The persistent radical effect (PRE) in
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 ...
describes and explains the selective product formation found in certain
free-radical cross-reactions. In these type of reactions, different radicals compete in secondary reactions. The so-called persistent (long-lived) radicals do not self-terminate and only react in cross-couplings. In this way, the cross-coupling products in the product distribution are more prominent.
The effect was first described in 1936 by Bachmann & Wiselogle.
They heated pentaphenylethane and observed that the main reaction product was the starting product (87%) with only 2% of tetraphenylethane formed. They concluded that the dissociation of pentaphenylethane into
triphenylmethyl
The triphenylmethyl radical (often shorted to trityl radical) is an organic compound with the formula (C6H5)3C. It is a persistent radical. It was the first radical ever to be described in organic chemistry. Because of its accessibility, the trit ...
and diphenylmethyl radicals was reversible and that persistent triphenylmethyl did not self terminate and transient diphenylmethyl did to a certain extent.
In 1964, Perkins
performed a similar reaction with phenylazotriphenylmethane in
benzene
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms, ...
. Again, the dimerization product of the persistent radical (phenylcyclohexydienyl) was absent as reaction product. In 1981, Geiger and Huber found that the
photolysis
Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...
of
dimethylnitrosamine into dimethylaminyl radical and
nitrous oxide
Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has a ...
was also completely reversible.
[ A similar effect was observed by Kräutler in 1984 for methylcobalamin.][
The PRE is a kinetic feature which provides a self-regulating effect in certain controlled/living radical polymerization systems such as ]atom transfer radical polymerization Atom transfer radical polymerization (ATRP) is an example of a reversible-deactivation radical polymerization. Like its counterpart, ATRA, or atom transfer radical addition, ATRP is a means of forming a carbon-carbon bond with a transition metal cat ...
and nitroxide mediated polymerization. Propagating radicals ''Pn*'' are rapidly trapped in the deactivation process (with a rate constant of deactivation, ''k''deact) by species ''X'', which is typically a stable radical such as a nitroxide. The dormant species are activated (with a rate constant ''k''act) either spontaneously/thermally, in the presence of light, or with an appropriate catalyst (as in ATRP) to reform the growing centers. Radicals can propagate (''k''p) but also terminate (''k''t). However, persistent radicals (''X''), as stated above, cannot terminate with each other but only (reversibly) cross-couple with the growing species (''k''deact). Thus, every act of radical–radical termination is accompanied by the irreversible accumulation of ''X''. Consequently, the concentration of radicals as well as the probability of termination decreases with time. The growing radicals (established through the activation–deactivation process) then predominantly react with ''X'' rather than with themselves.
References
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, refs=
[''Radicals: Reactive Intermediates with Translational Potential'' Ming Yan, Julian C. Lo, Jacob T. Edwards, and Phil S. Baran Journal of the American Chemical Society 2016 138 (39), 12692-12714 {{doi, 10.1021/jacs.6b08856]
[''The Persistent Radical Effect: A Principle for Selective Radical Reactions and Living Radical Polymerizations'' Hanns Fischer Chemical Reviews 2001 101 (12), 3581-3610 {{doi, 10.1021/cr990124y]
[Studer, A. (2001), ''The Persistent Radical Effect in Organic Synthesis''. Chem. Eur. J., 7: 1159–1164. {{doi, 10.1002/1521-3765(20010316)7:6<1159::AID-CHEM1159>3.0.CO;2-I]
[''THE RELATIVE STABILITY OF PENTAARYLETHANES. III.1 THE REVERSIBLE DISSOCIATION OF PENTAARYLETHANES'' W. E. BACHMANN and F. Y. WISELOGLE The Journal of Organic Chemistry 1936 01 (4), 354-382 {{doi, 10.1021/jo01233a006]
[''1145. The thermal decomposition of phenylazotriphenylmethane in p-xylene'' M. J. Perkins J. Chem. Soc., 1964, 5932-5935 {{doi, 10.1039/JR9640005932]
[''Mechanisms of free-radical aromatic substitution'' D.H. Hey, M.J. Perkins Gareth H. William Tetrahedron Letters Volume 4, Issue 7, 1963, Pages 445-452 {{doi, 10.1016/S0040-4039(01)90654-9]
[Geiger, G. and Huber, J. R. (1981), ''Photolysis of dimethylnitrosamine in the gas phase''. HCA, 64: 989–995. {{doi, 10.1002/hlca.19810640405]
[Kräutler, B. (1984), ''Acetyl-cobalamin from Photoinduced Carbonylation of Methyl-cobalamin''. HCA, 67: 1053–1059. {{doi, 10.1002/hlca.19840670418]
[''Unusual selectivities of radical reactions by internal suppression of fast modes'' Hanns. Fischer Journal of the American Chemical Society 1986 108 (14), 3925-3927 {{doi, 10.1021/ja00274a012]
[''The persistent radical effect: a prototype example of extreme, 105 to 1, product selectivity in a free-radical reaction involving persistent .cntdot.CoII acrocycleand alkyl free radicals'' Brian E. Daikh and Richard G. Finke Journal of the American Chemical Society 1992 114 (8), 2938-2943 {{doi, 10.1021/ja00034a028]
Organic reactions