How to explain an SN2 reaction?

Three methods which explain a bimolecular nucleophilic substitution reaction are discussed: (i) A traditional method using curved arrows provides a facile means for depicting how valence electrons move and are redistributed during the reaction. (ii) A method based on the orbital interaction theory deals with two independent orbitals of the two isolated reactants, one occupied and the other unoccupied. The energy gained by their stabilizing interaction is considered to assist the two reactants in surmounting the energy barrier of the substitution reaction. (iii) A method inspecting a set of molecular orbitals of a composite molecule that consists of the two reactants can describe any point on the reaction path. According to the last method, the reactant state, the intermediate states including the transition state, and the product state can be correlated with continuity. Although the methods (ii) and (iii) both deal with molecular orbitals, what they illuminate are different. Whereas the method (ii) based on the orbital interaction theory tractably predicts a driving force which arises only near the reactant state from the static viewpoint, the whole entire course of the reaction ought to be described by the method (iii), with which the ever-changing behaviors of the molecular orbitals are trailed along the reaction path. The present example on the simplest reaction course sounds a caution that chemists cannot be too careful about giving an interpretation based on the simple orbital interaction theory to the results obtained by calculations on a composite molecule with the whole reaction path being viewed.