Hydrocarbons Gomberg

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%. 

Fig. 1.8. Reversible formation reaction of the triphenylmethyl radical. The equilibrium lies on the side of the Gomberg hydrocarbon.

Treatment of triphenylmethyl chloride with silver gave not the expected hydrocarbon but an oxygen-containing compound later found to be the peroxide. The reaction run in an inert atmosphere did give a hydrocarbon, but one with unusual properties. It reacted rapidly with oxygen, chlorine, and bromine, and was quite different from tetra-phenylmethane or what was expected of hexaphenylethane. Gomberg... 

The initial product of the inhibition step is not known in this case and may be a molecular complex.8 The direct reaction of the ethane with the peroxy radical is an example of a covalent compound giving a reaction resembling that of a related free radical. The molecular weight determination by Gomberg was therefore a necessary part of the proof that he was dealing with radicals and not merely an unusually reactive hydrocarbon. The presence of free radicals has since been confirmed by measurements of the paramagnetic susceptibility and the paramagnetic resonance absorption.9-10 The latter evidence also rules out an alter-... 

Gomberg-Bachmann-Hey reaction org chem Production of diaryl compounds by adding alkali to a mixture of a diazonium salt and a liquid aromatic hydrocarbon or a derivative. gom,berk bak,man hT re.ak shan )... 

A seemingly minor technical problem, the ability of triphenylmethyl to pick up virtually any solvent as solvent of crystallization, occupied Gomberg for some time and led him into consideration of then fashionable structures involving tetravalent oxygen, which were later abandoned. Another sidetrack, more serious in view of the absence of a useful theory, was caused by experiments based on the known fact that triphenylchloromethane showed salt-like conductivity in solution in liquid SO2 It was thus definitively established that there are carbonium salts in the true sense of the definition applied to salts. When triphenylmethyl was dissolved in liquid SO2, it was found that it too conducted the electric current quite well. " How should one explain this strange phenomenon, a hydrocarbon behaving like an electrolyte ... 

Gomberg correctly hypothesized that the hydrocarbon is in equilibrium with triphenylmethyl radicals ... 

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... 

Only moderate yields of biphenyls result from the Gomberg reaction, in which an acidic solution of a diazonium salt is made alkaline with sodium hydroxide solution in a two-phase mixture with an aromatic hydrocarbon such as benzene (Scheme 8.20). A variation of this reaction uses a copper catalyst. The reaction is more successful in the intramolecular version and the Pschorr reaction (discussed in Chapter 12) offers a useful route to phenanthrene. 

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. ... 

In view of the subsequent work, the content of the actual monomeric diradicals in Schlenk hydrocarbons was rather miniscule. For Gomberg radical, the percentage of the monomeric radical in dilute solutions is relatively small (K msoc 10-4 M-1 at 293 K) 31 thus, the content of the monomer for the corresponding diradicals is likely to be much smaller that less than few percent for triphenylmethyl in dilute solutions. Also, the reported violet color of 8 is unlikely to correspond to monomeric diradical.63... 

In 1901 Gomberg discovered a hydrocarbon the behavior of which was best explained on the assumption that it contained a single trivalent carbon atom. The discovery aroused the interest of the whole chemical world as the existence of a hydrocarbon containing a trivalent carbon atom and an uneven number of hydrogen atoms was contrary to all previous experience. The intensive study of the new hydrocarbon, triphenylmethyl, (CeHs) sC, (439), and similar compounds has extended our knowledge of the valence of the carbon atoms. The compounds of this class, however, have a unique arrangement of atoms and their existence has not altered the older views as to the structure of unsaturated compounds which consist of straight chains of carbon atoms. 

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 ). 

Carbenes were the latecomers to the company of stable examples of organic reaction intermediates. The 100 anniversary of Gomberg s discovery of the stable triphenylmethyl radical was celebrated in 2000, and Gomberg and others were familiar with the triphenylmethyl carbocation by 1910. Stable anions derived from hydrocarbons like the fluorenyl anion were being investigated by Conant and McEwan in the decade 1930-40, but the first isolable species which might be considered to be carbenes were not reported until 1988 by Bertrand... 

---