Definition: What is Elimination Reaction?

The elimination reaction is an organic reaction in which two substituents are removed from a molecule to form a new product. The process takes place in the presence of acid, base, metal, and sometimes through heating. Through this process, it is possible to synthesize unsaturated (double or triple bond between carbon atoms) organic compounds from saturated (single C-C bonds) ones. Elimination reaction is mechanically the reverse of addition reaction ^[1-3].

The elimination reaction involves three fundamental steps ^[4].

1. Removal of proton
2. Formation of C=C pi bond
3. Breaking of the bond to the leaving group

Nomenclatures Used in Elimination Reaction

1. When a hydrogen atom is removed from the compound as a proton (H⁺), the process is called deprotonation.
2. If a halogen is removed, then it is called dehalogenation.
3. If both hydrogen and halogen are removed, then it is called dehydrohalogenation.
4. If hydrogen and oxygen are removed together, as in the case of alcohols, then it is dehydration reaction or β-elimination.

Types of Elimination Reaction

There are three types of elimination reaction ^{[4 – 5]}

1. E1 type

• two-step removal mechanism process
• also known as unimolecular elimination
• formation of an intermediate
• reaction rate is proportional to the concentration of the compound to be transformed – first-order kinetics
• regioselective, follows Zaitsev’s rule, and favors the formation of Zaitsev product

2. E2 type

• one step removal mechanism process
• also known as bimolecular reaction
• carbon-leaving group and carbon-hydrogen bonds break off simultaneously
• reaction rate is proportional to both the compound to be transformed and the transferring agent – second-order kinetics
• regioselective, follows Zaitsev’s rule, and favors the formation of Zaitsev product

3. E1cb type

• two-step elimination mechanism
• also known as unimolecular conjugate base elimination
• compound has an acidic hydrogen atom and a weak leaving group (e.g., -OH)
• standard in the dehydration reaction

Examples of Elimination Reaction ^{[2, 6]}

Mechanism of Elimination Reaction ^{[4 – 12]}

Conditions of Elimination Reaction

Elimination reactions are usually favored at high temperatures. For the dehydrohalogenation reaction, a strong base is required.

FAQs

Q.1. Where is the elimination reaction applied?

Ans. Application of elimination reaction can be found in Hofmann elimination to synthesize alkene during which the leaving group is a quaternary amine.

Q.2. What is the difference between a substitution and elimination reaction?

Ans. In the case of the elimination reaction, the agent responsible for elimination merely eliminates the group of atoms, whereas, in a substitution reaction, the agent substitutes one replacement group with another.

References:

1. Definition – Britannica.com
2. Definition – Chem.libretexts.org
3. Definition – Siyavula.com
4. Definition and mechanism – Byjus.com
5. Definition and mechanism – Nptel.ac.in
6. Examples – Chem.libretexts.org
7. Mechanism – Iitk.ac.in
8. Mechanism – Cliffsnotes.com
9. E1 reaction mechanism – Chem.ucalgary.ca
10. E1 reaction mechanism –Masterorganicchemistry.com
11. E2 reaction mechanism – Chem.libretexts.org
12. E2 reaction mechanism –Chem.ucalgary.ca