 Elimination, bimolecular reactions are one step, concerted reaction where both base and substrate participate in the rate limiting step.In an E2 mechanism, a base takes a proton near the leaving group, forcing the electrons down to make a double bond, and forcing off the leaving group-all in one concerted step.The rate law depends on the first order concentration of two reactants, making it a 2nd order (bimolecular) elimination reaction.