Ritchie equation

$log 10 k N = log 10 k 0 + N +$

applied to the reactions between nucleophiles and certain large and relatively stable organic cations, e.g. arenediazonium, triarylmethyl and aryltropylium cations in various solvents. $k N$ is the rate constant for reaction of a given cation with a given nucleophilic system (i.e. given nucleophile in a given solvent). $k 0$ is the rate constant for the same cation with water in water, and $N +$ is a parameter which is characteristic of the nucleophilic system and independent of the cation. A surprising feature of the equation is the absence of a coefficient of $N +$ , characteristic of the substrate (cf. the $s$ in the Swain–Scott equation), even though values of $N +$ vary over 13 log units. The equation thus involves a gigantic breakdown of the reactivity–selectivity principle. The equation has been extended both in form and in range of application.

Source:

PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1161