E1 and E2 reactions

E1 and E2 reactions

Jul 25, 2022, 16:45 IST

Unimolecular Elimination E1 Reaction

In unimolecular elimination, leaving group is first expelled in slow (rate determining) step followed by abstraction of a proton. Some important characteristics of E1 mechanism are:

(i) Better the leaving group, faster will be rate of E1 reaction because leaving group is expelled in rate determining step.

R―I > R―Br > R―Cl (E1 reactivity)

(ii) As the product of rate determining step is carbocation, we can interpret

that more the stability of carbocation, more will be rate of E1 reaction. 3o R―X>2o R―X>1o R―X

3o R―OH > 2o R―OH > 1o R―OH

Iii. When solvent polarity is increased, increase in solvation of neutral reactant is almost unaffected, solvation of partially charged transition state increases slightly but the solvation of charged intermediate increases considerably (as shown in energy profile of rate determining step). Therefore, increase in solvent polarity decreases the activation energy and increases the rate of E1 reaction. Hence, E1 reaction is highly favoured in polar solvent.

(iv) As a carbocation (containing six valence electrons and a vacant orbital) is formed, rearrangements are possible before the loss of H+.

(v) As proton has to be removed from adjacent atom of a carbocation (which is an easy process), a weak base like H2O, ROH etc. can also act as base. A strong base is not required for E1 reaction. Moreover, increase in basic strength of the base or its concentration will not increase the rate of E1 reaction.

(vi) As carbocation is planar, removal of H+ from both the sides of plane is equally favoured. Therefore, E1 reaction can be Anti Elimination (removal of two groups from opposite sides) or Syn Elimination (removal of two groups from same sides).

(vii)If removal of a proton from two different adjacent atoms is possible, removal of proton occurs almost exclusively from more electronegative atom. If adjacent atoms are same, more stable (more substituted alkene) product is major product.

Bimolecular Elimination E2 Reactions

In bimolecular elimination, both atoms or groups from adjacent atoms are removed simultaneously in a single step. Some important characteristics of E2 mechanism are:

(i) Better the leaving group, faster will be rate of E2 reaction because

leaving group is expelled in rate determining step.

R―I > R―Br > R―Cl

(ii) As the product of rate determining step in E2 reaction is alkene, more the stability of alkene, more will be rate of E2 reaction.

3o R―X>2o R―X>1o R―X

Iii. When solvent polarity is increased, increase in solvation of negatively

charged reactant and negatively charged products is considerable, while solvation of partially charged transition state is slight (as shown below in energy profile of rate determining step). Therefore, increase in solvent polarity increases the activation energy and decreases the rate of E2 reaction. Hence, E2 reaction is less favoured in polar solvent.

(iv) No rearrangements are observed during E2 elimination.

(v) As proton has to be removed from a carbon of neutral species (a difficult job), a strong base is required for E2 reaction . Moreover, increase in the basic strength or its concentration will increase rate of E2 elimination as base is involved in r.d.s.

(vi) E2 elimination is preferably ‘anti’ although ‘syn’ elimination may also take place if anti configuration is not possible.

Find below detail reaction Mechanims of E-1 & E-2 Reaction