The removal of two atoms or groups one generally hydrogen `(H^(+))` and the other a leaving group `(L^(-))` resulting in the formation of unsaturated compound is known as elimination reaction.
In `E_(1)` (elimination) reactions the C-L bond is broken heterolytically (in step 1) to form a carbocation (as in`S_(N^(1))` reaction) in which `(L^(-))` is lost (rate determining step). The carbocation (in step 2) loses a proton from the `beta-` carbon atom by a base (nucleophile) to form an alkene. `E_(1)` reaction is favoured in compounds in which the leaving group is at secondar `(2^(@))` or tertiary`(3^(@))` Position.
In `E_(2)` (elimination) reactions two sigma bonds are broken and a `pi-`bond is formed simultaneously.`E_(2)` reactions occur in one step through a transition state.
`E_(2)` reactions are most common in haloalkanes (particulary `1^(@)`) and better the leaving group higher is the `E_(2)` reaction. In `E_(2)` reactions, both the leaving groups should be antiplaner.
`E_(1) cb` (Elimination unimolecular conjugate base) reaction involves the removal of proton by a conjugate base (step 1) to produce carbanion which loses a leaving group to form an alkene (step 2) and is a slow step Neopentyl bromide undergoes dehydrohalogenation to give alkene even though it has no `beta-`hydrogen.This is due to :
A. `E_(2)`mechanism
B. `E_(1)` mechanism
C. due to rearrangement of carbocation by `E_(1)` mechanism
D. `E_(1)` cb mechanism
