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: $$S_f={\log }_{10}(\frac{f_p^{\text{X}}}{f_m^{\text{X}}})$$