E1 Reaction – two step mechanism where: the leaving group is lost 1st, followed by the formations of a carbocation intermediate which is then deprotonated. E2 Reaction – one step, concerted mechanism where bonds are broken and formed simultaneously.
The rate of E1 and E2 reactions increases with greater carbocation stability: 3°>2°>>1° (1° can only undergo E1) where E1 follows a unimolecular rate law while E2 is bimolecular. E2 reactions require a strong base to proceed, while E1 do not. Aldol condensation is an example of a commonly tested E2 reaction.