Aromatic hydrocarbons oxidation rates

Fig. 4 (a) Correlation of the standard oxidation potentials E%r of various alkylbenzenes with the irreversible CV peak potentials Ep at scan rate v = 100 mV s. (b) Correlation of the standard oxidation potentials r of various alkylbenzenes with the vertical ionization potentials IP. Numbers refer to the aromatic hydrocarbons identified in Tables I and III in Ref. 8. Reproduced with permission from Ref. 8. 

Katz, M., Chan, C., Tosine, H., Sakuma, T. (1979) Relative rates of photochemical and biological oxidation (in vitro) of polynuclear aromatic hydrocarbons. In Polynuclear Aromatic Hydrocarbons. Jones, P.W., Leher, P., Eds., pp. 171-189, Ann Arbor Science Publishers, Ann Arbor, MI. 

Another isomerization reaction of arene oxides is equilibrium with oxe-pins [5], Here, the fused six-membered carbocycle and three-membered oxirane merge to form a seven-membered heterocycle, as shown in Fig. 10.2. An extensive computational and experimental study involving 75 epoxides of monocyclic, bicyclic, and polycyclic aromatic hydrocarbons has revealed much information on the structural factors that influence the reaction rate and position of equilibrium [11], Thus, some compounds were stable as oxepins (e.g., naphthalene 2,3-oxide), while others exhibited a balanced equilibrium... 

Oxidation of aromatic hydrocarbons with lead tetraacetate develops as the second-order reaction. Hence, the rate-determined stage consists of the transformation of Pb" to Pb" with the participation of only one molecule of the hydrocarbon (Dessau et al. 1970). The formed hydrocarbon dication can, of course, react with the uncharged hydrocarbon ArH + + ArH —> ArH+ + ArH+. ... 

The cation-radicals ArH+ were detected, but they originated from the fast reaction of a one-electron transfer, which does not affect kinetic constants of the oxidation. The rate constant depends linearly on Brown s a constants of substituents (Dessau et al. 1970). All these data are in agreement with the formation of the strong polar dication of an aromatic hydrocarbon as an intermediate. Because PF salts (in particular the diacetate) are not reductants, the two-electron transfer reaction proceeds irreversibly. 

It is known that the oxidation of alkyl-substituted aromatic hydrocarbons in acetic acid on metal bromide catalysis follows the one-electron transfer mechanism (Sheldon and Kochi 1981). The rate-determining stage is the one-electron transfer from the substrate to the metal ion in the highest oxidation state (Digurov et al. 1986). As a result, an unstable cation-radical is formed that... 

The delocalised radical formed by protonation of the radical-anion is more easily reduced than the starting arene. For some polycyclic aromatic hydrocarbons, the redox potential for this radical species can be determined using a cyclic voltammetry technique [10]. Reduction in dimethylformamide is carried out to the potential for formation of the dianion. The dianion undergoes rapid monoprotonation and on the reverse sweep at a fast scan rate, oxidation of the monoanion to the radical can be observed. The radical intermediate from pyrene has E° = -1.15 V vs. see in dimethylformamide compared to E° = -2.13 V vs. see for pyrene,... 

Relative Rates of Photochemical and Biological Oxidation (in vitro) of Polynuclear Aromatic Hydrocarbons," in POLYNUCLEAR AROMATIC HYDROCARBONS, P.W. Jones and P. Leber (Editors), Ann Arbor Science Publishers, Inc., Ann Arbor, MI, 171-89. 

One possible problem peculiar to a quantitative study of the inhibition of oxidation of aromatic hydrocarbons by zinc dialkyl dithiophos-phates is that peroxide decomposition could yield a phenol during the initial-rate measurement. Rate curves for the zinc diisopropyl dithio-phosphate-inhibited oxidation of cumene are shown in Figure 7. In the initial presence of hydroperoxide the uninhibited rate is never reached, and the reaction soon exhibits autoinhibition, presumably caused by the... 

Relative oxidation rates of some aromatic hydrocarbons... 

Product inhibition is reported for reactions (1) and (2) in this scheme. Of interest are the relative overall oxidation rates for some aromatic hydrocarbons reported by the authors (Table 38). 

The rates of oxidation of aromatic hydrocarbons are shown in Table VI. The rates were determined by following disappearance of the original hydrocarbon rather than oxygen absorption—the technique used by most investigators. 

Neither the relative number of benzylic hydrogens nor the base strength accounts for the slow oxidation rate of the methylnaphthalenes. Formation of radicals in the presence of aromatic hydrocarbons can lead to radical attack on the aromatic ring. Addition of phenyl or methyl radical to the ring gives a cyclohexadienyl radical that may disproportionate or dimerize, or undergo hydrogen abstraction by another radical (3, 9,13). 

Table VI. Relative Rates of Oxidation of Aromatic Hydrocarbons ...

Even comprehensive mechanisms, however, must be utilized with caution. The GRI-Mech fails, for instance, under pyrolysis or very fuel-rich conditions, because it does not include formation of higher hydrocarbons or aromatic species. Its predictive capabilities are also limited under conditions where the presence of nitrogen oxides enhances the fuel oxidation rate (NO f sensitized oxidation), a reaction that may affect unbumed hydrocarbon emissions from some gas-fired systems, for example, internal combustion engines. 

Another synthetic method involves treating the parent aromatic hydrocarbon with sodium hypochlorite in water-chloroform, using phase transfer agents like tetrabutylammonium hydrogen sulfate or benzyltrimethylam-monium chloride.15 The epoxides are formed in high yields. The rate is pH dependent, and epoxide formation is most facile at pH 8-9. Many K-region arene oxides like 1,165, acenaphthylene 1,2-oxide (19), 1-azaphenanthrene 5,6-oxide (20), 4-azaphenanthrene 5,6-oxide (21), 1,10-phenanthroIine 5,6-oxide... 

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