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 $\ce{PhX}$ relative to that of a single position in benzene, then the selectivity factor \(S_{f}\) (expressing discrimination between p- and m-positions in $\ce{PhX}$) is defined as: \[S_{f}=\log _{10}(\frac{f_{p}^{\rm{X}}}{f_{m}^{\rm{X}}})\]