The Coexistence of the Reactivity–Selectivity Principle and of Linear Free Energy Relationships: A Diffusion Clock for Determining Carbocation Reactivities

Electrophilicity parameters of reactive carbocations and the rate constants of the reactions of such short-lived species with uncharged nucleophiles can be determined simply by competition experiments [Eq. (a); R = Ar1Ar2CH]. The only prerequisite for this is that suitable pairs of nucleophiles must be chosen so that one of the parallel reactions is diffusion controlled.