Swain–Scott equation

The linear free-energy relation of the form: \[\log _{10}(\frac{k}{k_{0}})=s\ n\] applied to the variation of reactivity of a given electrophilic substrate towards a series of nucleophilic reagents. n is characteristic of the reagent (i.e. a measure of its nucleophilicity) and s is characteristic of the substrate (i.e. a measure of its sensitivity to the nucleophilicity of the reagent). A scale of n values is based on the rate coefficients k for the reaction of methyl bromide with nucleophiles in water at 25°C, s being defined as 1.00 for these reactions and n being defined as 0.00 for the hydrolysis of methyl bromide. (Other scales have been devised.)
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1169 (https://doi.org/10.1351/pac199466051077)