Triarylmethyl ions

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. 

Table 3 E1/2 Values for three triarylmethyl ions in two solvents ...

Dirarylmethyl and triarylmethyl ions (trityl ions) are even more stable than the /er/-butyl ion which is impressively demonstrated by the commercial availability of solid [Ph3C] [BF4] and similar salts. Triphenylchloromethane dissociates in polar, inert solvents such as SO2, and therefore, it is not surprising that El spectra of triphenylmethyl compounds almost exclusively exhibit this ion together with some of its fragments, whereas the molecular ion peak is usually absent. Field desorption allows to circumvent this problem (Chap. 8.5). 

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. 

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. 

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... 

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

In the older literature there are reports of colorless but conducting solutions of triarylmethyl derivatives. It seems well-established that triarylmethyl cations are colored, but an oxonium ion formed by the covalent attachment of a carbonium ion to an ether oxygen atom, for example, would be colorless. 

The ionization of certain substances, notably triarylmethyl cyanides, is promoted by light, so that spectrophotometric analysis for carbonium ions is no exception to the rule that the analyst should be alert for any parallelism between the duration of exposure to light during the analysis and the result.1 7 -1 9... 

Just as interaction of the negative ion with solvent or with a catalyst can promote the ionization, the same is true of the interactions of the carbonium ion. The triaryl carbonium ions discussed so far should all be colored. Reports of colorless but conducting solutions of triarylmethyl derivatives are fairly common and may represent covalently solvated carbonium ions which are not expected to be colorless. For example, a colored solution of the ion XVI in acetic acid slowly fades when it is diluted with methanol containing enough acid to prevent the formation of any "color base. The fading is attributed to the formation of XVII.m... 

Ritchie was the first to directly measure the absolute reactivity of cations toward solvent and added nucleophiles. The cations were highly stabilized examples, triarylmethyl cations bearing stabilizing substituents such as 30 and 31, xanthylium ions (e.g., 32) and tropylium ions (e.g., 33). The feature (and requirement) of these cations was that they had a lifetime in water such that kinetics could be followed by conventional or stopped-flow spectroscopy whereby one solution containing the pre-formed cation was added to a second solution. The time required to mix these solutions was the important factor and limited measurements to cations with lifetimes longer than several milliseconds. The lifetimes in water for 30-33 are provided below. Lifetime is defined as the reciprocal of the first-order rate constant for the decay of the cation in solvent. 

Even for a series with varying aromatic substituents, the correlations with deviate significantly from linearity. Typical behavior is illustrated with data for monosubstituted triarylmethyl cations in Figure 1.2. Significant deviations are observed in the points for the para n donors. Moreover, these deviations are in the direction that indicates that these substituents have kinetic stabilizing effects greater than indicated by ct+. In fact, there are good correlations with a parameter based on NMR chemical shifts of benzylic-type cations obtained under stable ion conditions. 

In the presence of solvent alone, the lifetime of the intermediate of the stepwise reaction of X-l-Y in the narrow borderline between the S l and Sn2 substitution reactions of azide ion (—0.32 < a" " < —0.08, Fig. 2.2) is 1/ = 10 ° s. Azide ion is 10°-10 -fold more reactive than water toward triarylmethyl carboca-tions and related electrophiles, and this selectivity is independent of carbocation reactivity, so long as the reactions of both azide ion and solvent are limited by... 

Jencks and Richard, and others, had pioneered the use of the azide elock to quantitatively assess the lifetime of carbenium ions generated under solvolytic conditions. The method relies on the use of product yield data collected at varying [N3 ] to determine the N /solvent selectivity, expressed as the ratio of the second-order rate constant for trapping of the ion by N3 and the pseudo-lirst-order rate constant for trapping of the ion by solvent k Jk. The assumption is made that k z is diffusion limited at ca. 5 x 10 M"" s This assumption allows k to be estimated, and l/kg provides the lifetime of the ion in the solvent in the absence of added nucleophiles. McClelland and Steenken showed by direct measurement of k. , for a series of diarylmethyl and triarylmethyl carbocations that k is approximately constant at... 

If this is the case, substituent effects in the distal ring of a 4-biphenylyl-nitrenium ion should resemble those of a benzylic carbenium ion. Ren and McClelland showed that substituent effects on log for a series of distally substituted 4-biphenylylnitrenium ions (75n, 75p-v) were carbenium-ion-like. A plot of log kg vs. cr was not highly scattered but was non-linear and similar to those previously described for diarylmethyl and triarylmethyl... 

Accordingly, we might expect triphenylmethyl (or trityl) halides, (C6H5)3C—X, to be even more reactive. In fact, the C-X bonds of such compounds are extremely labile. In liquid sulfur dioxide, triarylmethyl halides ionize reversibly, although the equilibria are complicated by ion-pair association ... 

Most carbocations are too reactive to be directly observable in ordinary solvents, and until relatively recently evidence has been obtained indirectly, primarily through the study of reaction kinetics and trapping processes, experiments discussed in Sections 5.1, 5.2, and 5.4. Nevertheless, a few types of compounds have long been known to produce observable concentrations of positive ions relatively easily. The triarylmethyl derivatives were the first of this type to be investigated the halides ionize readily in non-nucleophilic solvents such as sulfur dioxide,70 and the alcohols yield solutions of the ions in concentrated sulfuric acid. Early observations by the freezing-point depression technique (see Section 3.2, p. 130) established that each mole of triphenyl carbinol yields 4 moles of ions in sulfuric acid, the reaction presumably being by way of Equation 5.14.71 Results in methane-sulfonic acid are similar.72... 

The cryoscopic method is also applicable to other triarylmethyl systems, to some diarymethyl and allylic ions, and, when ortho substituents are present, to aryl acylium ions (20) (Equation 5.15) 73 unfortunately, side reactions frustrate most attempts to generate carbocations in sulfuric acid. 

Although formally considered a 1,5-dication, 50 possesses a structure in which the carbenium centers are constrained at a distance of separation of 3.11 A. NMR studies show the carbenium ion centers at 513C 207.7, consistent with the carbocationic structure 50. In cyclic voltamographic analysis, the compound 51 shows an especially high oxidation potential (two-electron oxidation peak at 1.10 V), when compared to analogous dications and triarylmethyl monocations.19 It has also been shown that... 

This type of map can be used to discuss the different types of nucleophilic displacement reaction. Using the simplified version shown in Fig. 2 we have already seen that SN1 reactions, for instance the solvolysis of triarylmethyl halides, go through the separated ions in the top right-hand corner (Swain et al., 1953 Ritchie, 1971). At the opposite extreme, nucleophilic substitution at centres where the number of ligands can be increased may proceed over the bottom left-hand corner of the diagram. Examples are acyl transfer reactions and substitution at tetrahedral phosphorus centres (Alder et al., 1971) as well as substitution at square planar transition metal compounds (Wilkins, 1974). The nucleophilic reactions studied by Ritchie (1976), for which the rate... 

Thermal and photochemical decomposition of triarylmethyl azides proceeds by loss of molecular nitrogen. In contrast, the electron impact-induced fragmentation occurs by loss of N3, possibly because of the great stability of triarylmethyl cations. However, decomposition of the molecular ion of azidophenylacetic acid involved loss of N2 and CO2 as in the photochemical reaction (138). In the mass-spectrometric... 

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