Triphenylmethyl radical stability

The first conclusive qualitative evidence for the relative stability of aryldiazenyl radicals was the isolation of phenylazotriphenylmethane (8.50) in the thermolysis of a-(phenylazo)cumene in the presence of triphenylmethyl radicals by Porter et al. (1978), as shown in Scheme 8-32. 

The situation with polyarylmethanes is very similar. Due to the stabilization of free valence in arylmethyl radicals, the bond dissociation energy (BDE) of the bond C—02 for example, in triphenylmethyl radical is sufficiently lower than in alkylperoxyl radicals. This radical is decomposed under oxidation conditions (room temperature), and the reaction of Ph3C with dioxygen is reversible ... 

Since triphenylmethane is not stabilized by any resonance not already present in hexaphenylethane, the difference between the two heats of hydrogenation, or 22 kcal., might be a measure of the steric effect alone. The difference in the heats of dissociation into radicals when ethane and hexaphenylethane are compared is 62 kcal. This leaves about 40 kcal. to be accounted for as resonance stabilization of the radical.16 This degree of resonance stabilization for the triphenylmethyl radical does not violate quantum mechanical expectations. 

Ever since the discovery by Gomberg in 1900 of the dissociation of hexaphenylethane into triphenylmethyl radicals the search for a theoretical explanation of the phenomenon has been carried on. The modern theory of the stability of the aromatic free radicals attributes it in the main to the resonance of the free valence among many atoms.28... 

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). 

Although the foregoing reactions involving the triphenylmethyl radical seemed very unreasonable at the time they were discovered, the stability of the radical now has been established beyond question by a variety of methods such as... 

Just as several alkyl substituents increasingly stabilize a radical center (Table 1.2), so do two phenyl substituents. The diphenylmethyl radical ( benzhydryl radical ) is therefore more stable than the benzyl radical. The triphenylmethyl radical ( trityl radical ) is even more stable because of the three phenyl substituents. They actually stabilize the trityl radical to such an extent that it forms by homolysis from the so-called Gomberg hydrocarbon even at room temperature (Figure 1.8). Although this reaction is reversible, the trityl radical is present in equilibrium quantities of about 2 mol%. 

Today, we recognize that the ability of triphenylmethyl radicals to exist free in solution is due to two factors. First, the radical has considerable resonance stabilization. Second, and more important, there is considerable steric hindrance to the dimerization of the radical due to the three bulky phenyl groups. In fact, it has recently been shown that Gomberg s hydrocarbon is not hexaphenylethane but actually results from one... 

In fact, the stability of the triphenylmethyl radical we know to be due mainly to steric, rather than electronic, factors. X-ray crystallography shows that the three phenyl rings in this compound are not coplanar but are twisted out of a plane by about 30°, like a propeller. This means that the delocalization in this radical is less than ideal (we know that there is some delocalization from the ESR spectrum) and, in fact, it is little more delocalized than the diphenylmethyl or even the benzyl radical. 

Triphenylmethyl radicals couple to the Gomberg dimer 2.53, rather than the hexaphenylethane, PhsC-CPhs (2.54), as Gomberg originally proposed. The reason is that it is energetically more favourable for the dimeric compound to lose aromatic stabilization from one ring than to form the sterically strained 2.54. 

A marked distinction from the behavior of aliphatic azocompounds is the great thermal stability of azoarenes. Aryl radicals can be produced by thermolysis only with the assistance of a stable companion radical such as the triphenylmethyl radical in the case of phenylazotriphenyl methyl. 

The remarkable dissociation to form free radicals is the result of two factors. First, triphenylmethyl radicals are unusually stable because of resonance of the sort we have proposed for the benzyl radical. Here, of course, there are an even larger number of structures (36 of them) that stabilize the radical but not the hydrocarbon the odd electron is highly delocalized, being distributed over three aromatic rings. 

The experimental evidence for the formation of the triphenylmethyl radical was so overwhelming that the case for free radicals was firmly established. So why was Gomberg able to observe this particular radical Part of the reason for the stability of the triphenylmethyl radical can be attributed to the presence of three bulky benzene rings that effectively shield the central carbon atom bearing the radical and slow any reactions.9... 

The sulfonium salt was shown to spontaneously oxidize highly stabilized free radicals such as the triphenylmethyl radical to form the corresponding triphenylmethyl carbonium ion.(20) It would also appear that the dimethoxybenzylic free radical (a Norrish Type I photocleavage product of 2,2-dimethoxy-2-phenyl-acetophenone) is similarily oxidized by the arylsulfonium salt ( ). 

Unsymmetrical azo compounds must be used to generate phenyl radicals because azobenzene is very stable thermally. Phenylazotriphenylmethane decomposes readily because of the stability of the triphenylmethyl radical. 

We find in the majority of cases that the effects of P-NO2 and p-OCHa are in opposite directions (see Chapters VI and IX). However, they have the same effect on the stability of the triphenylmethyl radicals with respect to the ethane. The tris-(p-nitrophenyl)methyl is apparently completely in the free radical form in dilute solution at room temperatures, and perhaps even in the solid phase, owing to the additional forms in the free radical made possible by the presence of the nitro group. The most important of these forms are... 

Most radicals are transient species. They (f.. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylmethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section... 

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. 

Even a triphenylmethyl group distal to oxygen does not accelerate azoxyalkane thermolysis, as illustrated by 4, which decomposes with AG (290 °C) = 43.0 kcal/mol (ti/2 = 50 min.) When oxygen is placed proximal to the radical stabilizing substituent as in 5, homolysis of the benzhydryl to N bond is greatly facilitated. Now both radicals 6 and 7 benefit from resonance stabilization, reducing AG (150 °C) to 33.3 kcal/mol (ti/2 = 3.2 h). The same effect is... 

The distinguishing characteristic of a free radical is the presence of an unpaired electron. Species with an unpaired electron are said to be paramagnetic. The relative stability of the triphenylmethyl radical allows it to be studied by magnetic susceptibility measurement, which involves weighing it both inside and outside a magnetic field. The unpaired electron makes the radical paramagnetic, so the sample is drawn into the magnetic field. By this technique the dissociation of hexaphenylethane to the triphenylmethyl radical was determined to occur to the extent of 2% in a 0.1 M sample. 

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