Triphenylmethyl free radical

Steric effects have been discussed in free radical chemistry ever since the discovery of the first free radical, triphenylmethyl 1 by M. Gomberg in 19001. To what extent is the dissociation of its dimer, which was believed to be hexaphenylethane 23 till 19682 determined by electronic stabilization of triphenylmethyl 1 or by steric strain in its dimer ... 

The proposal by Moses Gomberg in 1900 of the formation of the stable and persistent free radical triphenylmethyl was a major landmark that set the stage for the rapid development of free radical chemistry in the 20th Century. Prior to Gomberg s proposal, the theory of free radicals had risen to prominence and then fallen into disrepute, but his work immediately attracted the attention of the world chemical community, and led to the ultimate acceptance of this once controversial concept. 

The experimental evidence presented above forces me to the conclusion that we have to deal here with a free radical, triphenylmethyl, On this... 

However, do these conclusions really express doubt about the existence of the free radical triphenylmethyl Or is it the nomenclature that is ambiguous With the correct answer at hand, one cannot state today that the chemical reactivity of a solution of ca. 2% trityl radical and 98% dimer 1 is entirely determined by the chemistry of the radical. Maybe Gomberg was talking about triphenylmethyl ... 

Schlenk was the one who first took triphenylmethyl-type radicals to the monomeric extreme and thus produced the final evidence for the existence of free radicals. The first example in this direction was phenylbis(biphenylyl)-methyl (11), which was isolated as white crystals from operations carried out in the apparatus described by Schmidlin. " Upon dissolution of 11 in benzene, a red color developed, and cryoscopic studies revealed that the monomeric phenylbis(biphenylyl)methyl constituted 80% of the equilibrium mixture. Trisbiphenylylmethyl (12) was even more extreme it formed black crystals and was a 100% monomeric free radical in an almost black solution. Finally, Schlenk et al. established the connection between the conducting solutions of triphenylhalomethanes and the free radical triphenylmethyl by showing that the cathodic reduction of triphenylbromomethane in liquid SO2 gave rise to triphenylmethyl. These findings were considered the definitive evidence for the free radical hypothesis, and Schlenck was nominated for the Nobel Prize in 1918 and several times afterwards for this achievement, amongst others (Table 2). 

What occurred was a surprising reaction forming a colored solution. Addition of iodine, for example, produced triphenylmethyl iodide and a colorless solution. Gomherg had generated a stable, yet reactive, free radical—triphenylmethyl radical ... 

About a century ago, Gomberg attempted to prepare hexa(phenyl)ethane(2) by treating a solution of triphenylmethyl chloride (1) with silver or zinc. He obtained a yellow solution of a stable species whose properties, though, did not appear compatible with those expectedfrom a hydrocarbon like hexa(phenyl)ethane. For example, the solution would decolorize rapidly when exposed to air, or treated with iodine or a number of other materials known to react with organic radical. Gomberg concluded in his paper of 1900 [1] The experimental evidence forces me to the conclusion that we have to deal here with a free radical, triphenylmethyl, (C6Hy)sC (3) ( Scheme ). 

From Table III we see that the difference between the free radical resonance energies of tribiphenylmethyl and triphenylmethyl is 0.07a. Hence X]/X2 = 37 = 2.2 X103. Ziegler and Ewald8 found that at 20°C the value of the dissociation constant for hexaphenylethane in benzene solution is 4.1 X10-4 and consequently we calculate for hexabiphenylethane a value of X = 2.2X103 X4.1 X 10 4 = 0.90. This value is probably too low as the compound is reported to be completely dissociated the error may not be large, however, since a dissociation constant of 0.90 would lead to 91 percent dissociation in 0.05M solution. 

Free radicals with resonance are definitely planar, though triphenylmethyl-type radicals are propeller shaped, like the analogous carbocations (p. 225). 

Some radicals (e.g., triphenylmethyl) are so unreactive that they abstract hydrogens very poorly if at all. Table 14.3 lists some common free radicals in approximate order of reactivity. ... 

The hazards of a rigid classification of substances which may modify the course of a free radical polymerization are well illustrated by the examples of inhibitors and retarders which have been cited. The distinction between an inhibitor or retarder, on the one hand, and a co-monomer or a transfer agent, on the other, is not sharply defined. Moreover, if the substance is a free radical, it is potentially either an initiator or an inhibitor, and it may perform both functions as in the case of triphenylmethyl. If the substance with which the chain radicals react is a molecule rather than a radical, three possibilities may arise (i) The adduct radicals may be completely unreactive toward monomer. They must then disappear ultimately through mutual interaction, and we have a clear-cut case of either inhibition or retarda-... 

Free radicals are classified from their lifetimes into long-lived (stable) and short-lived (intermediate) radicals. The most famous and first example of the former is triphenylmethyl (1), which was discovered just one hundred years ago by Gomberg [37]. 

The first organic free radical to be discovered was triphenylmethyl, the result of the effort of Gomberg to prepare hexaphenylethane.6 In geographical exploration, isolated white spaces on the map rarely contain anything strikingly different from the neighboring explored regions chemical exploration leads to all sorts of surprises. 

It will be noted that the non-planar propeller form lacks a plane of symmetry and that the production of optically active products by way of an intermediate radical might therefore be possible. Attempts to make precursory optically active phenylxenyl-a-naphthylmethyl halides failed, but the corresponding active thioglycolic acid has been prepared. It is racemized by the reaction with triphenylmethyl free radicals.19 Various other reactions believed to involve radical intermediates also give inactive products ... 

Triphenylmethyl reacts with atomic hydrogen and with methyl free radicals when a solution of it is exposed to a gas stream containing the other radicals.82... 

The decomposition of azobisnitriles is used to initiate polymerization. In the presence of quinones they give the same kind of addition product that is obtained from triphenylmethyl free radicals, strengthening the hypothesis that the decomposition is a radical one. 

Catalysis (initiation) by a free radical, on the other hand, is fairly conclusive evidence of a radical reaction, provided it is known that the catalyst is indeed a free radical and that it does not have pronounced polar properties as well. Many classes of compound once thought to decompose exclusively into ions or exclusively into radicals are now known to do both. Peroxides are one well-known example, AT-halo-amides are another. Catalysis by benzoyl peroxide probably does indicate a radical reaction since there is no evidence that this particular peroxide tends to give ions even under the most favorable conditions. But many other peroxides are known to decompose into ions, or at least ion pairs, as well as into radicals. The decomposition of azo compounds can also be either radical or ionic, the dialkyl azo compounds tending to give radicals, the diazonium compounds either radicals or ions. Catalysis by a borderline example of an azo compound would therefore be dubious evidence of either kind of mechanism. The initiation of the polymerization of octyl vinyl ether by triphenylmethyl chloride in polar... 

A bulky methacrylate, triphenylmethyl methacrylate (TrMA), is a unique monomer which gives an almost 100% isotactic polymer in anionic polymerization with n-butyllithium both in nonpolar and polar solvents. Moreover, even free-radical polymerization affords a highly isotactic polymer from this monomer.23 The isotactic specificity of TrMA polymerization is ascribed to the helical formation of the main chain. When TrMA is polymerized in toluene at —78°C... 

Free Radicals, Free atoms or larger fragments of stable molecules that contain one or more unpaired electrons are called free radicals. The unpaired electron is designated by a dot in the chemical symbol for the substance. Some free radicals are relatively stable, such as triphenylmethyl. 

The reaction of the triphenylmethyl radical with oxygen to form the peroxide discovered by Gomberg in 1900 (equation 1) was a strong piece of evidence for the radical structure 1, and the affinity of carbon centered radicals for oxygen remains one of their defining characteristics. Willstatter and Haber in 1931 also proposed a general role for free radicals in oxygen reactions in chemistry, which further stimulated interest in this field. 

The most serious obstacle for the free radical nature of triphenylmethyl was the series of experiments carried out to determine the molecular weight of triphenylmethyl . Cryoscopy was performed in a range of solvents and invariably showed that the molecular weight corresponded to that of the dimer, 486. Only in naphthalene, which in admixture with triphenylmethyl froze at about 80°C, was a lower value obtained, 410. This was a serious dilemma, but Gomberg in 1904 had a reasonable suggestion involving an equilibrium between a dimer and the free triphenylmethyl. 

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