Triphenylmethyl radicals, dimerization reactions

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). 

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 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. 

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. 

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.)... 

The idea that such carbon atoms with seven valence electrons could be involved in organic reactions took firm hold in the 1930 s. Two experimental studies stand out with historical significance in the development of the concept of free-radical chemistry. The work of Gomberg around 1900 provided evidence that when triphenyl-methyl chloride was treated with silver metal, the resulting solution contained PhsC in equilibrium with a less reactive dimeric molecule. It was generally assumed that the triphenylmethyl radical was in equilibrium with hexaphenylethane. Only... 

Consider the dimerization of the triphenylmethyl radical PhaC, which can be written as the reaction... 

For example, in the decomposition of phenylazotriphenylmethane in benzene, triphenylmethyl peroxide is formed if oxygen is present, indicating a radical reaction. The unstable phenyl radical does not persist long enough to dimerize but reacts instead with the solvent. 

Some early examples of a preference for unsymmetrical radical coupling were drawn together in a short paper by Perkins in 1964. ° Intriguingly, the first correct analysis of an example of this kind of behavior dates from Bachmann and Wiselogle clearly understood the role of triphenyl-methyl in accounting for their observations that, in solution at 100°C, pentaphenylethane dissociates rapidly and reversibly into triphenylmethyl and diphenylmethyl, but dimerization of the diphenylmethyl to form detectable quantities of tetraphenylethane (which is stable under the reaction conditions) occurs only very slowly. 

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. 

---