In the presence of polar solvents, tertiary alkyl halides undergo reaction with bases to generate alkenes by an E1 mechanism. In the E1 reaction, the rate-limiting step is the spontaneous ionization of the alkyl halide to give the tertiary carbocation which is then deprotonated by the base in a fast, second step. In the E1 reaction, only the alkyl halide is present in the rate-limiting transition state, making the reaction unimolecular and stepwise.