Elimination Reactions

As the name indicates, small molecule eliminates during these types of reactions.  This small molecule is generally water (-H2O) in the case of dehydration or HX in the case of dehydrohalogenation.  A 1, 2-elimination suggests that the atoms that are eliminated

come from adjacent or neighbor carbon atoms. In the organic chemistry, Elimination reactions are the important method for alkenes synthesis.

Following two are the widely described elimination reactions in organic chemistry:

  1. Dehydration: Elimination of water i.e.-H2O from the Alcohol.
  2. Dehydrohalogenation: Elimination of HX from alkyl halides. ( where X = Cl, F, Br or I)

Let’s see what happens during elimination process. There are three steps through which these reactions proceeds –

  1. Removal of a hydrogen or proton.
  2. Formation of the new Carbon-Carbon double bond i.e. pi bond.
  3. Breaking of Carbon -leaving group bond.

Now depending on facts, which step happen first, these reactions can be classified into two categories-

  1. Removal of leaving the group to form a carbocation, then removal of a proton and formation of the C=C bond. Reactions proceeds via this pathway are referred as E1 reactions.
  2. Another pathway, simultaneous proton removal and loss of the leaving group to form C=C bond. These types of reactions are referred as E2 reactions.

Now we will see the difference between these two mechanisms-

E1 mechanism

E1 means elimination and reaction.

Following two events happens during E1 reactions (shown in figure)-

E2 mechanism

E2 means elimination, bimolecular reaction.

This is a concerted pathway with the following mechanism (Shown in figure)-

So the factors which decide the pathway of reaction, means whether it would follow E1 pathway or E2 pathway are:

  1. Stability of Carbocation.
  2. Nature of leaving the group.
  3. Base: Strong or weak base.

On this basis, we can now differentiate E1 and E2 reactions-

E1 E2
Unimolecular/Bimolecular Unimolecular Bimolecular
Rate depends on The concentration of Substrate. The concentration of Substrate and concentration of Base as well.
Base Strong base not needed. Strong base needed
Stereochemistry requirement Not needed. Leaving group should be anti to hydrogen that has to remove.
Steps Two step reactions One step reaction.
Carbocation Should form a stable carbocation. Not needed.