reactivity–selectivity principle
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chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
the reactivity–selectivity principle or RSP states that a more reactive
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
or
reactive intermediate In chemistry, a reactive intermediate or an intermediate is a short-lived, high-energy, highly reactive molecule. When generated in a chemical reaction, it will quickly convert into a more stable molecule. Only in exceptional cases can these comp ...
is less selective in chemical reactions. In this context selectivity represents the ratio of
reaction rate The reaction rate or rate of reaction is the speed at which a chemical reaction takes place, defined as proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per unit ...
s. This principle was generally accepted until the 1970s when too many exceptions started to appear. The principle is now considered obsolete. A classic example of perceived RSP found in older organic chemistry textbooks concerns the
free radical halogenation In organic chemistry, free-radical halogenation is a type of halogenation. This chemical reaction is typical of alkanes and alkyl-substituted aromatics under application of UV light. The reaction is used for the industrial synthesis of chloroform ( ...
of simple
alkane In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which ...
s. Whereas the relatively unreactive
bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table (halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simila ...
reacts with 2-methylbutane predominantly to 2-bromo-2-methylbutane, the reaction with much more reactive
chlorine Chlorine is a chemical element with the Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate betwee ...
results in a mixture of all four regioisomers. Another example of RSP can be found in the selectivity of the reaction of certain
carbocation A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountere ...
s with
azide In chemistry, azide is a linear, polyatomic anion with the formula and structure . It is the conjugate base of hydrazoic acid . Organic azides are organic compounds with the formula , containing the azide functional group. The dominant applic ...
s and
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
. The very stable triphenylmethyl carbocation derived from
solvolysis In chemistry, solvolysis is a type of nucleophilic substitution (S1/S2) or elimination reaction, elimination where the nucleophile is a solvent molecule. Characteristic of S1 reactions, solvolysis of a chirality (chemistry), chiral reactant affor ...
of the corresponding
triphenylmethyl chloride Triphenylmethyl chloride or trityl chloride (TrCl) is a white solid with the chemical formula C19H15Cl. It is an alkyl halide, sometimes used to introduce the trityl protecting group. Preparation Triphenylmethyl chloride is commercially available ...
reacts 100 times faster with the azide anion than with water. When the carbocation is the very reactive tertiary
adamantane Adamantane is an organic compound with a formula C10H16 or, more descriptively, (CH)4(CH2)6. Adamantane molecules can be described as the fusion of three cyclohexane rings. The molecule is both rigid and virtually stress-free. Adamantane is the m ...
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 Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a d ...
s in their reactivity quantified by their pKa 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
Hammond's postulate Hammond's postulate (or alternatively the Hammond–Leffler postulate), is a hypothesis in physical organic chemistry which describes the geometric structure of the transition state in an organic chemical reaction A chemical reaction is a pro ...
. When reactivity-selectivity relationships do exist they signify different reaction modes. In one study ''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. ''The Journal of Organic Chemistry'', colloquially known as ''JOC'', is a peer-reviewed scientific journal for original contributions of fundamental research in all branches of theory and practice in organic and bioorganic chemistry. It is publ ...
; 2006; 71(26) pp 9723 - 9727; (Article)
the reactivity of two different
free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Ailments of unknown cause Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabo ...
species (A, sulfur, B carbon) towards addition to simple
alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
s such as
acrylonitrile Acrylonitrile is an organic compound with the formula and the structure . It is a colorless, volatile liquid although commercial samples can be yellow due to impurities. It has a pungent odor of garlic or onions. In terms of its molecular ...
,
vinyl acetate Vinyl acetate is an organic compound with the formula CH3CO2CH=CH2. This colorless liquid is the precursor to polyvinyl acetate and ethene-vinyl acetate copolymers, important industrial polymers. Production The worldwide production capacity of v ...
and
acrylamide Acrylamide (or acrylic amide) is an organic compound with the chemical formula CH2=CHC(O)NH2. It is a white odorless solid, soluble in water and several organic solvents. From the chemistry perspective, acrylamide is a vinyl-substituted primary ...
was examined. The sulfur radical was found to be more reactive (6*108 vs. 1*107 M−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 the
Bell–Evans–Polanyi principle In physical chemistry, the Evans–Polanyi principle (also referred to as the Bell–Evans–Polanyi principle, Brønsted–Evans–Polanyi principle, or Evans–Polanyi–Semenov principle) observes that the difference in activation energy between ...
with a factor \delta \, accounting for transfer of charge from the reactants to the
transition state In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked wi ...
of the reaction which can be calculated
in silico In biology and other experimental sciences, an ''in silico'' experiment is one performed on computer or via computer simulation. The phrase is pseudo-Latin for 'in silicon' (correct la, in silicio), referring to silicon in computer chips. It ...
: E_a = E_o + \alpha \Delta H_r + \beta \delta^2\, with E_a\, the
activation energy In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules pe ...
and \Delta H_r\, the reaction
enthalpy Enthalpy , a property of a thermodynamic system, is the sum of the system's internal energy and the product of its pressure and volume. It is a state function used in many measurements in chemical, biological, and physical systems at a constant ...
change. With the
electrophilic In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carri ...
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 In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
carbon radical on the other hand both enthalpy and polar effects have the same direction thus extending the activation energy range.


References


External links

* IUPAC
Compendium of Chemical Terminology The International Union of Pure and Applied Chemistry publishes many books which contain its complete list of definitions. The definitions are divided into seven "colour books": Gold, Green, Blue, Purple, Orange, White, and Red. There is also an ...
(Gold Book)
Reactivity–selectivity principle
{{DEFAULTSORT:Reactivity-selectivity principle Obsolete scientific theories Physical chemistry