Final answer:
E2 reactions occur in one step and are bimolecular, favored by steric hindrance and strong bases, whereas E1 reactions occur in two steps with the first being formation of a carbocation intermediate. The choice between E2 and E1 depends on the substrate and reaction conditions.
Step-by-step explanation:
E2 reactions occur in one step, making them bimolecular where the base and substrate interact in a single concerted step. This is consistent with a situation where a stable ionic intermediate is not formed, and factors like steric hindrance and strong bases favor E2 over S2 mechanisms.
On the other hand, E1 reactions occur in two steps. Initially, the leaving group departs, forming a stable carbocation intermediate; the second step involves the deprotonation to form the double bond. As tertiary substrates tend to react by E1 mechanism readily, E1 competes with the SN1 mechanism as both share the same rate-determining step.
The distinction between these mechanisms can be critical, especially when considering the rate laws and substrate types that dictate whether SN2, E2, or both will happen, with polar protic solvents generally being unsuitable for SN2 and E2 reactions.