Elimination Reactions/β-Elimination Reactions

Elimination Reactions/ β-Elimination Reactions

Definition

The reactions which involve the removal of two atoms or groups from adjacent carbon atoms of the molecule to form a multiple bond (alkene) are called elimination (E) reactions. In other words, the reaction in which β-hydrogen atom from β-carbon and an electronegative functional group (such as X^- or OH^-) from a-carbon are removed from organic compound yielding an unsaturated compound with a double bond between α and β-carbons is known as elimination reactions or β-elimination reactions.

e.g.

1.         Dehydrohalogenation of alkyl halides with a base

2.         Acid-catalyzed dehydration of alcohols

E and S_N reactions take place simultaneously and often competition occurs.

 General Pattern

Types of Mechanism of Elimination

Reaction (By Hughes and Ingold; 1941)

1.         Bimolecular   b-elimination reaction (E₂ reaction).

2.         Unimolecular b-elimination reaction (E₁ reaction).

Difference between E₂ and E₁ Reactions

Bimolecular b-Elimination Reaction

 

Definition

The single-step elimination reactions characterized by transition state involving the removal of b-hydrogen from b-carbon along with halide group, X^‒ (F.G) group from a-carbon of the molecule and formation of double bond between a and b-carbons simultaneously and the reaction rate is influenced by both the concentrations of substrate (alkyl halide) and attacking nucleophile or base are called Bimolecular b-Elimination Reaction; symbolized as E₂ Reactions (Where E stands for elimination and 2 stands for bimolecular).

 

General Representation

Example

Ethyl chloride on heating with alcoholic potash, undergo E₂ reaction, removing HX from adjacent carbon atoms (dehydrohalogenation) forming ethene.

 Mechanism

In E₂ reactions, the removal of outgoing nucleophilic halide (X^-) group and removal of b-hydrogen by the attack of strong base (e.g. OH^-) take place simultaneously through transition state in which attacking nucleophile becomes partially attached to electrophilic-acidic b-hydrogen of substrate as well as the halide group (X^-) is detached at the same time. This momentary unstable high-energy state is termed as transition state which readily changes to alkene by removing b-hydrogen and halide group.

As the base (B^- like OH^-) attacks or removes the b-hydrogen from b-carbon with simultaneous separation of halogen atom from a-carbon as halide (X^-) ion and formation of double bond, take place in the single step which is also the rate-determining step in which two molecules take part, therefore, it is a bimolecular reaction. The net result is the loss of HX from alkyl halide called dehydrohalogenation which is known as b-elimination reaction.  Thus one step elimination reaction involving two molecules in rate-determining step is termed as bimolecular b-elimination reaction designated as E₂.

Kinetics

The rate of E₂ reaction depends on the concentration of both the alkyl halides and attacking base (B-).  Thus the observed rate for E₂ reaction according to Law of Mass Action is:

Example

Primary alkyl halides (1°) generally undergo E₂ reactions.  Aprotic solvent favours E₂ reaction.

 Unimolecular b-Elimination (E₁) Reaction

 

Definition

The two steps elimination reactions characterized by slow reversible heterolytic cleavage of C – X bond of alkyl halide into carbonium ion and halide ion followed by fast removal of a b-hydrogen atom from b-carbon and halide group X^‒ (F.G) from a-carbon atom by the attack of base accompanied by the formation of double bond between a and b-carbon atoms and the reaction rate is influenced by the concentrations of only substrate molecule (alkyl halide) are called Unimolecular b-Elimination Reaction, symbolized as E₁.

 

Mechanism

The mechanism involves two steps:

Step-I (Formation of Carbonium Ion)

E₁ reaction proceeds through two steps mechanism. The first step involves the slow reversible heterolytic fission (ionization) of C–X bond of alkyl halide into carbonium ion (R⁺) and halide ion (X¯)

 

Step-II (Formation of Double Bond)

The second step comprises of fast removal of a proton (H⁺) or b-hydrogen from the b-carbon with the simultaneous formation of double bond between a and b-carbon by the attack of base (OH¯). 

The first step of C – X bond cleavage is slow and hence rate-determining step in which only one substrate molecule takes part, therefore, it is a unimolecular reaction. The net result is the loss of HX from alkyl halide called dehydrohalogenation which is known as b-elimination. Thus two steps elimination reaction involving only molecule in rate-determining step is termed as Unimolecular b-Elimination Reaction designated by E₁.

 

Kinetics

The rate of E₁ reaction depends on the concentration of only Tertiary Alkyl halide. Thus the observed rate equation for E₁ reaction according to Law of Mass Action is:

Example

Tertiary alkyl halides (3°) generally undergo E₁ reaction.  Polar protic solvent favours E₁ reaction. [Secondary alkyl halides give both E₂ and E₁ reaction depending upon the nature of solvent].