Triphenylmethyl radicals, dimerization

The radical dissociation of the Gomberg dimer , [3-(diphenyl-methylidene)-6-(triphenylmethyl)-l,4-cyclohexadiene] [48], is familiar to organic chemists as the original observation of carbon-carbon a bond dissociation in a solution (Gomberg, 1900 Lankamp et al., 1968). The yellow colour of the triphenylmethyl radical in the benzene solution should have been an observation convincing synthetic organic chemists of the stable existence of the triphenylmethyl radical [8-j. 

The importance of establishing the correct structure of the reaction product is best illustrated by the confusion that can result when this has been assumed, wrongly, as self-evident, or established erroneously. Thus the yellow triphenylmethyl radical (3, cf. p. 300), obtained from the action of silver on triphenylmethyl chloride in 1900, readily forms a colourless dimer (m.w. = 486) which was—reasonably enough—assumed to be hexaphenylethane (4) with thirty aromatic ... 

However, when m-DNB was added to a solution of triphenylchloromethane and potassium tcrt-butylate in 2,2-dimethoxypropane, the yield of the substitution product and dimer of the triphenylmethyl radical markedly increased and decreased, respectively (Simig and Lempert 1979). Therefore, the main pathway of the reaction does not involve the ion-radical step. These authors suggested an alternative pathway, which is conformed by a thorough structural analysis of the secondary products formed along with tert-butyl ester of triphenylcarbinole (Huszthy et al. 1982a, 1982b) (Scheme 4.21). 

In the light of the more complete study of ring-halogenated triphenylchloro-methanes in this paper, the free radical hypothesis was back - if it ever was excluded in the previous paper - in the final discussion of the constitution of triphenylmethyl , now with two tautomeric triphenylmethyl radical structures in equilibrium with each other and the Jacobson dimer 1 (Scheme 2). Note that the radical was symbolized by an open valence (a thick line is used here for clarity). The strong results obtained with 3 (Scheme 1) were explained by removal of the quinoid bromine atom from 4 giving a radical 6 which tautomerized to the triphenylmethyl analogue 7. By analogy with the... 

Triphenylmethyl peroxide, 300 rearrangement, 336 Triphenylmethyl radical, 43, 300 dimer, 44, 301, 311 shape,311... 

Historically, the triphenylmethyl radical (1), studied by Gomberg in 1987, is the first organic free radical. The triphenylmethyl radical can be obtained by the reaction of triphenylmethyl halide with metal Ag as shown in eq. 1.1. This radical (1) and the dimerized compound (2) are in a state of equilibrium. Free radical (1) is observed by electron spin resonance (ESR) and its spectrum shows beautiful hyperfine spin couplings. The spin density in each carbon atom can be obtained by the analysis of these hyperfine spin coupling constants as well as information on the structure of the free radical. 

The structure of dimer (2) was characterized by NMR. Thus, one triphenylmethyl radical reacts at the para-position of a phenyl group in another triphenylmethyl radical, not the central sp3 carbon (to form hexaphenylethane), to form dimer (2). However, rra(p-methylphenyl)methyl radical does not dimerize. So, the electronic effect in free radicals is quite large. 

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... 

Radical A, shown here, is related to the triphenylmethyl radical by having oxygen atoms bridging the ortho positions of the phenyl groups. This radical dimerizes much faster than the triphenylmethyl radical. Explain this observation. 

Whilst simple alkyl radicals are extremely short-lived, some other radicals survive almost indefinitely. Such radicals are known as persistent radicals. We mentioned the triphenylmethyl radical on p. 1022 this yellow substance exists in solution in equilibrium with its dimer, but it is persistent enough to account for 2-10% of the equilibrium mixture. 

The dissociation rate of the dimer of the triphenylmethyl radical in 28 solvents was studied by Ziegler el al. [167]. The decomposition rate of azobisisobutyronitrile in 36 solvents was measured by different authors [183-185, 562], Despite the great variety of solvents, the rate constants vary only by a factor of 2... 4. This behaviour is typical for reactions involving isopolar transition states and often indicates, but does not prove, a radical-forming reaction. The lack of any marked solvent effects in most free-radical forming reactions will become more apparent after an examination of some further reactions presented in Table 5-8. 

Ziegler et al. undertook their experiments with a compound which they believed to be hex-aphenylethane [167]. In 1968, it was shown that the dimer of the triphenylmethyl radical is not hexaphenylethane, but l-diphenyhnethylene-4-triphenylmethyl-2,5-cyclohexadiene [168, 169] in accordance with a proposal made by Jacobson in 1905 [170]. 

Fig. 5-9 shows that there is also a very rough, inverse correlation between g k/k(f) and 8l for the dissociation of the dimer of the triphenylmethyl radical [167]. It can be safely assumed that in this unimolecular reaction the molar volume of the activated complex is greater than the molar volume of the reactant, since a bond breaking must occur to some extent on activation. 

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. 

It was nearly ten years before Gomberg s proposal was generally accepted. It now seems clear that what happens is the following the metal abstracts a chlorine atom from triphenylchloromethane to form the free radical triphenyl-methyl two of these radicals then combine to form a dimeric hydrocarbon. But the carbon-carbon bond in the dimer is a very weak one, and even at room temperature can break to regenerate the radicals. Thus an equilibrium exists between the free radicals and the hydrocarbon. Although this equilibrium tends to favor the hydrocarbon, any solution of the dimer contains an appreciable concentration of free triphenylmethyl radicals. The fraction of material existing as free radicals is about 2% in a 1 A/ solution, 10% in a 0.01 M solution, and nearly 100 in very dilute solutions. We could quite correctly label a bottle containing a dilute solution of this substance as triphenylmethyl radicals. ... 

Problem 12.9 The for dissociation of the dimer I has been measured as 11 kcal/mole, the I act as 19 kcal/mole. (a) Draw the potential energy curve for the reaction, (b) What is the energy of activation for the reverse reaction, combination of triphenylmethyl radicals (c) How do you account for this unusual fact (Compare Sec. 2.17.)... 

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