partial rate factor
The rate of substitution at one specific site in an aromatic compound relative to the rate of substitution at one position in benzene. For example, the partial rate factor fpZ for para-substitution in a monosubstituted benzene C6H5Z is related to the rate constants k(C6H5Z) and k(C6H6) for the total reaction (i.e. at all positions) of C6H5Z and benzene, respectively, and % para (the percentage para-substitution in the total product formed from C6H5Z) by the relation: \[f_{p}^{\text{Z}} = \frac{6\ k\left(\text{C}_{6}\text{H}_{5}\text{Z}\right)}{k\left(\text{C}_{6}\text{H}_{6}\right)}\ \frac{\%\ \text{para}}{100}\] Similarly for meta-substitution: \[f_{m}^{\text{Z}} = \frac{6\ k\left(\text{C}_{6}\text{H}_{5}\text{Z}\right)}{2\ k\left(\text{C}_{6}\text{H}_{6}\right)}\ \frac{\%\ \text{meta}}{100}\] (The symbols p f Z, m f Z, o f Z are also in use.) The term applies equally to the ipso position, and it can be extended to other substituted substrates undergoing parallel reactions at different sites with the same reagent according to the same rate law.
See also: selectivity (of a reagent)
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1149 (