Inchemistrythereactivity–selectivity principleorRSPstates that a more reactivechemical compoundorreactive intermediateis less selective in chemical reactions. In this context selectivity represents the ratio ofreaction rates.

This principle was generally accepted until the 1970s when too many exceptions started to appear. The principle is now considered obsolete.[1]

A classic example of perceived RSP found in older organic textbooks concerns thefree radical halogenationof simplealkanes. Whereas the relatively unreactivebrominereacts with 2-methylbutane predominantly to 2-bromo-2-methylbutane, the reaction with much more reactivechlorineresults in a mixture of all fourregioisomers.

Another example of RSP can be found in the selectivity of the reaction of certaincarbocationswithazidesandwater. The very stable triphenylmethyl carbocation derived fromsolvolysisof the correspondingtriphenylmethyl chloridereacts 100 times faster with the azide anion than with water. When the carbocation is the very reactive tertiaryadamantanecarbocation (as judged from diminishedrateof solvolysis) this difference is only a factor of 10.

Constant or inverse relationships are just as frequent. For example a group of 3- and 4-substitutedpyridinesin their reactivity quantified by theirpKashow the same selectivity in their reactions with a group of alkylating reagents.

The reason for the early success of RSP was that the experiments involved very reactive intermediates with reactivities close tokinetic diffusion controland as a result the more reactive intermediate appeared to react slower with the faster substrate.

General relationships between reactivity and selectivity in chemical reactions can successfully explained byHammond's postulate.[Tetrazole-Derived Thiyl radical]When reactivity-selectivity relationships do exist they signify different reaction modes. In one study[2][3]the reactivity of two differentfree radicalspecies (A, sulfur, B carbon) towards addition to simplealkenessuch asacrylonitrile,vinyl acetateandacrylamidewas examined.

The sulfur radical was found to be more reactive (6*108vs. 1*107mole−1.s−1) and less selective (selectivity ratio 76 vs 1200) than the carbon radical. In this case the effect can be explained by extending theBell–Evans–Polanyi principlewith a factor [\delta \,] accounting for transfer of charge from the reactants to thetransition stateof the reaction which can be calculatedin silico:

[E_a = E_o + \alpha \Delta H_r + \beta \delta^2\,]

with [E_a\,] theactivation energyand [\Delta H_r\,] the reactionenthalpychange. With theelectrophilicsulfur 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 thenucleophiliccarbon radical on the other hand both enthalpy and polar effects have the same direction thus extending the activation energy range.