William 6. Smith Texas Christian University Fort Worth, Texas 76129
I I
The Hexaphenylethane Landmark Falls
The history of chemistry is replete with fundamental discoveries of such major importance that they form a body of facts found in all textbooks. Such certainly has been the case for Gomherg's discovery' that the treatment of triphenylmethyl chloride with silver in an inert solvent yields the relatively stable triphenylmethyl free radical and its association product hexaphenylethane. The details of the reaction are found in most elementary organic texts, while more advanced works discuss the various ways in which the equilibrium constant has been measured as well as measurements of the rate of association. From studies at various temperatures it has been calculated that the association process has the rather high activation energy of 8 kcal/mole--a fact usually ascribed to the steric effects of bringing two triphenylmethyl radicals close enough to form the central bond of the ethane. There seems to have been no serious reexamination over the years prior to 1968 of Gomberg's postulation that the correct process was 2(CaHdaC'
group of chemists in the Netherlands have reinvestigated the original reaction studied by Gomberg. Their resultsS clearly indicate that the dimer formed when triphenylmethyl chloride is treated with silver is not hexaphenylethane. The products have a uv maximum a t 315 mp well above that for monoalkylhenzenes. More pertintent u7as the nmr spectrum which was found to have aromatic protons at 6.8-7.4 ppm but a crude AB quartet at 5.8-6.4 ppm and a singlet a t 5 ppm. Further insight was gained from behavior of the di(2,6-dimethylpheny1)methyl system. Treatment of the appropriate chloride with silver gave an analog to the dimer above. Production of the radical by the V2+ ion acting on the carhinol in acetone-sulfuric acid gave a material having the uv and nmr expected of the ethane. Based on these data, it was proposed that the dimer generated from triphenylmethyl chloride and silver was
(CsHdaC-C(C6Hs)a
However, over the years data collected which suggested a possible flaw in the hypothesis. For instance, tri-(4-tbuty1phenyl)methyl does not associate to its corresponding ethane2 though it is not clear why the 1-hutyl groups in the para position should prevent this. Similarly, in the diphenylmethyl family the introduction of one para alkyl group does not affect the association constant, but the presence of two greatly diminishes the amount of ethane. Acting on suspicions engendered by these facts a GOMEERG, M., Chem. Be?., 33, 3150 (1900). SELWOOD, P. W., AND D o e ~ a sR. , M., J . Am. Chem. Soe., 72, 3860 (1950). * LANKAMP, H., NAUTA, W. T., AND h f A c L ~ . 4C., ~ , Tetrahedmn Letters, 249 (1968). ' SMITH,W. B., AND KMET,T. J . , J . Phys. Chm., 70, 4084 (1966).
The formation of the ethane by the reduction of the carbonium ion presumably involves a complexed metal ion. The results of the corresponding reaction with triphenyl carhinol have yet to be reported. The nmr spectrum of the triphenylmethyl dimer has been examined in more detail in our laboratories. The spectrum in carbon tetrachloride is as reported. Properly speaking the protons on the cyclohexadiene ring form an A2B& system. The spectrum is broadened by the presence of the paramagnetic triphenylmethyl radical. On spin decoupling the singlet C due to the aliphatic proton, the diene protons appear as a broadened AB quartet with chemical shifts a t 5.89 and 6.21 ppm and with J 10.5 cps. This value of J is in excellent agreement with other cross conjugated 1,4-cyclohexadienes4 and offers further proof that the assigned structure of the dimer is correct.
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The implications of these results are manifold. Clearly hexaphenylethane has yet to he synthesized. The data for associat,ion constants and rates need to be
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Journol o f Chemical Education
reviewed in the context of these new findings. I t might he said the whole area of triphenylmethyl chemistry has been reopened. A chemical landmark has fallen, and textbooks will have to he revised.
