- Reactivity–selectivity principle
In
chemistry the reactivity–selectivity principle or RSP states that a more reactivechemical compound orreactive intermediate is less selective in chemical reactions. In this context selectivity represents the ratio ofreaction rate s.This principle was generally accepted until the 1970s when too many exceptions started to appear. The principle is now considered obsolete ["Minireview The Reactivity-Selectivity Principle: An Imperishable Myth in Organic Chemistry " Herbert Mayr, Armin R. Ofial
Angewandte Chemie International Edition Volume 45, Issue 12 , Pages 1844 - 1854 [http://dx.doi.org/10.1002/anie.200503273 Abstract] ] .A classic example of perceived RSP found in older organic textbooks concerns the
free radical halogenation of simplealkane s. Whereas the relatively unreactivebromine reacts with 2-methylbutane predominantly to 2-bromo-2-methylbutane, the reaction with much more reactivechlorine results in a mixture of all fourregioisomer s.Another example of RSP can be found in the selectivity of the reaction of certain
carbocation s withazide s andwater . The very stable triphenylmethyl carbocation derived fromsolvolysis of the corresponding triphenylmethylchloride reacts a 100 times faster with the azide anion than with water. When the carbocation is the very reactive tertiaryadamantane carbocation (as judged from diminished rate of 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-substituted
pyridine s in their reactivity quantified by theirpKa show 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 to
kinetic diffusion control and 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 by the
Hammond postulate .was examined.
The sulfur radical was found to be more reactive (6*108 vs. 1*107 mole-1.s-1) and less selective (selectivity ratio's 76 vs 1200) than the carbon radical. In this case the effect can be explained by extending the
Bell–Evans–Polanyi principle with a factor accounting for transfer of charge from the reactants to thetransition state of the reaction which can be calculatedin silico :with the
activation energy and the reactionenthalpy change. 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.External links
* Reactivity–selectivity principle
Gold Book [http://www.iupac.org/goldbook/R05186.pdf Link]References
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