Anthracene radical attack

The most vexed subject in this field is the site of radical attack on substituted aromatic rings. Some react cleanly where we should expect them to. Phenyl radicals add to naphthalene 7.36, to anthracene 7.37 and to thiophene 7.38, with the regioselectivity shown on the diagrams. In all three cases, the frontier orbitals are clearly in favour of this order of reactivity (because of the symmetry in these systems, both HOMO and LUMO have the same absolute values for the coefficients). 

The most vexed subject in this field is the site of radical attack on substituted aromatic rings. Some react cleanly where we should expect them to. Phenyl radicals add to naphthalene (399), to anthracene (400)323 and to thiophene... 

Some indication of how theory compares with observation is given in Table II which shows the relative rates at which methyl radicals attack some of the compounds of interest (2). Notice that all rates are ordered inverse to delocalization energies. Values of DE quoted in Table II and hereafter have been nondimensionalized by the Hiickel factor 2/2 this nondimensional DE is also known as the Dewar number. The pattern of Table II is observed for a variety of other aromatic reactions such as nitration and sulfonation (I). Among the molecules considered in this study (Table I), benzene and anthracene represent the extremes of reactivity. 

The first reaction was found by Levy and Szwarc to be predominant when methyl radicals attacked isooctane. The second reaction is predominant, however, for aromatic hydrocarbons. The free radicals formed in the above two reactions will react with each other, with other free radicals, or with impurities. The affinity of the methyl radical to attack an aromatic increases in the following order benzene, diphenyl ether, pyridine, diphenyl, benzophenone, naphthalene, quinoline, phenanthrene, pyrene, and anthracene. The ability of free alkyl radicals to interact with isopropylbenzene and cyclohexene decreases in the following order methyl, ethyl, propyl, butyl, isopropyl, sec-butyl, and tertiary butyl. 

Free radicals such as the hydroxyl or hydroperoxyl radicals are expected to react with PAH by analogy to their reactivity with simple aromatics in the gas phase e.g. toluene is converted into cresols (Atkinson et al. (69)). The rate constant of hydroxyl radical attack on anthracene, pyrene and benzo(a)pyrene has been measured recently in aquous solution (Chekulaev and Shevchuk (90)). 

The reaction of benzyl radicals wdth several heterocyclic compounds W as more extensively studied by Waters and Watson, " - who generated benzyl radicals by decomposing di-tert-butyl peroxide in boiling toluene. The products of the reaction with acridine, 5-phenyl-acridine, 1 2- and 3 4-benzacridine, and phenazine were studied. Acridine gives a mixture of 9-benzylacridine (17%) (28) and 5,10-dibenzylacridan (18%) (29) but ho biacridan, w hereas anthracene gives a mixture of 9,10-dibenzyl-9,10-dihydroanthracene and 9,9 -dibenzyl-9,9, 10,10 -tetrahydrobianthryl. This indicates that initial addition must occur at the meso-carbon and not at the nitrogen atom. (Similar conclusions were reached on the basis of methylations discussed in Section III,C.) That this is the position of attack is further supported by the fact that the reaction of benzyl radicals with 5-... 

For instance, NOBF4 oxidation of benzo[a]pyrene (BP, the additional benzene ring is fused at positions 7 and 8 of pyrene) generates the BP+ BF4 salt. When this cation-radical salt is attacked with nucleophiles of various strengths, the pattern of nucleophilic substitution reflects the distribution of a positive charge in the cation-radical part of the salt. This positive charge is localized mainly at the meio-anthracenic position, that is, at the C-6 atom. Nucleophiles (Nu ) such as OH , AcO , and F enter this position (Scheme 3.68). 

This study indicates that the oxidation of dihydroanthracene in a basic medium involves the formation of a monocarbanion, which is then converted to a free radical by a one-electron transfer step. It is postulated that the free radical reacts with oxygen to form a peroxy free radical, which then attacks a hydrogen atom at the 10-position by an intramolecular reaction. The reaction then proceeds by a free-radical chain mechanism. This mechanism has been used as a basis for optimizing the yield of anthraquinone and minimizing the formation of anthracene. 

Some of the products and resin formation may be accounted for in this way. These heavily substituted benzene derivatives are likely to have absorption in the uv regions A < 300 nm. Since more HC1 is formed on keeping, the mode of polymerization seems to be as presented in (C) above and not by attack on another anthracene molecule by the AC1 radical. This is concluded from the low quantum yield of anthracene removal which does not predict chain reaction. 

The direct nature of attack of CN on radical cations of aromatic compounds has been demonstrated by CV [221]. The reversible one-electron oxidation of anthracene becomes irreversible in the presence of CN, and 2 F electrolysis gives a mixture of cyano and isocyano addition across the 9,10-position. Interestingly, it appears that cyanation of 9,10-diphenylanthracene gives the 9,10-diphenyl-9,10-dicyano-9,10-dihydroanthra-cene only [233]. 

For several of these molecules, however, there are /Sjmin associated with certain carbons, as well as with some of the bonds. As in the cases of guanine and cytosine, discussed earlier, such carbons are possible targets for attack by electrophiles (if appropriate channels of negative Vs(r) are present) or by radicals. Thus, we have proposed that anthracene, for example, should be represented by an additional structure, 6D [82], having two radical sites ... 

The mechanism for the photo-amination is reasonably suggested to involve nucleophilic attack of the amine on to the cation radical of the arene which is generated by photo-induced electron transfer to the dicyanobenzene. Irradiation at wavelengths longer than 500 nm of dichloromethane solutions of the charge-transfer complexes of various anthracenes with tetranitromethane leads to rapid... 

Domenech, A., Casades, L, and Garcia, H. 1999b. Electrochemical evidence for an impeded attack of water to anthracene and thianthrene radical ions located on the outermost layers of zeolites. Journal of Organic Chemistry 64, 3731-3735. 

What is the mechanism of attack of nucleophiles on radical cation species such as anthracene or thianthrene radical cations Extensive studies by a number of groups have been conducted on the reaction of radical cations with nucleophiles (6, 23, 24). Two main mechanisms have been proposed the disproportionation pathway, equations 6 and 7, in which the nucleophile,... 

The homol5dic acylation is practically unknown in the homocyclic aromatic series. The only intermolecular attack so far reported concerns the reaction of benzoyl radical and anthracene in which the position 9 is highly reactive towards free radicals. 

It has been proposed (e.g. Weinberg and Reddy, 1968) that the cation radical is adsorbed on the anode surface, in which state it might react directly with methanol or undergo further oxidation to a dication in a step prior to or concerted with attack by methanol. Methoxylations of anthracenes (Parker et al., 1971) are regarded as ECE processes. 

Although the attack on (or by) the aromatic ring that leads to substitution can be brought about by free radical, electron-rich (nucleophilic) or electron-poor (electrophilic) reagents, the surfeit of electrons associated with aromaticity provides a richness that dictates that the latter should be most common. Nonetheless, aU three substitution pathways will be considered. Indeed, the three that will be examined are benzene (CeHe) and the polycyclic arenes (naphthalene [CioH ], anthracene [C14H10], and phenanthrene [CmHjo]). 

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