selectivity factor, S f

A quantitative representation of selectivity in aromaticsubstitution 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 ( f p X f m X )
PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1162