Triphenylmethanol

The oxidation of TPA is a good example of the use of in-situ electrochemical ESR to provide details of electrode mechanisms but, at the same time, the above reveals how the results obtained and their subsequent interpretation are easily affected by poor cell design, as is observed from the work of Kondrikov. 

That the fate of the Ph3CO radical was further oxidation and not dimerization was confirmed by in-situ electrochemical ESR experiments. Day performed oxidising-reducing potential sequences as carried out with TPA. When the potential was held at + 2.20 V before stepping to - 1.80 V, the resultant ESR spectrum was identical to that shown in Fig. 29, i.e. a mixture of benzophenone and benzoquinone radical anions produced by the mechanism as detailed earlier for the fate of Ph3CO+ species from TPA oxidation. 

The oxidation of TPM shows how mechanistic detail can be confirmed by coupling electrochemistry with ESR, when looking at reactions where electrochemistry alone can only tentatively suggest details for the electrode mechanism. 

The three examples described above show the use of in-situ electrochemical ESR in the investigation of the mechanism of electrode reactions. The work of Compton et al. [67] upon the reduction of fluorescein in aqueous media illustrates how in-situ techniques can provide information of both electrode mechanism and kinetics. In buffered solutions in the pH range 9-10 Compton et al. observed an apparent two-electron reduction of fluorescein (F) to leuco-fluorescein (L). In-situ electrochemical ESR experiments using the channel electrode revealed an ESR spectra as shown in Fig. 32 which was attributed to semi-fluorescein (S) where 

The diffusion-limited current-solution flow rate behaviour for the reduction of fluorescein at about pH 10 was studied. The observed behaviour, shown in Fig. 33, revealed a transition from two-electron transfer at low flow rates to one-electron transfer at fast flow rates. This behaviour is characteristic of an ECE or DISP process (vide infra), where, at fast flow rates, the product of the first electron transfer is swept away from the electrode before the chemical step takes place and one-electron behaviour is observed. 

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The shifts at the para carbon position for substituted benzenes (and for fris-p-substituted triphenylmethanols) also support the scale as shown by the fitting results of sets 26 (and 27). 

Table 20-1. N-Substituted imidazoles and triazoles prepared by transfer reactions of azolides A—F with tertiary alcohols of the triphenylmethanol type and analogues.

Labelling the aliphatic carbon atom with (53%) allowed comparison of the nuclear magnetic C -shifts of the covalent sp -hybridized triphenyl-C -methanol (in tetrahydrofuran solution) with that of the labelled triphenylcarbonium ions, (C8Hg)3C HS07 (triphenyl-C -methanol in sulphuric acid solution). A shift of 129-6 p.p.m. to lower shielding was observed in the triphenylcarbonium ion, as compared with the covalent triphenylmethanol. 

Example The extraordinary stable trityl ion, PhsC, m/z 243, tends to dominate mass spectra (Chap. 6.6.2). Thus, neither the El spectrum of chlorotriphenyl-methane nor that of its impurity triphenylmethanol show molecular ions (Fig. 8.11). An isobutane PICI spectrum also shows the trityl ion almost exclusively, although some hint is obtained from the Ph2COH ion, m/z 183, that cannot be explained as a fragment of a chlorotriphenylmethane ion. Only FD reveals the presence of the alcohol by its molecular ion at m/z 260 while that of the chloride is detected at m/z 278. Both molecular ions undergo some OH or Cl loss, respectively, to yield the Ph3C fragment ion of minor intensity. 

Fig. 8.11. Comparison of El, PICI, and FD mass spectra of chlorotriphenyknethane containing some triphenylmethanol. By courtesy of C. Limberg, Humboldt University, Berlin.

Carbocations are a class of reactive intermediates that have been studied for 100 years, since the colored solution formed when triphenylmethanol was dissolved in sulfuric acid was characterized as containing the triphenylmethyl cation. In the early literature, cations such as Ph3C and the tert-butyl cation were referred to as carbonium ions. Following suggestions of Olah, such cations where the positive carbon has a coordination number of 3 are now termed carbenium ions with carbonium ions reserved for cases such as nonclassical ions where the coordination number is 5 or greater. Carbocation is the generic name for an ion with a positive charge on carbon. 

Triphenylmethyl fluoroborate is prepared by dissolving 27 g. (0.104 mole) of triphenylmethanol ( purum, Fluka A G) in 260 ml. of propionic anhydride by warming on a steam bath. With an acetone-dry ice bath the solution is cooled to 10° and maintained between 10° and 20° while 31 ml. of 43% w/w fluoroboric acid is added portionwise with swirling. The yellow solid is collected, washed well with dry ether, and dried in a desiccator under vacuum to yield 34 g. (90-99%). The product is very hygroscopic, taking up water with hydrolysis. It is desirable to prepare this reagent immediately before use. 

Triphenylmethanol (45.0 g, 0.17 mol) is added with swirling, giving a bright yellow-orange suspension. The amount of precipitate is increased by adding ethyl acetate (600 mL). Care must be takent to avoid use of an ether in place of ethyl acetate in this step, as this substitution promotes spectacular decomposition of the product. 

In a subsequent study, Shudo and co-workers244 showed that benzaldehydes with electron-withdrawing groups (N02, CF3) react with 2 equivalents of benzene in the presence of triflic acid to give substituted triphenylmethanes in good yields [Eq. (5.90)]. They also observed that pura-fluorobenzaldehyde and biphenyl-4-carboxaldehyde yield diphenylmethane and triphenylmethanol under similar conditions, and the same products were also isolated in the reaction of triphenylmethane (Scheme 5.29). 

Alternatively it may be prepared from triphenylmethanol (10 g) by heating under reflux in dry benzene (5 ml) with redistilled acetyl chloride (6.0 ml) for 30 minutes. The mixture is cooled, diluted with light petroleum (10 ml, b.p. 

Triphenylmethyl bromide, m.p. 153-154 °C, may be prepared in a similar manner from triphenylmethanol and acetyl bromide. 

The yield of phenyllithium generally exceeds 95 per cent. One interesting and instructive method of determination is to allow the phenyllithium to react with excess benzophenone and to weigh the triphenylmethanol formed which is assumed to be... 

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