Stable radical, definition

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. 

Somewhat more definitive evidence that a transient radical was produced is provided when the radical reacts with a trapping agent to give a stable radical product... 

The results definitely prove our hypotheses in the kinetic model for vinyl acetate emulsion polymerization (10), that vinyl radical, CH2=C-0Ac, is the major monomer radical formed and is a stable radical which reinitiates relatively slowly compared to the propagation step. 

Griller, D. Ingold, K. U. Acc. Chem. Res. 1976, 9, 13. In the context of this review, radicals are called persistent if their lifetimes in liquid solution exceed those of reactive radical species by many orders of magnitude. They may self-terminate slowly or disappear by other reactions, but these processes do not compete with the cross coupling with usual transient radicals. Stable radicals can be isolated in pure form. They are included in our definition of persistence. 

Detailed kinetic schemes also consist of several hundreds of species involved in thousands of reactions. Once efficient tools for handling the correspondingly large numerical systems are available, the extension of existing kinetic models to handle heavier and new species becomes quite a viable task. The definition of the core mechanism always remains the most difficult and fundamental step. Thus, the interactions of small unsaturated species with stable radicals are critical for the proper characterization of conversion and selectivity in pyrolysis processes. Parallel to this, the classification of the different primary reactions involved in the scheme, the definition of their intrinsic kinetic parameters, the automatic generation of the detailed primary reactions and the proper simplification rules are the important steps in the successive extension of the core mechanism. These assumptions are more relevant when the interest lies in the pyrolysis of hydrocarbon mixtures, such as naphtha, gasoil and heavy residue, where a huge number of isomers are involved as reactant, intermediate and final products. Proper rules for feedstock characterizations are then required for a detailed kinetic analysis. 

Stable radical cations have been observed optically and by ESR after irradiation in matrices containing an electron scavenger . However, ESR spectra have not been observed in the neat solids. Possible line broadening and overlap with radical spectra prevent definite conclusions about the absence of the ions from the ESR spectra. The optical absorption spectrum of the radical cation has been observed in neat 3-methyloctane at 6 K and 77 K = 600 nm) and in squalane (C30H62) at 77 K (A ax 1400nm) at 100ns after a 40 ns pulse ... 

As has already been indicated (see Chapter I), an extremely important shortcoming of inhibitors that contain readily labile hydrogen atoms and can form stable radicals is the fact that the possibility of transfer of the decomposition reaction chain is not excluded in their presence. Under definite conditions, in particular, when the temper-... 

Clearly, mechanistic investigations can provide circumstantial evidence for the participation of particular intermediates in a reaction but, here, we are concerned with the definitive observation of these species. If the intermediates are relatively stable then direct spectroscopic observation of the species during a room-temperature reaction may be possible As a rather extreme example of this, the zero-valent manganese radicals, Mn(CO>3L2 (L phosphine) can be photochemically generated from Mh2(CO)gL2, and, in the absence of O2 or other radical scavengers, are stable in hydrocarbon solution for several weeks (2, 3) However, we are usually more anxious to probe reactions in which unstable intermediates are postulated. There are, broadly speaking, three approaches - continuous generation, instantaneous methods and matrix isolation. 

As already mentioned, RNR is the metalloenzyme in which the first definitively characterized stable amino acid radical (1), later identified as a tyrosyl radical, was found in 1972. The RNR enzymes catalyse the reduction of ribonucleotides to their corresponding deoxyribonucleotides utilized in DNA biosynthesis. There are three unique classes of this enzyme, differing in composition and cofactor requirements all of them, however, make use of metal ions and free radical chemistry. Excellent reviews on RNRs are available (60, 61, 70, 89-97). 

However, radical intermediates cannot be definitely excluded, at least not in the reaction of pyiylium salts in pyridine. Steuber showed that such pyrylium salts as 2.4.6-triphenyl-pyrylium-or 2.4.6-tri-tert-butyl-pyrylium-tetrafluoroborate can be reduced to stable pyryl radicals 32 by pyridine this reduction proceeds particularly smoothly if traces of copper powder are added. 

Obviously there is a definite need in the fine chemical and pharmaceutical industry for catalytic systems that are green and scalable and have broad utihty [10]. More recently, oxidations with the inexpensive household bleach (NaOCl) catalyzed by stable nitroxyl radicals, such as TEMPO [17] and PIPO [18], have emerged as more environmentally friendly methods. It is worth noting at this juncture that greenness is a relative description and there are many shades of green. Although the use of NaOCl as the terminal oxidant affords NaCl as the by-product and may lead to the formation of chlorinated impurities, it constitutes a dramatic improvement compared to the use of chromium(VI) and other... 

As a rule, recrystallization cannot be used for the purification of organic metals. Recrystallization is usually performed under definite thermal influence and leads to dirtied, imperfect crystals. Ion radical salts are not thermally stable in solution. The direct donor-to-acceptor interaction is the best way to limit chemical impurities. In this case, the reaction mixture contains minimal amounts of substances that are not included in the structure of a given ion radical salt. The oxidation of donors in the presence of anions or ion exchange usually results in the formation of less pure crystals. 

The paradox implicit in the above description is that a successful replicator which rapidly produces long-lived, highly stable, perfect copies of itself cannot, by definition, evolve. Fortunately for us, evolution at the replicator scale occurs. If the replicator is a small molecule with perhaps only four or five subunits then one or two mistakes in the replication will generate a radically different product. It may be an even better replicator or completely inactive. If, however, the replicator has many hundreds or thousands of subunits then, while one key error might destroy its function, it is more likely that a few errors will have a marginal effect on its activity. Gradual evolution of this type would lead to a family of similar replicators... 

The most definite and indisputable evidence for the existence of free radicals is obtained from spectroscopy. Physicists are able to interpret band spectra without ambiguity on the basis of such units as OH, CN, BeCl, SiO, CH2, C2 and others. There is a whole host of radicals of this type which can explain quantitatively all the lines of a complex band spectrum and there is no other way to explain them. Moreover calculations based on quantum mechanics show that many of these free radicals, which violate all rules of the classical theories of valence, are stable and do not necessarily decompose at ordinary or even at moderately high temperatures. The difficulty in finding them is not that they are too unstable but rather that they are so very reactive that they combine immedi-... 

The Consequences of Different Stability Definitions How Stable Are Ethyl and Fluoromethyl Radicals ... 

While alcohols also form alcoholafes (p. 79), the phenolates are more stable and more definitely salt-like in character due to the more strongly acid nature of the phenyl or other aryl radical. The phenolates, however, are decomposed by carbon dioxide and react alkaline to some indicators. 

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