The present work is focused on studying chlorine and fluorine identity SN2 substitutions on a methyl center, within the framework of the newly introduced reaction electronic flux J(xi), that allows one to identify charge transfer and polarization mechanisms that take place along the reaction coordinate. The main results concern the discovery of different charge transfer mechanism, despite both reactions have the same energetic pattern with simultaneous bond breaking and formation. It turns out that the chlorine substitution is mainly driven by polarization effects and characterized by through bond interactions while intermolecular charge transfer dominates the fluorine exchange reaction, that is characterized by through space interactions.