What is elimination reaction ?

What is elimination reaction ?

The reaction ,in which two atoms or groups or one atom and one group ,attached to adjacent carbon atom, are eliminated from a molecule, resulting in the formation of unsaturated compounds, then the  reaction is called elimination reaction.

Elimination reaction is the opposite reaction of addition reaction.

There are three types of elimination reaction .

(I)1,2 elimination or β elimination

(II) α-elimination 

(III)Ƴ-elimination

Examples: 

Most common example of elimination reaction  is 1,2 elimination or β elimination, which given below.

There are two types of mechanism of 1,2 elimination or β elimination reaction.

(I) E1 reaction mechanism ( elimination unimolecular reaction)

(II) E2 reaction mechanism (elimination bimolecular reaction )

(I)E1 reaction mechanism: 

This mechanism is parallel to SN1 mechanism, in that the rate determining step is slow ionization of substrate( alkyl halide) to a carbocation, but the second step is now elimination of a β proton to form an alkene.

The reaction is carried out in the presence of a base and the proton is transferred to this.
For example, t-butyl chloride gives isobutene in aqueous ethanolic KOH.

The rate of a reaction depends on slow step. Hence , the E1 mechanism  follow fisrt order kenetics.

                    Rate of reaction = k[ t-butyl chloride]

E1 reaction mechanism:

The energy profile diagram of E1 mechanism  are shown below.

 E2 reaction mechanism:

The E2 elimination is parallel to SN2 mechanism. E2 elimination take place in a single concerted step in which two sigma bonds break and a pi bond forms simultaneously with the simultaneous departure of two groups, one of which is usually a proton. 

An E2 reaction require an added base which pulls out the proton.

In E2 elimination reaction the proton and leaving group are eliminated from opposite side. So it is called trans elimination.

The E2 mechanism follow second order kinetics.

Thus , the rate of  elimination reaction = k[substrate][base] .

An example of E2 elimination reaction is the action of ethanolic KOH on isopropyl bromide.

The energy profile diagram for E2 mechanism are shown below. 

Summary :

What is elimination reaction ? 
What is 1,2 elimination or β-elimination reaction ?
What is α-elimination reaction ? 
What isㄚ-elimination reaction ?

Click here: E1CB mechanism, alpha and gama elimination.