Final answer:
An elimination addition reaction goes through an intermediate, which varies depending on the mechanism: E1 involves a carbocation intermediate, E2 is bimolecular involving the base and substrate, and E1cB involves a stabilized carbanion intermediate.
Step-by-step explanation:
An elimination addition reaction typically occurs via an unstable intermediate that forms during the reaction process. For example, in an E1 elimination reaction, a carbocation intermediate is formed when the leaving group, such as a halide, dissociates. This mechanism is unimolecular, involving the rearrangement within a single molecule to form the carbocation. Subsequently, a base removes a proton from a carbon adjacent to the carbocation, leading to the formation of an alkene.
In the case of an E2 reaction or elimination bimolecular, it is a bimolecular mechanism involving the substrate and a base. The base deprotonates a hydrogen from a carbon adjacent to the carbon bearing the leaving group, which simultaneously leaves, forming an alkene. This reaction step involves two species and is therefore bimolecular.
Lastly, an E1cB mechanism involves first deprotonating the substrate to form a carbanion intermediate, which then leads to the loss of the leaving group. This type of mechanism is often preferred when the carbanion intermediate is stabilized by resonance.