If bond breaking and bond formation is simultaneous, the reaction takes place in a single step and the same is the rate determining step.

Since both alkyl halide and nucleophile(base) are involved in this step it is bimolecular and of second order. Hence it is termed Substitution Nucleophilic Bimolecular ( S_N2).

In this process there is no intermediate involved. The reaction proceeds through a transition state wherein both the nucleophile and the leaving group are loosely bonded to the central carbon.

The rate of the reaction depends on both the alkyl halide and the base.