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.

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-