Triarylmethyl

The triarylmethyl cations are particularly stable because of the conjugation with the aryl groups, which delocalizes the positive charge. Because of their stability and ease of generation, the triarylmethyl cations have been the subject of studies aimed at determining the effect of substituents on carbocation stability. Many of these studies used the characteristic UV absorption spectra of the cations to determine their concentration. In acidic solution, equilibrium is established between triarylearbinols and the corresponding carbocations. 

The diarylmethyl cations listed in Table 5.1 are 6-7pATr+ units less stable than the corresponding triarylmethyl cations. This indicates that the additional aryl group has a cumulative, although not necessarily additive, effect on stability of the carbocation. Primary benzylic cations (monparylmethyl cations) are generally not sufficiently stable for determination of pATr+ values. A particularly stable benzylic ion, the 2,4,6-trimethyl-phenylmefliyl cation, has a pATr+ of — 17.4. 

A wide range of caibocation stability data has been obtained by measuring the heat of ionization of a series of chlorides and cafbinols in nonnucleophilic solvents in the presence of Lewis acids. Some representative data are given in Table 5.4 These data include the diarylmediyl and triarylmethyl systems for which pX R+ data are available (Table 5.1) and give some basis for comparison of the stabilities of secondary and tertiary alkyl carbocations with those of the more stable aryl-substituted ions. 

P-p-C H4Ph2CCl, Pyr, 25°, 5 days, 90%, where P = styrene-divinyl-benzene polymer. Triarylmethyl ethers of primary hydroxyl groups in glucopyranosides have been prepared using a polymeric form of triphenylmethyl chloride. Although the yields are not Improved, the workup is simplified. 

This group was developed for protection of the 5 -OH group in nucleosides. The derivative is prepared from the corresponding triarylmethyl chloride and is cleaved by reductive cleavage (Zn/AcOH) of the phenacyl ether to the / -hydro-xyphenyldiphenylmethyl ether, followed by acidic hydrolysis with formic acid." ... 

The cleavage is an S jl reaction that occurs by protonation of the oxygen atom followed by loss of the stable triarylmethyl carbocation. 

There will be a gradual loss of stable radical with these systems as the di- or triarylmethyl radicals produced from the macroinitiator can add monomer, albeit slowly.99 100 This side reaction provides a mechanism for mopping up the excess stable radical formed as a consequence of termination between propagating radicals and may be essential to maintaining polymerization rates. 

A plot depicting isokinetic relationships, (a) The thermal rearrangement of triarylmethyl azides, reaction (7-35) is shown with different substituents and solvent mixtures. The slope of the line gives an isokinetic temperature of 489 K. Data are from Ref. 8. (b) The complexation of Nr by the pentaammineoxalatocobalt(III) ion in water-methanol solvent mixtures follows an isokinetic relationship with an isokinetic temperature of 331 K. The results for forward (upper) and reverse reactions are shown with the reported standard deviations. Data are from Ref. 9. 

On the other hand, the stability of positive and negative triarylmethyl ions is due to the same resonance effect as that of the neutral radicals discussed in this paper. 

A number of benzylic cations have been obtained in solution as SbFg salts, Diarylmethyl and triarylmethyl cations are still more stable. Triphenylchloro-methane ionizes in polar solvents that do not, like water, react with the ion. In SO2, the equilibrium... 

Triarylmethyl cations (14) are propeller shaped, though the central carbon and the three ring carbons connected to it are in a plane The three benzene rings cannot all be in the same plane because of steric hindrance, though increased resonance energy would be gained if they could. 

There is some evidence in favor ° of the captodative effect, some of it is from ESR studies. However, there is also experimental and theoretical evidence against it. There is evidence that while FCH2 and p2CH are more stable than CH3, the radical Cp3- is less stable that is, the presence of the third F destabilizes the radical. " Certain radicals with the unpaired electron not on a carbon are akso very stable. Diphenylpicrylhydrazyl is a solid that can be kept for years. We have already mentioned nitroxide radicals. Compound 29 is a nitroxide radical so stable that reactions can be performed on it without affecting the unpaired electron (the same is true for some of the chlorinated triarylmethyl radicals mentioned above ). ot-Trichloromethylbenzyl(rer/-butyl)aminoxyl (30) is extremely stable. In... 

For a review of crystal-structure determinations of triarylmethyl cations and other carbocations that can be isolated in stable solids, see SundaraUngam, M. Chwang, A.K. in Olah Schleyer, Ref. 2, vol. 5, p. 2427. 

In the paper published in 1900, he reported that hexaphenylethane (2) existed in an equilibrium mixture with 1. In 1968, the structure of the dimer of 1 was corrected to be l-diphenylmethylene-4-triphenylmethyl-2,5-cyclohexadiene 3, not 2 [38]. Since Gomberg s discovery, a number of stable radicals have been synthesized and characterized, e.g., triarylmethyls, phenoxyls, diphenylpicryl-hydrazyl and its analogs, and nitroxides [39-43]. The radical 1 is stable, if oxygen, iodine, and other materials which react easily with it are absent. Such stable radicals scarcely initiate vinyl polymerization, but they easily combine with reactive (short-lived) propagating radicals to form non-paramagnetic compounds. Thus, these stable radicals have been used as radical scavengers or polymerization inhibitors in radical polymerization. 

In addition to Gomberg s original method using the reaction of the triarylmethyl halide with copper, zinc, or silver amalgam in an inert solvent, a number of other methods have been used. Table II shows some of them. Some of the methods amount to oxidation of a... 

Reaction with other radicals is characteristic of the triarylmethyls and the chain reaction with oxygen has already been discussed. The absorption of the radical nitric oxide is also used as an analytical tnethod for triphenylmethyl. 

When one of the aromatic groups of the triarylmethyl free radical is replaced by an alkyl group, a decrease in stability due to a loss of resonance stabilization is to be expected. The paramagnetism and reactions associated with these less stable radicals will therefore appear only when the ethane is heated well above room temperature, the dissociation being endothermic. The rate of formation, but not the equilibrium constant, is experimentally accessible for these radicals since the radical once formed is subject to rearrangement, cleavage, and disproportionation reactions ... 

The same thing happens when alkyl halides are treated with silver in the presence of oxygen.41 The reaction with silver in the absence of oxygen is of course one of the methods used to prepare the stable triarylmethyl free radicals. 

The decomposition of t.ert-bvA.y triarylmethyl peroxides is particularly well suited to the measurement of migration aptitudes in oxygen radicals because the nature of the product leaves no doubt that the reaction is in fact a rearrangement rather than a /9-cleavage. 

It seems reasonable to assume that Ka, the dissociation constant of the ion pairs, is very nearly the same for all the various triarylmethyl... 

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