Triphenylmethyl a stable free radical

In the case of the triphenylmethyl radical shown in Figure 4.86, it is possible to write many different resonance structures but in a small free radical such as the methyl radical there is only one possible structure. The reactivity of the radical decreases as the unpaired spin density at each site decreases, and the radical also becomes more stable because of the resonance energy. This resonance stabilization is zero for the phenyl radical, since the unpaired electron resides in an orbital which is orthogonal to the it system. By contrast, the methylphenyl radical has a resonance stabilization energy of some 10 kcalmol-1, and the larger methylnaphthyl radical is stabilized by about 15 kcalmol-1. These resonance stabilizations can have important consequences for the energy balance of photochemical reactions (see e.g. sections 4.4.2 and 4.4.4). 

Stable Free Radicals. Stable free radicals are a small minority of the more than 6 million chemical compounds known by 2005. The oxygen molecule is paramagnetic (S = 1). In 1896, Ostwald stated that "free radicals cannot be isolated." Only four years later, Gomberg123 made triphenylmethyl (Fig. 11.63), the first proven stable and persistent free radical [48] An infinitely stable free radical used as a reference in EPR is diphenyl-picryl hydrazyl (DPPH). Other persistent free radicals are Fremy s124 salt (dipotassium nitrosodisulfonate K+ O3S-NO-SO3- K+) 2,2-diphenyl-l-picrylhydrazy (DPPH)l, Galvinoxyl (2,6-di-tert-butyl-a-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-l-ylidene)-p-... 

Covalent bonds must be broken homolytically to form free radicals. As a rule, the less energy that this requires, the more stable will be the free radicals. Stable free radicals such as the triphenylmethyl radicals are not usually capable of starting a polymerization. 

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 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. 

For the field I was inadvertently entering (Walling had asked Kharasch if he could join his group), 1937 was a landmark year. Free radicals of course first entered organic chemistry in 1900 with Gomberg s preparation and identification of triphenylmethyl, but the chemistry and properties of such stable or persistent species had remained largely a chemical curiosity... 

With resonance possibilities, the stability of free radicals increases 149 some can be kept indefinitely.150 Benzylic and allylic151 radicals for which canonical forms can be drawn similar to those shown for the corresponding cations (pp. 168, 169) and anions (p. 177) are more stable than simple alkyl radicals but still have only a transient existence under ordinary conditions. However, the triphenylmethyl and similar radicals152 are stable enough to exist in solution at room temperature, though in equilibrium with a dimeric form. The concen-... 

The reactivity of a free radical can be defined by the rate constants of its reactions with other molecules and other free radicals. In general this reactivity depends on the extent of localization of the unpaired electron. When it is highly localized on a single atom, as in the methyl radical for example, this site is highly reactive. However, delocalization of the unpaired electron over aromatic rings reduces the reactivity to the point where some free radicals can be kept virtually for ever in the form of stable, unreactive samples. The triphenylmethyl radical is the best known example. 

Mechanism and kinetics of cationic poiymerization initiation. Unlike free-radical and anionic polymerization, initiation in cationic polymerization employs a true catalyst that is restored at the end of the polymerization and does not become incorporated into the terminated polymer chain. Initiation of cationic polymerization is brought about by addition of an electrophile to a monomer molecule. TVpical compounds used for cationic polymerization include protonic acids (e.g., H2SO4, H3PO4), Lewis acids (e.g., AICI3, BF3, TiCl4, SnCl4), and stable carbenium-ion salts (e.g., triphenylmethyl halides, tropylium halides) ... 

Thus the dimer undergoes its surprising reactions by first dissociating into triphenylmethyl, which, although unusually stable for a free radical, is nevertheless an exceedingly reactive particle. 

Even though adjacent lone pairs, tt bonds, and cr bonds do not stabilize radicals as much as they stabilize carbocations, the cumulative stabilizing effect of several such groups on a radical can be considerable. Benzylic radicals are particularly low in energy, as the radical center is stabilized by three tt bonds. The triphenylmethyl (trityl) radical, a triply benzylic radical, was the first free radical to be recognized as such. This remarkably stable radical is in equilibrium with the dimer that results from combination of the methyl carbon of one radical with a para carbon on another radical. (The structure of the dimer was originally misidentified as hexaphenylethane.)... 

Gomberg was the first to characterize a free radical when, in 1900, he generated triphenylmethyl radical 5 by reacting chlorotriphenylmethane (4) with zinc metal.l Triphenylmethyl radical 5 is unusual in that it is quite stable and its formation is probably the first experimental verification of a free radical. Frankland, however, may have been the first to generate transient methyl and ethyl radicals in the reaction of iodomethane and iodoethane with zinc, in 1849.2 In the last 30 years, attention has been focused on radicals, their reactivity, and their applications to organic synthesis. Excellent monographs by Davies and Parrott, Lazar et al., Hay, ... 

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 ). 

Interest in free radicals began with the woik of Gomberg (1900), who demonstrated the existence of the triphenylmethyl radical (PhsC ). The triphenylmethyl radical is stable enongh to exist in solution at room temperature, often in equilibrium with a dimeric form. At room temperature, this concentration of PhsC in benzene is 2%. Although triphenylmethyl-type radicals are stabilized by resonance, it is steric hindrance to dimerization and not resonance that is the cause of their stability. 

Based on these conclusions. Compound A might be expected to be converted into Compound B simply by heating how ever, it appeared to be stable to heat. When A was melted (about 335° C) in a sealed, exhausted tube, heated for 20 hours, and then allowed to cool, the pale yellow oil crystallized almost immediately and appeared to be unchanged A. In the work of Schlenk and Mark, ° Kipping found support for a postulate that the structure of Compound A, containing tervalent silicon, need not be prone to cyclize or polymerize. These workers had found that the free radical species, triphenylmethyl and pentaphenylethyl, did not combine to give octaphenylpropane, nor did pentaphenylethyl dimerize to produce deca-phenylbutane. 

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