The reaction involves a one-step mechanism where carbon-hydrogen and a carbon-halogen bond break to form a double bond. The rate of the E2 reaction depends on both substrate and base, since the rate-determining step is bimolecular. A strong base is generally required, one that will allow for displacement of a polar leaving group. The hydrogen that is to be removed must be opposing to that of the leaving group. The pair of electrons from the breaking C-H bond donate into the antibonding orbital of the C-(leaving group) bond, leading to its loss as a leaving group.