Toluene cations

Since the latter conditions pertain to aromatic nitration solely via the homolytic annihilation of the cation radical in Scheme 16, it follows from the isomeric distributions in (81) that the electrophilic nitrations of the less reactive aromatic donors (toluene, mesitylene, anisole, etc.) also proceed via Scheme 19. If so, why do the electrophilic and charge-transfer pathways diverge when the less reactive aromatic donors are treated with other /V-nitropyridinium reagents, particularly those derived from the electron-rich MeOPy and MePy The conundrum is cleanly resolved in Fig. 17, which shows the rate of homolytic annihilation of aromatic cation radicals by NO, (k2) to be singularly insensitive to cation-radical stability, as evaluated by x. By contrast, the rate of nucleophilic annihilation of ArH+- by pyridine (k2) shows a distinctive downward trend decreasing monotonically from toluene cation radical to anthracene cation radical. Indeed, the... 

In acetonitrile (AN), the toluene cation-radical has high thermodynamic acidity, its pK is between -9 and -13 (Nicholas and Arnold 1982). In the same solvent (AN), neutral toluene has... 

To elevate p-selectivity in nitration of toluene is another important task. Commercial production of p-nitrotoluene up to now leads with twofold amount to the unwanted o-isomer. This stems from the statistical percentage of o m p nitration (63 3 34). Delaude et al. (1993) enumerate such a relative distribution of the unpaired electron densities in the toluene cation-radical—ipso 1/3, ortho 1/12, meta 1/12, and para 1/3. As seen, the para position is the one favored for nitration by the attack of NO (or NO2 ) radical. A procednre was described (Delande et al. 1993) that used montmorillonite clay supported copper (cupric) nitrate (claycop) in the presence of acetic anhydride (to remove excess humidity) and with carbon tetrachloride as a medinm, at room temperature. Nitrotoluene was isolated almost quantitatively with 23 1 76 ratio of ortho/meta/para mononitrotoluene. 

Formation of radicals having a lower energy than that of the starting cation radicals is obviously favorable for their deprotonation. The cation radicals of toluene and other alkylbenzenes are illustrative examples. As shown (Sehested Holcman 1978), these cation radicals lose protons even in very acidic aqueous solutions. The deprotonation rate does not, in general, depend on the medium acidity. In acetonitrile (AN), the toluene cation radical has high thermodynamic acidity, its pKa is between 9 and -13 (Nicholas ... 

Medium acidity is not essential for the deprotonation of the toluene cation radical, but medium basicity provokes the abstraction of the cation radical proton. Although this effect is obvious, special experiments were undertaken to define it more accurately (Bernstein 1992 and references therein). The studies show that at least three water molecules are re-... 

This reaction scheme is applicable to C H bond activation by strong electron acceptors. For example, the formation of benzyl acetate from toluene using Co111 as the oxidant has been shown to occur via the toluene cation radical [41] ... 

Aryl and aliphatic amines are perhaps the best known D molecules primarily due to the availability of the nonbonded electrons on nitrogen for PET processes. The simplest bond cleavage reaction for amine cation-radicals is heterolytic C—H cleavage or deprotonation. It is well known that the acidity of protons on a species after loss of an electron, i.e., after one-electron oxidation, is greatly enhanced [7]. For example, the pKa of the benzylic protons in the toluene cation-radical have been estimated to be —10 [7] compared to pKa value of 35 before oxidation. The process of proton loss can be extremely fast, on the time scale of picoseconds [8]. 

The product distribution in the decomposition of the perHuoro-toluene Cation salt similarly indicates that the stoichiometries are as follows... 

Similarly, toluene was found to be quite differently sorbed on the two zeolites investigated. The most striking differences were seen in the IR spectra of the sorbed species in the region of the CH stretching vibrations (see Fig.2). In both cases, the coverage of toluene per sorption site (Si-OH-Al, alkali cation) was lower than for methanol under otherwise identical conditions (0.5 toluene/site in HZSM5 and 0.25 toluene/cation in Rb-X at p, i e t = 0.001 mbar). 

The flash photolysis of toluene cations has been shown to yield two forms of C7H7+ ions, one of which can react to give C8H9+ [reaction (33)].61 The reactive form was shown to be the benzylium form, the non-reactive one the tropylium ion. 

Ledwith and Russell (1974b) have found that chlorination of benzene, toluene and other aromatic molecules is easily achieved in aqueous acetonitrile containing sodium peroxydisulfate and copper(II) chloride. Toluene, for example, gives no benzyl chloride but a mixture of chlorotoluenes (58% o-, 4% m-, and 38% p-) consistent with the spin distribution in the toluene cation radical. The amount of copper chloride used can be catalytic provided another source of chloride ion (LiCl) is added. Reaction is attributed to the very fast transfer of chlorine atom from copper(II) chloride to the cation radical (132) the metal halide is thus regarded as a trap for the aromatic cation radical. In the absence of copper(II) chloride, reactions of toluene with peroxydisulfate ion and chloride ion give... 

Scheme VIILI2. Transformations of the toluene cation radical.

Organic solvents, i.e., acetone, hexane, or toluene cationic and anionic detergents... 

Table 1. Mean PL decay time in nanoseconds for as received CdSe/ZnS QDs (Jo) in toluene, cationic QDs (r+), anionic QDs (j°) in aqueous solutions and in aqueous solutions with oppositely charged polyelectrolytes (t+pss) and (

Bott, S.G. Coleman, A.W. Atwood, J.L. Inclusion of both cation and neutral molecule hy a calixarene. Structure of [p-rerr-butylmethoxycalix[4]arene-sodium-toluene] cation. J. Am. Chem. Soc. 1986. 108. 1709-1710. 

Weisshaar and co-workers recorded ZEKE spectra of toluene through different intermediate resonances of and they are presented in Figure 9. From the assignment it can be shown that there is a positive with lower in energy than the >a2 torsional state. The torsional states of the toluene cation (0, 15, 54 and 75 cm i) are not very different from the torsional states in the state (1,15, 55 and 77 cm i), leading to the conclusion that the torsional barriers in the cation and in are quite similar. 

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