Elimination - Beta Elimination, E1 Mechanism, Rate, Energy Profile Diagram, Stereochemistry, Practice Problems and FAQs

Imagine a beauty pageant contest in which contestants from various countries participated. In the finale, contestants from five countries are left and are made to stand in a line on a stage. In the second last round, contestants from the Philippines and France get eliminated such that those from Venezuela, South Africa and Colombia, come closer. The ultimate aim of the contest is to filter the best suitable candidate.

Similarly, in some organic reactions, atoms from two adjacent carbon atoms are eliminated to form a stronger bond containing product.

Depending upon the mechanism of how two atoms are eliminated, Majorly they are of three types, E1, E2, E1cB. Let's study the E1 mechanism in detail.

TABLE OF CONTENTS

Elimination
Mechanism of Elimination
E1 Elimination
Reactions That Follow E1 Mechanism
Rate of the Reaction in E1 Mechanism
Energy Profile Diagram for E1 Elimination Reaction
Stereochemistry of E1 Mechanism
Practice Problems
Frequently Asked Questions - FAQs

-Elimination

In elimination, two atoms/substituents are removed from the adjacent atoms ( and positions) to form a new multiple bond. It is also known as elimination.

For example, when a haloalkane is made to react with a base, , in the presence of heat (, the elimination of the halogen (from the and proton ( from the takes place to give the corresponding alkene.

Mechanism of -elimination

Generally, the base () abstracts a proton ( ) from the followed by the removal of halide ion from the . This can be understood with the help of the mechanism given below.

One of the important examples of Elimination is dehydrohalogenation. In dehydrohalogenation, hydrogen from the and a halogen from the get eliminated when heated with an alcoholic solution of

Based on the fact that either abstraction of a proton or the removal of halogen is the first step or there will be simultaneous removal, elimination reactions can be classified into three types namely, E1 E2 and E1cB.

E1 Elimination

In E1 elimination, the leaving group leaves the substrate first to form a carbocation intermediate. Then, the proton abstraction takes place to form an alkene. E1 elimination can be explained in detail by the following reactions. Example- Dehydrohalogenation of an alkyl halide in the presence of water.

Reactions That Follow E1 Mechanism

Dehydrohalogenation of alkyl halide

As the name dehydrohalogenation suggests, halogen and hydrogen are removed. The reaction occurs in the presence of a weak base like It is a two-step process. Let's consider a reaction of with in the presence of heat. The reaction occurs in two steps as follows:

Step 1: Formation of carbocation

being electronegative leaves as to form a tertiary carbocation.

Step 2: Attack of base

being a weak base abstract the from the to give.

Dehydration of Alcohol

Let us consider the dehydration reaction of .

The reaction occurs in three steps.

Step 1: Protonation of alcohol

Oxygen being electron rich accepts the to give protonated alcohol.

Step 2: Loss of leaving group

Protonated alcohol removes a water molecule to give carbocation. The carbocation formed here is a stable secondary carbocation.

3. Deprotonation to form alkene

There is the removal of to give an alkene. There are two types of available i.e. a and b.

Case 1: Removal of proton ‘a’ gives but-1-ene

Case 2: Removal of proton ‘b’ gives but-2-ene.

The reaction results in the formation of two products. The major product is decided with the help of Saytzeff's rule. According to this rule, the most substituted alkene is the major product. Generally, it is the most preferred one. being the most substituted of the two will be the major product.

Rate of the Reaction in E1 Mechanism

The formation of the carbocation is the rate-determining step in the E1 elimination reaction.

Thus, E1 elimination is a first-order reaction, i.e., a unimolecular reaction. The more the stability of the carbocation intermediate, the more the rate of the reaction. The increasing order of stabilities of carbocations is:

Energy Profile Diagram for E1 Elimination Reaction

Let us take the example of . It undergoes E1 elimination such that two products are obtained, and .

According to the Saytzeff rule, will be the major product. In the E1 reaction of , the first step involves the formation of carbocation intermediate via (Transition State - 1). The removal of leads to the formation of two products. being the stable alkene will have less energy than the unstable alkene.

Note:

The energy of is greater than the energy of . This is because in the reaction is moving towards a less stable and reactive carbocation, but in the reaction is moving closer to the formation of stable alkene.

Stereochemistry of E1 Mechanism

According to the Saytzeff rule, but-2-ene will be the major product. But-2-ene exits in two isomeric forms, that is trans and cis. The trans isomer of but-2-ene would be more stable than the cis isomer. So, trans-but-2-ene would be the overall major product.

Practice Problems

Q1. The major product formed will be:

Both A and B
None of the above

Answer: A

Solution:

According to the Saytzeff rule, A being the more substituted alkene, will be the major product.

So, option A) is the correct answer.

Q2. E1 elimination occurs in presence of:

Heat
Weak base
Polar protic solvent
All of the above

Answer: D

Solution: E1 elimination occurs in the presence of a weak base like , and heat. A polar protic solvent solvates the halide ion (leaving group) formed due to thebond breaking and also helps in the breaking of bond. Therefore, a polar protic solvent favours an E1 elimination.

So, option D) is the correct answer.

Q3. According to which rule, the more substituted alkene will be formed as a major product?

Markovnikov’s rule
Saytzeff rule
Anti markovnikov’s rule
Hofmann rule

Answer: B

Solution: According to the Saytzeff rule, the most substituted alkene is the major product. Generally, it is the most preferred one.

So, option B) is the correct answer.

Q4. What are the factors that determine the stability of the product in an E1 elimination reaction?

Stability of carbocation
Stability of alkene formed
Both A and B
None of the above

Answer: C

Solution: The formation of the carbocation is the rate-determining step in the E1 elimination reaction. The more the stability of the carbocation intermediate, the higher the rate of the reaction. The increasing order of stabilities of carbocations is:

Primary carbocation Secondary carbocation Tertiary carbocation

The major product out of the two forms is decided with the help of the Saytzeff rule. According to this rule, the most substituted alkene is the major product. Generally, it is the most preferred one.

So, option C) is the correct answer.

Frequently Asked Questions - FAQs

Q1. Why does the dehydration of alcohol take place in an acidic medium but not in a basic medium?
Answer: In an acidic medium, gets converted into which is a good leaving group. In a basic medium is the leaving group, and it is a poor leaving group. So, the dehydration of alcohols generally takes place in an acidic medium.

Q2. Why are alcohols a poor starting material for the synthesis of 1-alkene?
Answer: In the synthesis of 1-alkene, a primary carbocation () is formed that would rearrange to form alkene. Even if 1-alkene is also formed, it would rearrange in the acidic medium to alkene. So, 10 alcohols are poor starting materials for the synthesis of alkene.

Q3. What is the difference between an E1 and reaction?
Answer:

Q4. What is a ?
Answer: Carbon that is directly attached to the carbon which is attached to the functional group, is called.

Related Topics

Electrophiles and Nucleophiles	Types of Reactions
Nucleophilic Substitution Reaction-SN2	Nucleophilic Substitution Reaction-SNi, SNNGP
Substitution vs Elimination	Factors Affecting SN1 and SN2