The elimination reactions mainly involve three mechanism E1,E2 and E1cB. If the leaving group departs before `beta`-proton (`H^t=Theta` ion) then it is E1 mechanism. If proton is taken off first before leaving group it is E1cB mechanism.The pure E2 involves both `beta`-Hydrogen and leaving group departing simutaneously. If acidity of `beta`-Hydrogen increases and leaving group ability decreases then E1cB mechanism increases.
Which of the following substrate will undergo fastest reaction through E1cB route