Several theoretical approachs could not define clearly the strong local attacks between the electrophilic and nucleophilic reagent when the local attack was predicted in the various methodologies, the various solvents and by the various substituents of reagents. For this, we have proposed a new descriptor, named ΔZ. Indeed, when ΔZ difference is very weaker, the local interaction is very strong and vice versa. This descriptor serves to select the best methodology, the best solvent or the possible geometries of reagents. This descriptor has been succefully evaluated.