The reactivity–selectivity principle: redefinition and reexamination on an experimental basis

Abstract

The Reactivity-Selectivity Principle (RSP) has been redefined with more precision for several cases, all restricted to rate constants but with different variable factors This principle is formulated in terms of simple mathematical expressions, involving only rate constants and avoiding any σ constants, equilibrium constants or other parameters. With a given reaction series four types of behaviour are possible: (i) a valid RSP when the selectivity decreases with reactivity; (ii) anti-RSP when the reverse is true; (iii) indifferent behaviour when the change in selectivity is negligible; (iv) a cross-over when RSP is valid in one part of the series and invalid in the other. All four cases have been observed in the available experimental data. A statistical examination has been carried out on two sets, each containing 100 reaction series. In a set concerning the original narrow definition -two reagents reacting with one series of similar substrates – RSP is evidently an invalid rule: the numbers of positive and negative cases is approximately equal. In the second set, where temperature is the variable factor, RSP holds but not without exceptions (five convincingly documented exceptions from 100 cases): generally a reagent is less selective at a higher temperature. Some relations to the Hammond postulate are mentioned.