Triphenylmethane carbonium ion

Arylmethane leuco dyes are converted into di- or triarylmethane dyes on oxidation. This class of dye precursors sometimes is referred to as leuco di- or triphenylmethane dyes, or di- or triphenylmethane leuco dyes. The use of the term di- or triarylmethane dyes can be misleading as the central carbon atom is a carbonium ion. Instead, the term di- and triarylmethine dye is recommended for this class as it correlates with the well-known polymethine dyes. Nevertheless, it has not been commonly used. 

Hirschler and Hudson (36/6), however, favor the opinion that Bronsted sites are exclusively responsible for the activity of silica-alumina. In studying the adsorption of perylene and of triphenylmethane, they concluded that carbonium ions are not formed by a hydride abstraction mechanism as claimed by Leftin (362). Instead, triphenylmethane is oxidized by chemisorbed oxygen to triphenylcarbinol in a photo-catalyzed reaction, followed by reaction with a Bronsted acid giving water and a triphenylmethyl carbonium ion. After treatment with anhydrous ammonia, the organic compound was recovered by extraction as triphenylcarbinol. About thirteen molecules of ammonia per assumed Lewis site were required to poison the chemisorption of trityl ions. The authors explain the selective inhibition of certain catalyzed reactions by alkali by assuming that only certain of the acidic protons will ion-exchange with alkali ions. 

Hydride Abstraction from Organic Ligands The removal of a hydride ion from an organic radical is an important method of generating carbonium ions stabilized by metal carbonyl systems. Dauben and Honnen (61) in 1958 were the first to exploit this method by use of the powerful hydride abstractor, triphenyl methyl (or trityl) carbonium ion, which is converted thereby into triphenylmethane. 

Oxidmion of ketone acetals and ethers. Ketones can be regenerated from the ethylene acetal derivatives by treatment with trityl fluoroborate in dry dichloro-methanc (Nj) at room temperature. Thus the reaction of trityl fluoroborate with cyclohexanone ethylene acetal results in cyclohexanone (80%) and triphenylmethane. The reaction thus involves hydride transfer to the triphenyl carbonium ions. Triethyl-oxonium fluoroborate can also be used but is somewhat less effective than trityl fluoroborate. 

A study was made of the ultraviolet spectra of benzene, alkyl-, amino-, and nitro-derivatives of benzene, diphenyl-amine, triphenylmethane, triphenylcarbinol, and anthra-quinone adsorbed on zeolites with alkali exchange cations, on Ca- and Cu-zeolites, and on decationized zeolites. The spectra of molecules adsorbed on zeolites totally cationized with alkali cations show only absorption bands caused by molecular adsorption. The spectra of aniline, pyridine, triphenylcarbinol, and anthraquinone adsorbed on decationized zeolite and Ca-zeolite are characterized by absorption of the corresponding compounds in the ionized state. The absorption bands of ionized benzene and cumene molecules appear only after uv-excitation of the adsorbed molecules. The concentration of carbonium ions produced during adsorption of triphenylcarbinol on Ca-zeolite and on the decationized zeolite depends on the degree of dehydroxyla-tion of the zeolite. 

That the observed spectrum was the result of a chemical reaction between the hydrocarbon and the catalytically active centers of the silica-alumina surface (chemisorption), and not due to a general sur-fatochromic spectral shift, was demonstrated from the spectrum of this compound adsorbed on a nonacidic or very weakly acidic silica gel (29). The spectrum (Fig. 30, Curve B) of silica gel exposed to triphenylmethane vapor for 1000 hours at 100°C was identical to the spectrum (Curve A) of an alcoholic solution of this hydrocarbon. The close agreement between these spectra suggested that on silica gel the triphenylmethane was physisorbed. This was further evidenced by the marked loss of spectral intensity (Curve C) attendant to a four hour evacuation at 100°C. In contrast, on silica-alumina where the hydrocarbon was chemisorbed as the carbonium ion, no decrease in absorbance was noted even after 48 hr evacuation at 275°C. These data constituted the first direct demonstration of the formation of carbonium ions as a consequence of chemisorption of a tertiary hydrocarbon on the surface of a cracking catalyst by a reaction involving the rupture of an aliphatic C-H bond. The generality of this process of carbonium ion... 

Using the triphenylcarbonium ion as a probe it was possible to demonstrate (82) that mechanism A-l could obtain and to exclude A-2 and B from further consideration. It should be noted that if prior olefin adsorption were a necessary prerequisite for paraffin adsorption, then carbonium ions could not be formed from paraffin molecules in a system rigorously freed from olefin or olefin-forming impurities. As triphenylmethane itself is certainly not an olefin precursor, there remained only two possible sources of olefinic impurity to be considered namely, the catalyst surface and the reagents employed. The former was discounted on the grounds that it was highly improbable that either olefins or carbonium ions on the surface survived the ex-... 

Fio. 32. Chemisorption of triphenylmethane (1.8 X 10 4gm) on silica-alumina (8.9 meter ). Complete chemisorption would correspond to 5 X 101 carbonium ions per square centimeter of surface. 

Triphenylme thane dyes, such as xylenol orange and methylthymol blue, are widely used in spectrophotometiic determination of rare earths. Veber (1979, 1981) studied several triphenylmethane dyes by polarographic methods. It was found that the quinoid group and the carbonium ion are responsible for the reduction wave in polarography. Zhang et al. (1984) investigated and compared the polarographic... 

Some of the intermediate steps in the synthesis of a dipyrrylmethene have been studied chemically (36) and the spatial changes are similar to those postulated by Michaelis (93) for the triphenylmethane dyes. The condensation of the pyrrole-a-aidehyde (5) goes through an intermediate carbinol stage (4) in which the central carbon atom is bound tetrahedrally. The carbinol readily loses hydroxyl ion in the presence of very weak acid to form the planar carbonium ion 5), which represents one of the resonance forms of dipyrrylmethene usually written as (6),... 

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