Reactivity–selectivity principle

[1] A classic example of perceived RSP found in older organic chemistry textbooks concerns the free radical halogenation of simple alkanes.

Whereas the relatively unreactive bromine reacts with 2-methylbutane predominantly to 2-bromo-2-methylbutane, the reaction with much more reactive chlorine results in a mixture of all four regioisomers.

General relationships between reactivity and selectivity in chemical reactions can successfully be explained by Hammond's postulate.

In one study [2] the reactivity of two different free radical species (A, sulfur, B carbon) towards addition to simple alkenes such as acrylonitrile, vinyl acetate and acrylamide was examined.

accounting for transfer of charge from the reactants to the transition state of the reaction which can be calculated in silico:

With the electrophilic sulfur radical the charge transfer is largest with electron-rich alkenes such as acrylonitrile but the resulting reduction in activation energy (β is negative) is offset by a reduced enthalpy.

With the nucleophilic carbon radical on the other hand both enthalpy and polar effects have the same direction thus extending the activation energy range.