The linear free-energy relation applied to the reactions between nucleophiles and certain large and relatively stable organic cations, e.g. arenediazonium, triarylmethyl and aryltropylium cations in various solvents. is the rate constant for reaction of a given cation with a given nucleophilic system (i.e. given nucleophile in a given solvent). is the rate constant for the same cation with water in water, and 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 , characteristic of the substrate (cf. the in the Swain–Scott equation), even though values of 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.