These mechanisms depend on the structure of the reactant and nucleophile, the solvent, temperature.

Let's consider the transition state in the reaction between iodomethane and chloride ion.

By applying Newton's second law for rotational systems, the equation of motion for the pendulum is obtained.

In the substitution reaction, the nucleophile Cl- attacks the reactant iodomethane and forms product chloromethane and leaves I- as leaving group. The intermediary state where nucleophile Cl- forms a bond and I- breaks the bond with the iodomethane simultaneously is known as the transition state. This mechanism is known as SN2 type, which is stereospecific, i.e., it gives a product inverse in configuration with the reactant. Usually, aprotic solvents like acetone, DMSO, DMF, etc., favor this reaction.