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 Sf (expressing discrimination between p- and m-positions in PhX) is defined as
Sf = lg (fpX/fmX).
STOCK and BROWN (1963).
