Unimolecular elimination, which is abbreviated E1, is a two-step process proceeding through a carbocation intermediate.
The rate of reaction is dependent on the concentration of only one species, namely the substrate.
In the first step, the leaving group departs, producing a carbocation. In the second step, a proton is removed by a base.
E1 is favored by the same factors that favor SN1 : highly polar solvents, highly branched carbon chains, good leaving groups, and weak nucleophiles in low concentration. These mechanisms are therefore competitive, and directing a reaction toward either E1 or SN1 alone is difficult, although high temperatures tend to favor E1.