a strong base, since there is no leaving group that needs to be displaced. As a result, the stereochemistry of the starting material is not required.

Conditions of E1 reaction:

The factors that favor E1 reaction is the same factors that favor SN1  reaction which is as follows:

• Alkyl halide:

If there is a more stable carbocation intermediate having a lower activation barrier, the E1 reaction proceeds more quickly.

3^o > 2^o > 1^o in terms of carbocation.

• Leaving group:

The group that leaves with a complete negative charge is always ejected. As a result, the best groups that leave are those that can best stabilize an anion. Mostly we can include water as a leaving group in this reaction. 

• Solvent:

The solvent usage causes the positive charge stabler, by surrounding the carbocation with high electron density. The solvent can be protic or Aprotic but it must be polar as polar solvent favors E1 reaction.

• Nucleophiles:

Those nucleophiles that undergo the E1 mechanism are weak neutral molecules. Any nucleophile that is more powerful than this is a strong base that causes elimination reactions.

The E1 reaction always favors the Zaitsev product.

---

E2 Reaction

[Click Here for Sample Questions]

It's a one-step reaction whose rate is determined by the base and substrate concentrations. Both strengths of the base and the nature of the alkyl halide influence the reactivity. Order of reactivity for the E1 reaction is 3^o > 2^o > 1^o. (Tertiary > Secondary > Primary). This reaction proceeds at room temperature.

E2 reaction

Stereochemistry of E₂ reaction

E₂ eliminations could lead to more stable stereochemistry, although it's not completely certain. The initial material for this reaction has two sp³ hybridized carbons, which form two sp² hybridized carbons on rehybridization. The C-X bond and the C-H bond line up on the same plane and face each other in opposite directions.

Conditions of E₂ reaction

The following are the conditions that favor SN2 reaction over E2 reactions 

• Alkyl halide

If there is steric congestion around the carbon atom undergoing the inversion process, the SN2 reaction will be slowed down.

To put it another way, we conclude that if there is less congestion, the reaction will be faster.

CH₃X > RCH₂X >> R₂CHX >>> R₃CX

• Leaving group

The leaving group is always thrown out if it has a complete negative charge. As a result, the best-leaving groups are those that can stabilize an anion the most effectively.

• Solvent

The environment in which the solvent exists can speed up or slow down a reaction. Let’s see the role of solvent in the E2 reaction. The solvent is of two types: Protic solvent and aprotic solvent.

Protic solvent: The nucleophile is saved when a shield is formed because of the bonding of hydrogen or protic solvent with an anion that is attempting to attack the alkyl halide. As a result, the reaction takes longer. Alcohol, water are some of the examples of protic solvents.

Aprotic (polar) solvent: Those solvents that are incapable of giving H's are known as aprotic solvents. (i.e, no O-H or N-H bonds.) Due to which it's favorable in SN2reaction because it's a 1 step reaction and it doesn't require forming or having H's to stabilize the Carbocation step. Acetone, DMSO are some of the examples of aprotic solvents.

Therefore, A polar aprotic solvent favors the E2 reaction. And same as the E1 reaction, the E2 reaction always favors the Zaitsev product.

---

Things to Remember

• Elimination reaction is of two types: E₁ and E₂.
• E1 is a unimolecular reaction with first-order kinetics. E2 is a bimolecular second-order reaction.
• Both E1 and E2 reactions yield alkene as the product.
• The factors that favor E1 reaction are the same factors that favor SN1  reaction.
• A polar aprotic solvent favors the E2 reaction.
• In the case of E1 reaction, the solvent can be protic or Aprotic but it must be polar as polar solvent favors E1 reaction.
• Both E1 and E2 reactions always favor the Zaitsev product.
• Those nucleophiles that undergo the E1 mechanism are weak neutral molecules.
• Mostly water is a leaving group in the E1 reaction.
• In the case of the E2 reaction, the leaving group is the one having a negative charge.

Also Read:

---