REACTIVITY.We address the concept of steric effect in the pursuit of establishing quantitative correlations between reactivity of molecules and their molecular shape.To this end, a set of series of S N 2 reactions were modeled to compute the energy barrier height and the molecular volume, atomic charges and dipole moment of molecules.We introduced the Kappa index to characterize the shape of molecules.Quantitative correlations between the change in the energy barrier with properties of molecules and Kappa index were presented and discussed.