Free radicals triarylmethyl

When one of the aromatic groups of the triarylmethyl free radical is replaced by an alkyl group, a decrease in stability due to a loss of resonance stabilization is to be expected. The paramagnetism and reactions associated with these less stable radicals will therefore appear only when the ethane is heated well above room temperature, the dissociation being endothermic. The rate of formation, but not the equilibrium constant, is experimentally accessible for these radicals since the radical once formed is subject to rearrangement, cleavage, and disproportionation reactions ... 

The same thing happens when alkyl halides are treated with silver in the presence of oxygen.41 The reaction with silver in the absence of oxygen is of course one of the methods used to prepare the stable triarylmethyl free radicals. 

Most photochemical reactions are not very polar and often involve free radical intermediates. But initiation of a reaction by light is no infallible indication of a radical mechanism, as is clear from the example of the photo-ionization of certain triarylmethyl cyanides.448... 

Gomberg reaction org chem The production of free radicals by reaction of metals with triarylmethyl halides. gom.berk re,ak-sh3n ... 

There were several attempts to gain better control on the free radical polymerization process [18, 19], One of these methods was named the iniferter method. The compounds used in this technique can serve as m/tiator, trans/er agent and terminating agent [20-22], Another technique is based on the use of bulky organic compounds such as diaryl or triarylmethyl derivatives [23-25], The main disadvantages of these systems comprise slow initiation, slow exchange, direct reaction of counter radicals with monomers, and their thermal decomposition. Therefore, these techniques did not offer the desired level of control over the polymerization. 

Triarylmethyl compounds also form rather stable triarylmethyl radicals, and indeed the first stable carbon free radical to be reported was the triphenyl-... 

Free radicals may be generated by oxidation, reduction, or by homolytic cleavage of one or more covalent bonds, such as C—C bonds e.g. dimers of triarylmethyl radicals), N—N bonds e.g. tetrasubstituted hydrazines), O—O bonds e.g. hydroperoxides, dialkyl and diacyl peroxides, peroxycarboxylic esters), C—N bonds e.g. dialkyl azo compounds), and N—O bonds (as in the thermolysis of nitrogen pentoxide O2N— O—NO2). Two typical examples, which have been investigated in different solvents, are given in Eqs. (5-56) and (5-57) cf. also reaction (5-39a) in Section 5.3.2. 

The basic significance of Gomberg s work remains unchanged. Many dimers have been prepared, and the existence of free triarylmethyl radicals has been substantiated in a number of ways indeed, certain of these compounds seem to exist entirely as the free radical even in the solid state. The most convincing evidence for the free-radical nature of these substances lies in properties that arise directly from the odd electron that characterizes a free radical. Two electrons... 

Gomberg free radical reaction. Formation of free radicals by abstraction of the halogen from triarylmethyl halides with metals. 

Action of free radicals on metals Schlenk and Marcus25 investigated the action of triarylmethyl radicals on metallic sodium, whereby compounds of the type R3CNa were formed. Paneth and Loleit26 studied the formation of organometallic compounds by reaction of methyl and ethyl radicals with arsenic, antimony, and bismuth. 

Although it might be expected that reactions which employed triarylmethyl halides would occur very readily, such reactions are rendered potentially more complex by the known nature of the halides and their propensity for involvement in free radical reactions. Whereas normal alkylation proceeds between sodium diethyl phosphite and diphenyl-methyl halides, success, or otherwise, in the use of the triphenylmethyl halides depends to some extent on the individual halide and on the metal in the phosphite salt. Thus, in an early study (in 1939), Arbuzov found that in reactions between silver dialkyl phosphites and triphenylmethyl bromide, dialkyl triphenylmethylphosphonates were indeed formed, but the use of the corresponding alkyl chloride provided the phosphite triester instead (metal dialkyl phosphites possess ambident anions ). A later study confirmed the behaviour of the silver salts towards the chloride, but also showed that, whereas dialkyl phophites with primary alkyl groups yielded phosphonic diesters (as had already been found), those with secondary alkyl groups afforded phosphite triesters moreover, the presence and nature of aromatic substituents were also able to control the course of the reaction. Reactions which involve triarylmethyl halides and sodium dialkyl phosphites may well be of a free radical nature since repeated studies have demonstrated the forma-... 

The light absorption and fluorescence of triarylmethyl free radicals. J. Am. Chem. Soc. 66 (1944) 1985. 

An unusual reaction consists of the reductive dehalogenation of a triarylmethyl chloride.1 5 xhe metal is silver and the product is a stable free radical (Eq. 40). 

Odd-electron molecules (often called free radicals) can behave either as electrophiles or electrodotes or even as both simultaneously. Probably the outstanding examples are the triarylmethyl... 

Moses Gomberg, the father of free radical chemistry in 1900 identified the triphenylmethyl radical, the first organic free radical it was found to be highly reactive with a very short half-life. Due to its reactivity toward oxygen, Gomberg could not isolate the triarylmethyl radical, but a yellow peroxide product. Despite this problem, Gomberg proposed the formation of the triarylmethyl radical, a trivalent carbon species. 

Hexaarylbiimidazoles (HABIs) were discovered in the early 1960s and aroused worldwide interest because of their unusual physical properties various stimuli such as heat, light and pressure converted these molecules to colored species, which were found to be relatively stable free radicals. Unlike Gomberg s triarylmethyl radicals, triarylimidazolyl radicals were oxygen insensitive. A research program directed at finding commercial applications for these materials at the DuPont Company resulted in the discovery that HABIs were useful as photooxidants and photopolymerization initiators. This in turn led to a number of very successful commercial products for the graphic arts, electronics, displays and communication industries. 

Figure 4.5 shows NBMO coefficients for several arylmethyl, diaryl-methyl, and triarylmethyl radicals. It will be seen that the NBMO coefficients at the exocyclic carbon atoms decrease rapidly with the number of attached groups. Dissociation should therefore occur most readily in the case of hexaarylethanes. This is the case. Solutions of hexaphenylethane at room temperature contain free triphenylmethyl radicals, whereas tetraarylmethanes dissociate to an appreciable extent only at much higher temperatures and diarylethanes are still more stable. 

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