Elimination reactions
Elimination
When a loss of two groups occurs from the adjacent carbon atoms and results in the formation of multiple bonds. This process is known as elimination. When elimination occurs from the two adjacent carbon atoms then this is also known as β-elimination or 1,2-elimination.
β-elimination |
Sometimes elimination occurs from the same carbon atoms. Two groups remove from the same carbon atom then this elimination is known as 1,1-elimination or α-elimination.
α-elimination |
In
elimination one group leaves along with bonding electrons are known as leaving group, other group leaves
without electrons. Most of the time it is hydrogen. It has two types.
·
E1
mechanism
·
E2
mechanism
These reactions are closely related to SN1 and SN2.
E1 mechanism
This mechanism consists of two steps. It closely resembles SN1.`
- Ø
In
the 1st step ionization of substrate take place results in the
formation of a carbocation.
- Ø
In
the 2nd step, carbocation rapidly loses a proton from β-carbon to a
base, usually, this base is a solvent.
- The first step is the same for both mechanisms, so the rate of both mechanisms is the same. It is the second step where both the reactions are different from one another. In E1 solvent pulls the proton from the carbocation and in SN1 solvent substitutes the positively charged carbon atom.
Kinetics
of E1 mechanism
In the rate-determining step, only one molecule (substrate) is involved so the rate depends upon the concentration of substrate only. It is a first-order reaction.
Kinetics of E1 mechanism |
E2
mechanism
This mechanism is a bimolecular reaction. It occurs in
one step only. It resembles with SN2
mechanism. In this mechanism, attacks base on the proton and departure of leaving
group occur at the same time. A transition form exists in which carbon attaches
to five groups. Base attack on proton from one side and at a time leaving
group leave the substrate from the other side.
E2 mechanism |
This mechanism involves only one step so this step is
known as the rate-determining step.
Kinetics
of E2 mechanism
Two molecules are involved in rate-determining steps so
rate of this mechanism depends upon the concentration of substrate and base both.
So this reaction is a second-order reaction.E2 mechanism
Factors
affecting the eliminations reaction
There are many factors which affect these mechanisms
which are as follow;
Effect
of substrate
As I already discussed, in both the mechanism the rate
of the reaction depends on the concentration of substrate. Substrate structure
is very important in this regard. E1 mechanism consist of two steps. In the 1st
step carbocation is formed. E1 mechanism
are favorable for tertiary substrate as compared to secondary and primary
substrate.
Tertiary
< < secondary < primary < methyl
E2 mechanism occurs in one step so, the substrate
which has less hindrance is suitable for this mechanism because in this mechanism
the attacking of base and departure of leaving group occur at a time. E2
mechanism is more favorable for primary substrate as compared to tertiary and
secondary substrate.
Effect
of base
Base does not effect on the rate of E1 mechanism
because it is not the part of rate determining step. It attacks on the 2nd
step. It effect on the rate of E2
mechanism because it is the part of rate determining step. Those molecules
which are good electron donors are better bases. Most of the time solvent act
as base in elimination reactions.
Effect
of leaving group
In both the mechanism, leaving groups leave along with
their electron pair so those leaving groups which easily accommodate the
electron pair after separating from the substrate are considered as good
leaving group. Conjugate bases of strong bases are good leaving groups. Those
leaving which have ability to delocalize the electron pair are also considered
as good leaving group. PhO-
is better leaving group as compared to RO- because
delocalization occur in phenoxide ion. Good leaving group favors E1 mechanism
because here ionization becomes easier and a poor leaving group favors E2
mechanism.
Effect
of solvent
Both the mechanism occur in solution form so reaction
medium also play important role in the rate of mechanism. E1 mechanism is more
favorable in polar solvents because it involves an ionic intermediate and polar
solvents are more helpful in the ionization (1st step). E2 mechanism
favorable in nonpolar solvents.
Effect
of temperature
In all the reactions, there is a competition between
substitution and elimination reaction. In elimination reaction many new bonds
are formed so it need high activation energy. So when temperature is high
elimination will favors more than substitution, whether the reaction is
bimolecular or unimolecular.
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