Moses Gomberg - Discovery of Persistent Radicals

Discovery of Persistent Radicals

Seeking to prepare hexaphenylethane (5), Gomberg attempted a Wurtz coupling of triphenylmethyl chloride (1). Elemental analysis of the resultant white crystalline solid, however, uncovered discrepancies with the predicted molecular formula:

Hypothesizing that (1) had combined with molecular oxygen to form the peroxide (4), Gomberg found that treatment of (1) with sodium peroxide was another means of synthesizing (4).

By performing the reaction of triphenylchloromethane with zinc under an atmosphere of carbon dioxide Gomberg obtained the free radical (2). This compound reacted readily with air, chlorine, bromine and iodine. On the basis of his experimental evidence Gomberg concluded that he had discovered the first instance of a persistent radical and trivalent carbon. This was a controversial conclusion for many years as molecular weight determinations of (2) found a value that was double that of the free radical. Gomberg postulated that some non-tetravalent carbon structure existed in solution because of the observed activity towards oxygen and the halogens. Gomberg and Bachmann later found that treatment of "hexaphenylethane" with magnesium resulted in a Grignard reagent, the first instance of the formation of such a compound from a hydrocarbon. Studies of other triarylmethyl compounds gave results similar to Gomberg's, and it was hypothesized that (2) existed in equilibrium with its dimer hexaphenylethane (5). However this structure was later disproven in favor of the quinoid dimer (3).

At the end of his first report of trivalent carbon "On Trivalent Carbon"reference 5 Gomberg wrote "This work will be continued and I wish to reserve the field for myself." While nineteenth-century chemists respected such claims Gomberg found that the field of chemistry he founded was too rich to reserve for himself.