Final answer:
The elimination reaction with a unimolecular rate law determination is the E1 reaction. This reaction involves the formation of a carbocation in the rate-determining step, which competes with the SN1 mechanism that shares a similar step. E1 reactions contrast with E2 and SN2 mechanisms, which are bimolecular.
Step-by-step explanation:
The elimination reaction with a unimolecular rate law determination is known as an E1 reaction, where 'E' stands for elimination and '1' represents the unimolecular aspect of the mechanism. In an E1 reaction, the rate-determining step is the dissociation of the leaving group to form a carbocation. This is a unimolecular process as it involves only one molecular entity in the rate-determining step. E1 always competes with SN1, since both reactions share a common first step involving the formation of a carbocation intermediate.
In comparison, E2 reactions are bimolecular, involving both the substrate and a base in the rate-determining step. SN2 reactions are also bimolecular, including the interaction between the nucleophile and the substrate. For SN1 reactions, like E1, the rate-determining step involves the formation of a carbocation from the substrate. Ultimately, whether a reaction follows E1, E2, SN1, or SN2 mechanisms depends on factors including the substrate structure, the nature of the nucleophile/base, and the solvent system.