Polynuclear aromatic hydrocarbon radical cations

It is now well established that a variety of organic molecules such as polynuclear aromatic hydrocarbons with low ionization energies act as electron donors with the formation of radical cations when adsorbed on oxide surfaces. Conversely, electron-acceptor molecules with high electron affinity interact with donor sites on oxide surfaces and are converted to anion radicals. These surface species can either be detected by their electronic spectra (90-93, 308-310) or by ESR. The ESR results have recently been reviewed by Flockhart (311). Radical cation-producing substances have only scarcely been applied as poisons in catalytic reactions. Conclusions on the nature of catalytically active sites have preferentially been drawn by qualitative comparison of the surface spin concentration and the catalytic activity as a function of, for example, the pretreatment temperature of the catalyst. Only phenothiazine has been used as a specific poison for the butene-1 isomerization on alumina [Ghorbel et al. (312)). Tetra-cyaonoethylene, on the contrary, has found wide application as a poison during catalytic reactions for the detection of active sites with basic or electron-donor character. This is probably due to the lack of other suitable acidic probe or poison molecules. 

Polynuclear aromatic hydrocarbons are the most easily oxidized on catalyst surfaces. Perylene and anthracene have been the most commonly used hydrocarbons, and their cation radicals have been characterized in such work by esr and absorption spectroscopy. Naphthalene (Rooney and Pink, 1962) and naphthacene (Flockhart et al., 1966) have been used, but their oxidation to the cation radical goes only poorly. 

Although many polynuclear aromatic hydrocarbons give stable cation radical solutions (e.g. in sulfuric acid), not many give stable... 

The PET-generated arene radical cations also undergo nucleophilic substitution via the a-complex. Photocyanation of arenes may be cited in this context as a very early example [139], where hydrogen served as the group undergoing displacement. This concept is further extended [140] for the direct amination of polynuclear aromatic hydrocarbons with ammonia or primary amines via the arene radical cation produced by irradiating arenes in die presence of DCNB. Another potentially useful application of this methodology is... 

So far, only a few examples of cationic photopolymerizations using PET corresponding to Scheme 3 have been described [10,13,165]. In the ternary system cyclohexene oxide, 9.10-dicyano anthracene and polynuclear aromatics, the polymerization of the former is initiated by the radical cations of the aromatic hydrocarbons formed via the PET with the dicyano compound. 

Three studies on radical cations discuss the characterization of polynuclear aromatic radical cation salts as organic metals (8), the reactions of cation radicals with neutral radicals (9), and the magnetic-electrical properties of perfluoroaromatic radical-cation salts (10). Chapters on polynuclear aromatic compounds in nonvolatile petroleum products (II) and in coal-based materials (12) present reviews of the subject and new findings. The remaining chapters in this book discuss the thermal conversion of polynuclear aromatic compounds to carbon (13), the nitration of pyrene by mixtures of N02 and N204 (14), the spectra, structures, and chromatographic retention times of large polycyclic aromatic hydrocarbons (15), the desulfurization of polynuclear thiophenes correlated with tt electron densities (16) and simple theoretical methods to predict and correlate polynuclear benzenoid aromatic hydrocarbon reactivities (IT). 

Some aromatic hydrocarbons (e.g. perylene) give cation radicals in (neat) antimony trichloride solutions at 75° (Porter et al., 1970 Johnson, 1971). However, the cation radicals are not formed in the absence of oxygen. In fact, molten antimony trichloride can be used as a solvent at 99° for the anodic oxidation of perylene, naphthacene, and other polynuclear aromatics, provided that the electrolyte (e.g. KC1) is highly dissociated (Bauer et al., 1971). When a more covalent electrolyte (e.g. A1C13) is used, the solvent system itself becomes the oxidant [(17) and (18)]. 

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