Radicals terminology

In radical terminology, the opposite of transience is not stability but persistence.14 Persistent radicals do not react with themselves at diffusion-controlled rates however, they may still react readily with other radicals or with triplet oxygen. Thus, persistence is a kinetic property that is more often related to sterically hindered recombination than to electronic stabilization. Persistent radicals typically also lack P-C—H bonds and they cannot disproportionate. Several persistent radicals are illustrated in Scheme 4. Persistent radicals are rarely present in synthetic applications, but when they are, there will be important consequences. 

The stabilizing effects of vinyl groups (in allylic radicals) and phenyl groups (in benzyl radicals) are very significant and can be satisfactorily rationalized in resonance terminology ... 

Earlier sections have already provided several examples of radical fragmentation reactions, although this terminology was not explicitly used. The facile decarboxylation of acyloxy radicals is an example. 

It is the aim of this chapter to describe the nature, selectivity, and efficiency of initiation. Section 3.2 summarizes the various reactions associated with initiation and defines the terminology used in describing the process. Section 3.3 details the types of initiators, indicating the radicals generated, the byproducts formed (initiator efficiency), and any side reactions (e.g. transfer to initiator). Emphasis is placed on those initiators that see widespread usage. Section 3.4 examines the properties and reactions of the radicals generated, paying particular attention to the specificity of their interaction with monomers and other components of a polymerization system. Section 3.5 describes some of the techniques used in the study of initiation. 

Most monomers have an asymmetric substitution pattern and the two ends of the double bond are distinct. For mono- and 1,1-disubstituted monomers (Section 4,3.1) it is usual to call the less substituted end "the tail" and the more substituted end "the head". Thus the terminology evolved for two modes of addition head and tail and for the three types of linkages hcad-to-tail, hcad-to-hcad and tail-to-ta.il. For 1,2-di-, tri- and tetrasubstituted monomers definitions of head and tail are necessarily more arbitrary. The term "head" has been used for that end with the most substituents, the largest substituents or the best radical stabilizing substituent (Scheme 4.4). 

The terminology describing the action of antioxidants is unfortunately not clear. Terms such as antioxidant power , antioxidant effectiveness , antioxidant ability , antioxidant activity , and antioxidant capacity are often used interchangeably and without discrimination. Here we use the term antioxidant activity as meaning a measure of the rate of antioxidant action, and the term antioxidant capacity as meaning a measure of the extent of antioxidant action, i.e. the amount of radicals or intermediates and products produced during oxidation that are quenched by a given antioxidant. Thus antioxidant activity is related to the kinetics of the antioxidant action and antioxidant capacity to the stoichiometry. 

The term persistent is typically applied to long-lived radicals7" which in a majority of cases can be characterized by ESR spectroscopy. " However, the lifetime of those radicals can vary rather broadly from several minutes to months, depending on the environment around the radical center. In this contribution, we will use the terminology of persistent radicals for those radical species with a relatively long lifetime however, those persistent radicals that can be isolated as individual room temperature stable compounds and in many cases characterized by X-ray crystallography will be specifically named as stable radicals (for the chemistry of stable radicals, see Section 2.2.4). 

Note on terminology Many authors write stable when they mean unreactive or inert but stable is the opposite of unstable , and the opposite of reactive is unreactive or inert . Methyl radicals are very stable, but very reactive hexaphenyl ethane is unstable, but very inert. 

Now, just the same sort of rationalization can be applied to the radical addition, in that the more favourable secondary radical is predominantly produced. This, in turn, leads to addition of HBr in what is the anti-Markovnikov orientation. The apparent difference is because the electrophile in the ionic mechanism is a proton, and bromide then quenches the resultant cation. In the radical reaction, the attacking species is a bromine atom, and a hydrogen atom is then used to quench the radical. This is effectively a reverse sequence for the addition process but, nevertheless, the stability of the intermediate carbocation or radical is the defining feature. The terminologies Markovnikov or anti-Markovnikov orientation may be confusing and difficult to remember consider the mechanism and it all makes sense. 

Every new aspect of a science involves a revolution in the technical terms of that science. This is best shown by Chemistry, where the whole of the terminology is radically changed about once in twenty years, and where you will hardly find a single organic compound that has not gone through a whole series of dilferent names. 

The recent TUPAC Compendium of Chemical Terminology—The Gold Book recommends that the name of compounds having the structure R2N—O" R2N +—0 is more appropriately that of aminoxyl radicals . The synonymous terms nitroxyl radical or nitroxide are accordingly not desirable, even though quite popular in various fields of science and technology. This chapter follows a previous chapter of the series and, for this historical reason, retains the old terminology of the compounds in the title, but this use will be discontinued from now on in the text. 

The initiator is present in the water phase, and this is where the initiating radicals are produced. The rate of radical production if, is typically of the order of 1013 radicals L-1 s-1. (The symbol p is often used instead of Rj in emulsion polymerization terminology.) The locus of polymerization is now of prime concern. The site of polymerization is not the monomer droplets since the initiators employed are insoluble in the organic monomer. Such initiators are referred to as oil-insoluble initiators. This situation distinguishes emulsion polymerization from suspension polymerization. Oil-soluble initiators are used in suspension polymerization and reaction occurs in the monomer droplets. The absence of polymerization in the monomer droplets in emulsion polymerization has been experimentally verified. If one halts an emulsion polymerization at an appropriate point before complete conversion is achieved, the monomer droplets can be separated and analyzed. An insignificant amount (approximately <0.1%) of polymer is found in the monomer droplets in such experiments. Polymerization takes place almost exclusively in the micelles. Monomer droplets do not compete effectively with micelles in capturing radicals produced in solution because of the much smaller total surface area of the droplets. 

H atom elimination is of lower importance from this state, 81 82 IC in the mechanism suggested by Wichramaaratchi et al. is practically identical with 8i 8x used in the terminology of Orlandi s group. The authors of Refs. 55, 83, 121, and 122 all agree that there is a dissociative triplet state, T , which can easily be populated by ISC from 81. We show the potential energy curve of this state (T ) as it has been suggested by Orlandi et al. [83,121]. This state gives only radical decomposition products. In order to explain the results of the biphotonic sensitization experiments (Sec. 2.1) another triplet state —1 eV below 81 (Ti) is also needed. Most probably, this triplet is also dissociative. 

Continuing research in radical ion chemistry will yield many advances the continuing development of new techniques and their application to previously studied systems may add some new facets to the reaction mechanisms. Of course, progress will also include the rediscovery of facts, structures, and mechanisms, derived or formulated over the past five decades, which can now be veiled in new terminology. In short, research in radical ion chemistry will flourish for some years to come. 

Using Saveanfs terminology, such a process is called redox catalysis in its proper meaning, while Shono formed the expression homomediatory system . This type of mechanism was already schematically presented in the case of an oxidation in Eqs. (2) to (4). To this category of redox catalysts belong, for example, the radical anions and cations of aromatic and heteroaromatic compounds and some reactions of triaryl amine radical cations. 

Because radicals are uncharged, one would expect that electron donation and withdrawal would be less important than in ionic reactions. Although this expectation is certainly correct, there is nevertheless ample evidence that transition states of radical reactions do have some polar character. In resonance terminology, one might describe the transition state of a homolysis as a resonance hybrid (25) of covalent, radical, and ionic structures, with importance of the ionic forms subject to influence by substituents in A and B. It should therefore... 

Fig. 1.14. NaBH4, reduction of (/Thydroxyalkyl)mercury(II) acetates to alcohols and radical fragmentation of (/f hydroxy-alkyl)mercury (II) hydrides. According to the terminology used in Figure 1.2 it is a "substitution by fragmentation."...

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