E2 and E1 are two different mechanisms of elimination reactions in organic chemistry. E2 (Elimination Bimolecular) refers to the elimination reaction in which a substrate reacts with a base to produce a double bond and a leaving group. The reaction occurs in a single step and is facilitated by the presence of a strong, bulky base. The reaction rate depends on the concentration of both the substrate and the base. E1 (Elimination Unimolecular) refers to the elimination reaction in which the substrate first undergoes a change in stability, such as the loss of a leaving group, and then the remaining molecule rearranges to form a double bond. Unlike the E2 reaction, the reaction rate depends only on the concentration of the substrate, not the base. The major difference between the two elimination mechanisms is the rate-determining step and the type of transition state involved. In an E2 reaction, the rate-determining step is the formation of the transition state, which is characterized by