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| In [[chemistry]] the '''reactivity–selectivity principle''' or '''RSP''' states that a more reactive [[chemical compound]] or [[reactive intermediate]] is less selective in chemical reactions. In this context selectivity represents the ratio of [[reaction rate]]s.
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| This principle was generally accepted until the 1970s when too many exceptions started to appear. The principle is now considered obsolete.<ref>''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]</ref>
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| A classic example of perceived RSP found in older organic textbooks concerns the [[free radical halogenation]] of simple [[alkane]]s. 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 [[regioisomer]]s.
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| Another example of RSP can be found in the selectivity of the reaction of certain [[carbocation]]s with [[azide]]s and [[water]]. The very stable triphenylmethyl carbocation derived from [[solvolysis]] of the corresponding [[triphenylmethyl chloride]] reacts 100 times faster with the azide anion than with water. When the carbocation is the very reactive tertiary [[adamantane]] carbocation (as judged from diminished [[reaction rate|rate]] of solvolysis) this difference is only a factor of 10.
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| Constant or inverse relationships are just as frequent. For example a group of 3- and 4-substituted [[pyridine]]s in their reactivity quantified by their [[pKa]] show the same selectivity in their reactions with a group of alkylating reagents.
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| 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.
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| General relationships between reactivity and selectivity in chemical reactions can successfully explained by [[Hammond's postulate]].
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| [[Image:ReactivitySelectivity.png|right|300px|Tetrazole-Derived Thiyl radical]]When reactivity-selectivity relationships do exist they signify different reaction modes. In one study <ref>''Search for High Reactivity and Low Selectivity of Radicals toward Double Bonds: The Case of a Tetrazole-Derived Thiyl Radical'' Jacques Lalevée, Xavier Allonas, and Jean Pierre Fouassier [[J. Org. Chem.]]; '''2006'''; 71(26) pp 9723 - 9727; (Article) {{DOI|10.1021/jo061793w}}</ref><ref>Sulfur [[tetrazole]] radical derived from [[photolysis]] of [[disulfide]] and carbon radical derived from photolysis of [[t-butylperoxide]] followed by proton abstraction from [[triethylamine]]</ref> the reactivity of two different [[free radical]] species (A, sulfur, B carbon) towards addition to simple [[alkene]]s such as [[acrylonitrile]], [[vinyl acetate]] and [[acrylamide]] was examined.
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| The sulfur radical was found to be more reactive (6*10<sup>8</sup> vs. 1*10<sup>7</sub> mole<sup>−1</sup>.s<sup>−1</sup>) and less selective (selectivity ratio 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 <math>\delta \,</math> accounting for transfer of charge from the reactants to the [[transition state]] of the reaction which can be calculated [[in silico]]:
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| <math>E_a = E_o + \alpha \Delta H_r + \beta \delta^2\,</math>
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| with <math>E_a\,</math> the [[activation energy]] and <math>\Delta H_r\,</math> the reaction [[enthalpy]] change. With the [[electrophile|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 [[nucleophile|nucleophilic]] carbon radical on the other hand both enthalpy and polar effects have the same direction thus extending the activation energy range.
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| ==External links==
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| * Reactivity–selectivity principle [[Gold Book]] [http://www.iupac.org/goldbook/R05186.pdf Link]
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| ==References==
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| {{Reflist}}
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| {{DEFAULTSORT:Reactivity-selectivity principle}}
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| [[Category:Obsolete scientific theories]]
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| [[Category:Physical chemistry]]
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