Triphenylmethyl cation stability

The N+ relationship, as discussed above, is a systematization of experimental facts. The equation of Scheme 7-4 has been applied to nearly 800 rate constants of over 30 electrophiles with about 80 anionic, neutral, and even cationic nucleophiles covering a range of measured rate constants between 10-8 and 109s 1 (Ritchie, 1978). Only about a dozen rate constants deviated from the predicted values by more than a factor of 10, and about fifty by factors in the range 5-10. It is therefore, very likely that this correlation is not purely accidental. Other workers have shown it to be valid for other systems, e.g., for ferrocenyl-stabilized cations (Bunton et al., 1980), for coordinated cyclic 7r-hydrocarbons (Alovosus and Sweigart, 1985), and for selectivities of diarylcarbenes towards alkenes (Mayr, 1990 Mayr et al., 1990). On the other hand, McClelland et al. (1986) found that the N+ relationship is not applicable to additions of less stable triphenylmethyl cations. 

One of the most stable carbocation structures is the employing all three rings. Trityl chloride ionizes read-triphenylmethyl cation (trityl cation). In this struc- ily, and can capture an available nucleophile, ture, the positive charge is stabilized by resonance... 

Additional phenyl substituents stabilize carbocations even more. Triphenylmethyl cation is particularly stable. Its perchlorate salt is ionic and stable enough to be isolated and stored indefinitely. 

The relationship between charge density and the NMR chemical shifts is, however, only qualitative and should be used with caution. Other factors, such as neighboring anisotropic effects of the substituents, should also be considered. The cationic center of triphenylmethyl cation (<5I3C 211.2 ppm), for example, is much shielded from that of tricyclopropylmethyl cation (270.9 ppm), which may erroneously lead to the conclusion that a phenyl is more stabilizing than the cyclopropyl group. 

Photoheterolytic cleavage of benzyl alcohol in neutral media occurs only when it gives rise to highly stabilized cations, such as triphenylmethyl cations (compare Sec. 15.1 the reaction may be at least in part adiabatic in this case and yield the excited cation) [79-81], xanthyl [82] or fluorenyl cations [83]. However, acid catalysis is effective and methyl ethers are by far the main products from benzyl alcohols in acidified aqueous methanol [84-88]. Electron donating substituents in the ortho and, to a lesser extent, in the meta position enhance the quantum yield (acid catalysis may be... 

Swain et al. (1953b) noted that a qualitative relationship exists between the stability of a carbocation and its selectivity. For example, the selectivity of a number of carbocations in aqueous solution and in the presence of azide ion was enhanced with increasing carbocation stability the ratio An /Aw, where An and Aw are the specific rate constants with azide ion and water respectively, was found to increase from 3-9 for the t-butyl cation to 170 for the diphenylmethyl cation, to 240 for the 4,4 -dimethyldiphenylmethyl cation, and to 280,000 for the highly stabilized triphenylmethyl cation. Sneen et al. (1966a) observed that this relationship could be quantified. It was found that a plot of log (An /Aw ) against log A (where A is the solvolytic rate constant) for a number of alkyl chlorides gave a linear correlation. Sneen made the first attempt to utilize such a relationship as a mechanistic tool. The selectivity of... 

Initiation with Triphenylmethyl Cation. Triphenylmethyl (trityl) cation derives its stability (7, 30) from resonance between the electro-... 

If a benzylic cation is bonded to more than one phenyl group, the stabilizing effects are additive. An extreme example is the triphenylmethyl cation. This cation is exceptionally stable, with three phenyl groups to stabilize the positive charge. In fact, triphenylmethyl fluoroborate can be stored for years as a stable ionic solid. 

An exceptionally stable cation is formed when three benzene rings can help to stabilize the same positive charge. The result is the triphenylmethyl cation or, for short, the trityl cation. The symbol Tr (another of these organic elements ) refers to the group Ph3C. Trityl chloride is used to form an ether with a primary alcohol group by an SnI reaction. Here is the reaction. 

Triphenylmethyl cation (10 ) is effectively stabilized by electron donating substituents. B. W. Laursen et al. reported the highly stable carbocation 11 with a value of 19.7 10). Recently, they reported a similar carbocation with a much higher p R value (23.7) II). In our continuing efforts to prepare extremely stable carbocations, we have investigated the effect of introduction of electron donating substituents into each azulenyl group. 

The results are appreciably different from those based on the analysis of the reactions of monosubstituted triphenylmethyl cations (McClelland et al., 1989). Obviously, this is due to the difficulties of correlating the substituent effects in a simple manner. The preferred results on the trisarylmethyl cation lead to the conclusion that both the forward process of the k ionization and the reverse process of the solvent-recombination step of the carbocation with various nucleophiles can be described to a good approximation by a Y-T a scale with an r value of the transition state which is essentially identical with the intrinsic r value of the thermodynamic stabilities. However, we will consider the triarylmethyl system further following similar analysis of a-arylethyl cations. 

Greater stabilization can be achieved through the attachment of a second phenyl ring, producittg the diphenylmethyl cation. Ph CH . A third ring gives the triphenylmethyl cation, Ph C whidi is even more stable. The triphenylmethane dyes, e.g. 

Carbonium ions1 are of the type R1R2R3C+. The triphenylmethyl cation, one of the earliest known, owes its stability primarily to the fact that the positive charge is highly delocalized, as indicated by canonical structures of the type lO-I(a-d). It behaves in some respects like other large univalent cations (Cs+, R4N+, R4As+, etc.) and forms insoluble salts with large anions... 

The 1,2,3-tri-f-butylcyclopropenium cation is so stable that the perchlorate salt can be recrystallized from water. An X-ray study of triphenylcyclopropenium perchlorate has verified the existence of the carbocation as a discrete ion. ° Quantitative estimation of the stability of the unsubstituted cyclopropenium ion can be made in terms of its pXjj+ value of —7.4, which is intermediate between such highly stabilized ions as triphenylmethyl cation and the M-(4-methoxyphenyl)methyl cation. (See Section 4.4.1 for the definition of pXr+). An HF/6-31G MO calculation on the following isodesmic reaction ... 

Protonation of the olefin, or protonation and subsequent dehydration of the parent alcohol, gives cations which are then subjected to laser excitation or steady-state irradiation. Cations generated in this way were identified by their characteristic absorption spectra, which also indicated cation stability over the time scale of the individual experiments by the lack of change in their absorption spectra. Among the numerous cations generated in acidified solution for photochemical studies are the xanthyl and thioxanthyl [7-15], dibenzosuberenyl [10], triphenylmethyl [10,15], a,co-diphenylpolyenyl [16], and 1,1-diarylethyl [17] cations. Media included acetonitrile acidified with trifluoroacetic acid (TFA-ACN) or aqueous sulfuric acid [7-9,11,14,15], TEA in 2,2,2-trifluoro-ethanol (TFA-TFE) [10,12,13], n-heptane acidified with TFA [9], and BFj-etherate in methylene chloride [16]. The absorption spectral data for several cations have been previously reviewed [6]. Characterization of the cation excited states will be discussed in Section III. 

Carbenium ions have three bonds to the central carbon and are planar, with the bonds directed toward the corners of a triangle (sp hybridization). They have six electrons in outer shell of carbon and a vacant p orbital. Carbenium ions are important intermediates in a number of organic reactions, notably the S l mechanism of NUCLEOPHILIC SUBSTITUTION. It is possible tO produce stable carbenium ions in salts of the type (C H5)3C C1", which are orange-red solids. In these the triphenylmethyl cation is stabilized by delocalization over the three phenyl groups. It is also possible to produce carbenium ions using SUPERACIDS. 

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