Persistent free radicals

For analyzing more deeply the origin of free-radical persistence and the eventual activation barrier of their dimerization reactions, we have wondered whether the latter obey the Evans-Polanyi relation ... 

The issue of stability vs. persistence of free radicals is an important one that dates back to the birth of the field. In 1900 Gomberg prepared the triphenylmethyl or trityl radical according to Eq. 2.13. Under appropriate conditions, the free radical persists in solution indefinitely at room temperature. This initially controversial result was arguably the birth of reactive intermediate chemistry, and it spurred volumes of work. The trityl radical is in equilibrium with a dimer that, for decades, was assumed to be hexaphenylethane. However, nuclear magnetic resonance (NMR) and ultraviolet (UV) studies in 1968 revealed that the actual dimer was the unsymmetrical structure shown in Eq. 2.13, in which one trityl center added to the para position of a ring of another radical. 

Although the existence of the stable and persistent free radicals is of significance in establishing that free radicals can have extended lifetimes, most free-radical reactions involve highly reactive intermediates that have fleeting lifetimes and are present at very low concentrations. The techniques for study of radicals under these conditions are the subject of the next section. 

The last two reactions are useful for esr studies involving free radicals. Until recently, the only trialkyltin radical that had been observed directly, in solution, by esr was MesSn- (295), but many more have now been reported (e.g., EtsSn-, PrsSn-, and BusSn ) (296). Bulky ligands [e g., (PhCMejCHjlaSn ] increase the persistence of the radicals, so that esr observation is easier (297), and tris(2,3,5-trimethylphenyl)tin and tris(2,3,5-triethylphenyl)tin radicals, at 180° and 100°, respectively, are in thermal equilibrium with the corresponding hexaaryldi-tins (298). 

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radicals trapped in this manner. Even under these conditions, the methyl radical decomposes with a half-life of 10-15 min in a methanol lattice at 77 K. Since the lifetime of a radical depends not only on its inherent stabihty, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

Studies on the formation and reactivity of P-centered radicals continue to be a versatile source of mechanistic information and reactions of interest in synthetic chemistry. Various new persistent or stable P-centered radicals have been described and could find applications as paramagnetic probes. The possibility of influencing the properties of organic free radicals bearing an appropriately located phosphorus group should find interesting applications. 

Other Oxidants—Combinations of the above oxidants and other oxidants such as persulfate compounds are also being used to treat MTBE and other oxygenates. These and other combinations and other oxidants are being developed to maximize the generation of highly oxidizing free radicals, increase oxidant persistence, or otherwise enhance in situ oxidation. 

Free-Valence Persistence in Reactions of Free Radicals with Molecules... 

Thus, free valence persists whenever an atom or a radical undergoes a unimolecular reaction or interacts with valence-saturated molecules (possessing an even number of electrons). This is a natural consequence of conservation of the number of electrons in chemical reactions. Therefore, free valence cannot persist when a radical reacts with a radical. Both reactants have an odd numbers of electrons, and the product formed has an even number of electrons, for example,... 

Although some radical species may persist for prolonged periods, most are generally unstable and will attempt to donate their unpaired electron to a nearby molecule or to remove or abstract a second electron, usually in the form of a hydrogen atom, from a neighboring molecule to pair with their free electron. Free-radical reactions are intrinsic to a majority of the metabolic and... 

Formation of both adduct 106, mesitylene adduct 109 and heterocycle 104 can be ascribed to homolysis (Scheme 21, pathway (i)). Decarboxylation of acyloxyl radical gives radicals, R , which either combine with the persistent free radical 102 giving 106 or, by abstracting hydrogen from solvent, give 3,5-dimethylbenzyl radicals, which combine with 102 to give 109. 

This article concerns a simple expedient whereby short-lived reactive free radicals may be transformed into more persistent paramagnetic species, thus enabling esr techniques to be applied to systems in which the concentration of the reactive radical remains below normal detection limits. The principle is a simple one. It depends upon the addition to the reaction system of a small quantity of a diamagnetic substance (the spin-trap ) having a particularly high affinity for reactive radicals the product of this trapping reaction must be a particularly persistant free radical (the spin adduct ) whose concentration will build to readily detectable levels (>ca. 10—7—10-6 M). The general reaction is represented by equation (1). 

Referring to a reaction intermediate or free radical that has a lifetime longer than that of a transient species, typically on the time-scale of at least several minutes in dilute solution in inert solvents. Persistence is therefore a kinetic property related to reactivity. The stability of an intermediate or free radical is a thermodynamic property, often expressed in terms of the appropriate bond strengths. See Transient Chemical Species D. Griller and K. U. Ingold (1976) Acc. Chem. Res. 9, 13. 

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