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Elimination Reaction Definition, Types and Mechanism - Difference Between E1 and E2 Reaction

 

Introduction

The elimination reaction is a chemical process involving removing substituents from a compound to produce a new molecule. The reaction occurs in the existence of acid, base, metal, and, in some instances, high temperature. Using this method, it is feasible to make unsaturated (double or triple carbon bonds) organic molecules from saturated (single C-C bonds) organic substances. Structurally, the elimination process is the inverse of the addition reaction.

Elimination Reaction Steps

There are three primary stages in the elimination reaction.

  • Proton ejection
  • C=C pi bond formation
  • Breaking the bonds with the departing group

Some Terms Used in Elimination Reaction

  • Deprotonation is the act of eliminating a hydrogen atom from a chemical as a proton (H+).
  • Dehalogenation is the process of removing a halogen.
  • Dehydrohalogenation occurs when both hydrogen and halogen are eliminated.
  • A dehydration process or β-elimination occurs when hydrogen and oxygen are simultaneously eliminated, as in alcohols.

Elimination Reaction Types

Elimination reactions can be divided into three categories.

1. Type E1

  • The rate of the reaction is directly related to the amount of the chemical to be converted.
  • This is done via a two-step expulsion process, sometimes called unimolecular elimination.
  • The two steps are deprotonation and ionization; due to ionization, a carbon cation forms as an intermediate product. This carbon cation loses a proton during deprotonation.
  • It results in the production of an intermediary.
  • The first-order dynamics are regioselective. Regioselectivity is defined as the partiality of breakage of chemical bonds in one direction over another.
  • It is a first-order reaction as the reaction rate is dependent on only one molecule.

Example of E1 reaction
Alcohol E1 reactions consist of three steps: protonated alcohol production, carbon cation production, and alkene creation (Deprotonation). Therefore, it is also known as alcohol dehydration.

2. Type E2

  • Bimolecular reactivity is another name for this one-step elimination mechanism.
  • In this elimination reaction type, both the ionization and the deprotonation steps of the reaction co-occur.
  • Second-order kinetics are stereoselective and regioselective. Stereoselectivity is where the formation of one stereoisomer is preferred concerning another.
  • The departing carbon group and C-H bonds split at the same time. Moreover, the reaction rate is directly related to both the molecule to be converted and the transmitting reagent.
  • The base in E2 reactions must be powerful enough to extract a mildly acidic hydrogen atom.

E2 Reaction Mechanism

A strong base will assault the substrates in E2 elimination, producing a transitory state wherein the C-X and C-H bonds begin to dissolve, and the pi bond and the bond between the strong base and the hydrogen begin to form. Alkene is formed as a result of this transition state.

3. Type Reaction E1cB (Conjugate Base)

  • E1cB reactions are elimination reactions that occur in a basic solution with moderately acidic hydrogen to be eliminated and a comparatively feeble leaving group.
  • This is a two-step procedure.
  • The first base forms a stable anion that travels to an adjacent atom and ejects the departing group, resulting in a double or triple bond formation.
  • Elimination Unimolecular conjugate Base is the expansion of the abbreviation E1cb.

Steps in E1cB reaction

E1cB is a two-stage process, with the first step either being reversible or not. To create a stable anion, a base firstly extracts the comparatively acidic proton. The anion's electrons subsequently travel to a nearby atom, discharging the departing group and establishing a double or triple bond in the process.

Differences between E1 and E2 Reaction

The following tabular column enlists the differences between E1 and E2 reactions.

E1 Reaction E2 Reaction
1. The mechanism of the reaction occurs in two steps i.e., deprotonation and ionization. 1. The mechanism of the reaction occurs in one step only. Both the leaving group and the departing group are simultaneously eliminated.
2. The order of the reaction is unimolecular. 2. This has a bimolecular order of reaction.
3. Rate of the reaction proceeds as R = kr [substrate] 3. Rate of the reaction proceeds as R = kr [substrate][base].
4. Reaction proceeds in the attendance of a weak base 4. Reaction proceeds in the attendance of a strong base only.
5. Stereochemistry is not important. 5. The leaving group must be opposite to the hydrogen being removed.
6. Formation of carbocation follows the following order of stability. Tertiary carbocations > Secondary carbocations >> Primary carbocations 6. There is no such order of stability in E2 reaction.
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