A quantitative representation of selectivity in aromatic substitution reactions (usually electrophilic, for monosubstituted benzene derivatives). If the partial rate factor, f, expresses the reactivity of a specified position in the aromatic compound PhX relative to that of a single position in benzene, then the selectivity factor S f (expressing discrimination between p- and m-positions in PhX) is defined as: \[S_{f}=\log _{10}(\frac{f_{p}^{\text{X}}}{f_{m}^{\text{X}}})\]