Elimination - Beta Elimination, E1cB 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 India, 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 pi bond containing product.

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

TABLE OF CONTENTS

Elimination
Mechanism of Elimination
E1cB Elimination
Conditions for E1cB Elimination
Mechanism of E1cB Elimination
Rate of the Reaction in E1cB Mechanism
Practice Problems
Frequently Asked Questions - FAQ

Elimination Reactions

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.

E1cB Elimination

In E1cB, E stands for elimination, 1 for unimolecular, and cB for conjugate base. In E1cB, the proton is abstracted to form the conjugate base. The anion that results is stable enough to exist because it can

be delocalised on to the electron-withdrawing group. Although the anion is stabilized by the electron-withdrawing group, it still prefers to lose a leaving group and become an alkene.

Conditions for E1cB Elimination

There are two conditions for any molecule to give E1cB elimination reaction:

1. A good electron withdrawing group must be present at the β-position to the leaving group.

Example- Carbonyl (nitro (cyano ( sulphonyl (

Phenyl ( ester (, and other carbonyl stabilizing groups.

2. Generally, the poor leaving group shows E1cB reaction.

Example-

Mechanism of E1cB-Elimination

The mechanism of E1cB reaction is a two-step process.

Step 1: Abstraction of proton to form carbanion

Base, abstracts the proton from the carbon to which electron withdrawing group, nitro(is attached. The removal of the proton gives the carbanion intermediate.

Step 2: Removal of leaving group to give an alkene

Leaving group, attached to the adjacent carbon of the carbon containing negative charge, leaves such that π bond is formed between the two carbons containing the leaving group and negative charge.

Rate of the Reaction in E1cB Mechanism

Second step, i.e. removal of the leaving group to form alkene, is a rate-determining step of the E1cB reaction. The reaction proceeds through the conjugate base of the starting material that’s why it is called as unimolecular elimination via conjugate base.

The leaving group is not lost from the starting molecule, but from the conjugate base of the starting molecule. This sort of elimination, which starts with a deprotonation, is called E1cB (cB for conjugate Base).

Practice Problems

Q1. If the mechanism is E1cB then the possible products will be:

Answer: B
Solution:

Q2. What are the essential conditions for any reaction to show E1cB mechanism?

A). Presence of electron withdrawing group

B). Presence of electron donating group

C). Poor leaving group

D). Both A and C

Answer: D

Solution: There are two conditions for any molecule to give E1cB elimination:

1. A good electron withdrawing group must be present at the β-position to the leaving group.

2. Presence of poor leaving group.

Q3. Which of the following substrates undergo elimination via the E1cB mechanism?

A).

B).

C).

D). Both A and B

Answer: D

Solution: There are two conditions for any molecule to give E1cB elimination:

1. A good electron-withdrawing group must be present at the β-position to the leaving group.
2. Presence of poor leaving group.

Both A and B have electron-withdrawing groups at as well as the poor leaving group at . So both the conditions of the E1cB mechanism is satisfied.

Therefore, options A and B undergo the E1cB mechanism.

Q4. Which of the following is rate-determining step in the E1cB mechanism?

A).Abstraction of proton to form carbanion
B). Removal of leaving the group to form alkene
C). Both A and B
D). None of the above

Answer: B

Solution: The mechanism of the E1cB reaction is a two-step process.

1. Abstraction of proton to form carbanion
2. Removal of leaving the group to form alkene

Step two out of two steps is the rate-determining step.

Frequently Asked Questions - FAQs

Q1. What is a reaction?
Answer. In , two atoms/substituents are removed from the adjacent atoms (adjacent positions) to form a new multiple bonds. It is also known as .

Q2. Is the presence of a poor leaving group a necessary condition for any compound to show an E1cB reaction?
Answer. It is not necessary that there is always a poor leaving group. Good leaving group (like halogen also shows E1cB reaction.

Q3. Ifis used as a solvent, then deuterium exchange takes place in the E1cB mechanism. Why?
Answer. In the E1cB mechanism, carbanion formed as an intermediate in the first step is reversible in nature.

Q4. How does an electron-withdrawing group stabilize the carbanion?
Answer. The electron-withdrawing group stabilize the carbanion by effect.

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