ELIMINATION REACTION- E1 MECHANISM; E1cB MECHANISM (ORGANIC CHEMISTRY)

E1- Mechanism:

This elimination takes place (without the participation of a base) in 2 steps,  unimolecular ionization,  being rate-determining step. 

The main feature of this mechanism is that under the influence of solvation forces the electron attracting group (leaving group) breaks away along with the bonding electrons. The resulting Carbocation subsequently loses a proton to the solvent or to some other proton acceptor.  

The reaction has 2 stages,  of which the first is the rate determining step and as a result, the reaction rate depends only on the concentration of the first reactant. In E1 reactions,  a Proton is eliminated from the carbon adjacent to the positive, electron deficient carbon and the pair of electrons formerly shared by this hydrogen is available for the formation of a π-Bond. 

The Carbocations are planar species and their formation at the bridgehead position will be a difficult process.  Thus, bicyclic structures prevent the bridgehead carbon becoming planar even though the cation would be tertiary.  It is not formed due to very high energy. Such compounds will not undergo E1 reactions. 

The direction of elimination: 

2-Bromo-2-methylbutane  on reaction with water in ethanol gives substitution as the major product when both water and ethanol can act as a nucleophile to give an alcohol or an ether.  The major alkene formed is more highly substituted. 

 Thus, from among alkyl halides with the same alkyl group the alkyl fluorides display the least reactivity in E1 reaction.

                               RF < RCl < RBr < RI

E1 elimination from cyclic compound: 

Since a carbon is formed in the first step of the E1 reaction, the relative stereochemistry of the leaving groups  (anti coplanarity)  is not important. When menthyl chloride undergoes E2 reaction only one alkene is formed in hundred percent yield due to the need for the departed group to attend diaxial positions. When menthyl chloride is subjected to E1 reaction conditions 2 alkenes are formed, the major product is in accord with the saytzeff rule. 

Curtin-Hammett principle:

That Curtin Hammett principle applies to a conformationally heterogeneous reactant where the products must be non-equilibrating.  The Curtin Hammett principle implies that in a chemical reaction which gives one product from one conformer and a different product from another conformer.  The product composition is not determined by the relative population of the ground state conformer but largely depends on the relative energies of the corresponding transition state involved.

E1cB Mechanism:

In the E1cB mechanism,  the base rapidly removes the proton from the β carbon resulting in the formation of carbanion, which loses the leaving group in the rate-determining step.  Since the conditions of base catalysed elimination reaction does not allow the formation of an unstabilized carbanion,  it is reasonable to presume that if formed,  they must be either rapidly reconverted to the substrate is converted to the alkene. In this mechanism, the overall rate is limited to that of the slower state 2nd stage,  which depends only on the concentration of the conjugate base of the reactant.  This mechanism is called as E1cB as the leaving group is lost from the conjugate base of the starting material and the reaction is Unimolecular. 

The first step of is reversible, and hence, when the reaction is carried out in C₂H₅OD instead of C₂H₅OH, the intermediate carbanion should pick up deuterium.

If the E1cB mechanism is correct,  we recover  2-phenyl-ethyl bromide after a partial transformation to styrene.  On the other hand, there should be no incorporation of deuterium if the E2 mechanism is operative.  Actual experiments have shown that there is no deuterium incorporation and hence the E1cB mechanism does not operate in this case.  

However, this mechanism does operate under special circumstances.  1,1,1-trifluoro-2,2-dichloroethane (3), for instance, undergoes a base-catalyzed exchange of β hydrogen atom with the solvent deuterium faster than dehydrofluorination. 

A strong carbon-fluorine bond (and the consequent poor leaving ability of fluoride ion) coupled with the electron-withdrawing effect of halogens explains the formation of carbanion before elimination.

The removal of a proton and loss of the leaving group occurs simultaneously in E2 mechanism whereas removal of a Proton is the first step in the E1cB reaction.  In the subsequent rate-determining step of the E1cB reaction, the leaving group departs from the conjugate base of the substrate.