Hydrocarbon radical cations reaction mechanisms

A general theory of the aromatic hydrocarbon radical cation and anion annihilation reactions has been forwarded by G. J. Hoytink 210> which in particular deals with a resonance or a non-resonance electron transfer mechanism leading to excited singlet or triplet states. The radical ion chemiluminescence reactions of naphthalene, anthracene, and tetracene are used as examples. 

The mechanistic aspects of aromatic121 and alkene122 radical cation reactions have been reviewed. A second review article covers the structure and properties of hydrocarbon radical cations, as revealed by low-temperature ESR and IR spectroscopy.123 A review of the reactivity of divalent phosphorus radical cations has appeared which discusses ionic and SET processes and their kinetics.124 The structure and reactivity of distonic radical cations have been reviewed, including experimental and calculated heats of formation, structures, reactivity, and mechanisms.122125... 

Another way of carrying out electron-transfer mediated oxidation reactions is to use semiconductors as catalysts (Mozzanega et al., 1977). Titanium dioxide will, photocatalyse the oxidation of substituted toluenes to benz-aldehydes by electron transfer from toluene into the photogenerated hole. The electron in the conduction band will reduce oxygen giving the superoxide anion. Reaction of the superoxide anion with the hydrocarbon radical cation produces the aldehyde. A similar mechanism has been used to explain the observation that dealkylation of Rhodamine B (which contains N-ethyl groups) occurs when the dye is irradiated in the presence of cadmium sulphide (Watanabe et al., 1977). 

We postulated a reaction mechanism with participation of an aromatic radical cation which was formed by one electron transfer from an aromatic hydrocarbon to copper(II) chloride. Activated alumina has electron-acceptor properties, and formation of a radical cation of an aromatic hydrocarbon adsorbed on alumina has been observed by ESR (ref. 13). Therefore, it seemed to us that alumina as a support facilitates the generation of the radical cation of the aromatic hydrocarbon. 

Energetic electron transfer reactions between electrochemically generated, shortlived, radical cations and anions of polyaromatic hydrocarbons are often accompanied by the emission of light, due to the formation of excited species. Such ECL reactions are carried out in organic solvents such as dimethylformamide or acetonitrile, with typically a tetrabutylammonium salt as a supporting electrolyte. The general mechanism proposed for these reactions is as follows. 

A review has focused on differentiation between polar and SET mechanisms through kinetic analysis.82 hi two separate reviews, the effects of solute-solvent interactions on electron-transfer reactions have been described.83,84 A review of the behaviour of radical cations in liquid hydrocarbons has given particular emphasis to those with high mobility.85 A paper presents selected studies in the formation of radicals by oxidation with manganese- or cerium-based reagents and then- application to C—C bond formation by SET processes.86... 

A similar mechanism (Eq. 4) is operative in reactions of saturated hydrocarbons with closed-shell oxidizing electrophiles (E = Haln+, NOz+, etc.) where the H-trans-fer from a C-H bond is accompanied by an ET through the linearly H-coupled fragment (H-coupled electron transfer) [13]. The hydrocarbon part of the transition structures resembles the respective radical cation (that for the reaction of adamantane with Cl7+ is shown (6) in Scheme 2) [13]. 

In the twentieth century, Kehrmann applied this reagent for the one- and two-electron oxidation of phenothiazine and characterized semiquinoid and holoquinoid species by UV/VIS spectroscopy (Fig. 4) [44,45], In the 1950s colored solutions of aromatic hydrocarbons (perylene, anthracene, etc.) in sulfuric add were found to be paramagnetic [46] and, shortly thereafter, their radical cations were postulated based on optical [47] and ESR spectroscopic data [48]. The detailed reaction mechanisms of these oxidations are still in question molecular oxygen may well be a necessary ingredient... 

The reactions of DPAt and radical cations of other aromatic hydrocarbons with pyridine and substituted pyridines are among the most intensively studied electrode reactions of positive ions. The first definitive study of the mechanism of the reaction employed the rotating disk electrode (Manning et al 1969). Data were found to fit ECE working curves (Fig. 21) for the reactions of DPA7 with 4-cyanopyridine, 4-acetoxypyridine, pyridine and 4-methylpyridine. Pseudo first order rate constants of about 3, 10, 30, 300... 

Spectra and kinetics were also determined for many other species. The solvated electron was observed and its spectrum was determined in a wide variety of solvents, from ethers and alcohols to hydrocarbons and even supercritical fluids. Other radicals, including the benzyl radical, the first species studied in pulse radiolysis, were observed. Excited states, both singlet and triplet, anions and cations, were determined for aromatic species. The number and variety of species is large. The importance of these studies was that it was now possible to observe the intermediate states in the radiation-chemical reactions and thus confirm or refute reaction mechanisms that had been proposed based on product yield data. 

This mechanism does not constitute a major reaction pathway for the anodic oxidation of alternant aromatic hydrocarbons (AAH) or alkyl-substituted AAH under common experimental conditions [12,16-19], but has been found to be operative, for example, for tetra-arylethylenes [20-23]. The difference in the reaction pattern of AAH and tetraarylethylenes is related to the rotational freedom around the central C-C bond experienced by the radical cations of the latter [24]. This results in a small or even negative difference between the standard potentials for the first and second one-electron transfer, E and E2, and, accordingly, the value of AT(j,spr [Eq. (15)] may take relatively large values. 

Since cation-radical formation in the chemisorption of hydrocarbons has not previously been considered in the catalytic literature, the nature, reactions, and mechanism for formation of such species should be of considerable importance to the elucidation of catalytic reaction mechanisms particularly in view of the fact that Webb (20) has found spectral evidence for the formation of species other than carbonium ions from butene-2 adsorbed on silica-alumina. It is not possible at the present time to define either the role of cation-radicals in acid catalysis or the chemical nature of the electrophilic surface sites involved in their formation. 

Dicyanobenzene was converted into 3-cyanobenzoic acid by using the bacterium Rhodococcus rhodochrocus ". Intramolecular electron transfer involving isomeric forms of dicyanobenzene was investigated in detaiP and the mechanisms of electroreduction of all three isomers were studied. When dicyanobenzene was used as catalyst in photoamination reactions of arenes, the first step of the reaction was the formation of an aryl radical cation by electron transfer from the aromatic hydrocarbon to m-dicyano-benzene. ... 

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