Elimination reactions/ types/ factors affecting elimination reactions

                                   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 S_N1 and S_N2. 

E1 mechanism

       This mechanism consists of two steps. It closely resembles S_N1.`                      

1. Ø In the 1^st step ionization of substrate take place results in the formation of a carbocation.
2. Ø In the 2^nd step, carbocation rapidly loses a proton from β-carbon to a base, usually, this base is a solvent.

E1 mechanism

1. 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 S_N1 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 S_N2 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 1^st 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 2^nd  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 (1^st 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.