Aromatics Oxidation Kinetics

With respect to the kinetics of aromatic oxidations, (extended) redox models are suitable, and often provide an adequate fit of the data. Not all authors agree on this point, and Langmuir—Hinshelwood models are proposed as well, particularly to describe inhibition effects. It may be noted once more that extended redox models also account for certain inhibition effects, for mixtures of components that are oxidized with different velocities. The steady state degree of reduction (surface coverage with oxygen) is mainly determined by the component that reacts the fastest. This component therefore inhibits the reaction of a slower one, which, on its own, would be in contact with surface richer in oxygen (see also the introduction to Sect. 2). 

Tetrakis phosphino complexes of nickel(O) readily react with aliphatic and aromatic nitro compounds RN02 to afford the corresponding nitroso complexes of nickel(0) [Ni(PR3)2(RNO)] and the phosphine oxide. Kinetic studies have been carried out to elucidate the mechanism of this oxygen transfer reaction. The reaction mechanism shown in equations (30)-(32) has been postulated.193... 

The degradation schemes of four aromatic hydrocarbons benzene, toluene, /7-xylene and 1,3,5-trimethylbenzene, have been updated on the basis of new kinetic and mechanistic data from current literature and conference proceedings and are available as part of the latest version of the Master Chemical Mechanism (MCMv3.1) via the MCM website thttn //mcm.leeds.ac.uk/MCM). The performance of these schemes concerning ozone formation from tropospheric aromatic oxidation has been evaluated using detailed environmental chamber datasets from the two EU EXACT measurement campaigns at EUPHORE (EXACT I - September 2001 and EXACT II - My 2002 (Pilling et al, 2003)). 

Among the metal acetates which have been studied recently, Co(III) acetate engages in electron transfer more easily than Pb(IV), Mn(III), and Ce(IV) acetates (Heiba et al., 1968a, 1969a, b Heiba and Dessau, 1971). Kinetics have shown, both with Mn(III) and Co(III) acetates, that electron transfer is fast and reversible and is followed by a rate-determining step whose nature depends on the aromatic. Oxidation of 1-methoxynaphthalene in acetic acid at 100°, for example, leads to l-acetoxy-4-methoxynaphthalene [eqns... 

The use and importance of aromatic compounds in fuels sharply contrasts the limited kinetic data available in the literature, regarding their combustion kinetics and reaction pathways. A number of experimental and modelling studies on benzene [153, 154, 155, 156, 157, 158], toluene [159, 160] and phenol [161] oxidation exist in the literature, but it would still be helpful to have more data on initial product and species concentration profiles to understand or evaluate important reaction paths and to validate detailed mechanisms. The above studies show that phenyl and phenoxy radicals are key intermediates in the gas phase thermal oxidation of aromatics. The formation of the phenyl radical usually involves abstraction of a strong (111 to 114 kcal mof ) aromatic—H bond by the radical pool. These abstraction reactions are often endothermic and usually involve a 6 - 8 kcal mol barrier above the endothermicity but they still occur readily under moderate or high temperature combustion or pyrolysis conditions. The phenoxy radical in aromatic oxidation can result from an exothermic process involving several steps, (i) formation of phenol by OH addition to the aromatic ring with subsequent H or R elimination from the addition site [162] (ii) the phenoxy radical is then easily formed via abstraction of the weak (ca. 86 kcal moT ) phenolic hydrogen atom. 

Table 3-18 DSC Oxidation Kinetics according to ASTM E 698-79 of Aromatics Atmosphere Air Pressure 7 bar... 

The kinetic data listed in Chapter 2, Table la show a very fast reaction of these compounds with OH radicals, while the NO3 reactions were found to be slow. Furthermore, an extremely rapid UV photolysis was determined irradiation with VIS lamps did not result in photolysis. The major atmospheric sinks of 2,4-hexadienedials will therefore be the reaction with OH radicals. The fast OH reactions and rapid UV photolysis can explain why it has not yet been possible to unambiguously identify 2,4-hexadienedials in aromatic oxidation systems, especially where the UV photolysis of H2O2 is used as the OH radical source. 

Tropospheric Degradation of Aromatics Laboratory Kinetic Studies of some First Steps of the OH Initiated Oxidation... 

Organic sulfur compounds such as sulfurized spermaceti oil, terpene sulfides, and aromatic disulfides have been used. Encumbered phenols such as di-tertiary-butylphenols and amines of the phenyl-alphanaphthylamine type are effective stopping the kinetic oxidation chain by creating stable radicals. 

Radical Scavengers Hydrogen-donating antioxidants (AH), such as hindered phenols and secondary aromatic amines, inhibit oxidation by competing with the organic substrate (RH) for peroxy radicals. This shortens the kinetic chain length of the propagation reactions. 

Oxidation of benzene (and also chlorobenzene and toluene) by Mn(III) acetate in glacial acetic acid gives a mixture of products including benzyl acetate (from benzene) indicating an initial attack on the aromatic by CH2C02H . The kinetics and actual rate of disappearance of Mn(III) are the same for CgHs and... 

One-step hydroxylation of aromatic nucleus with nitrous oxide (N2O) is among recently discovered organic reactions. A high eflSciency of FeZSM-5 zeolites in this reaction relates to a pronounced biomimetic-type activity of iron complexes stabilized in ZSM-5 matrix. N2O decomposition on these complexes produces particular atomic oj gen form (a-oxygen), whose chemistry is similar to that performed by the active oxygen of enzyme monooxygenases. Room temperature oxidation reactions of a-oxygen as well as the data on the kinetic isotope effect and Moessbauer spectroscopy show FeZSM-5 zeolite to be a successfiil biomimetic model. 

For some organic compounds, such as phenols, aromatic amines, electron-rich olefins and dienes, alkyl sulfides, and eneamines, chemical oxidation is an important degradation process under environmental conditions. Most of these reactions depend on reactions with free-radicals already in solution and are usually modeled by pseudo-first-order kinetics ... 

The formation of the Wheland intermediate from the ion-radical pair as the critical reactive intermediate is common in both nitration and nitrosation processes. However, the contrasting reactivity trend in various nitrosation reactions with NO + (as well as the observation of substantial kinetic deuterium isotope effects) is ascribed to a rate-limiting deprotonation of the reversibly formed Wheland intermediate. In the case of aromatic nitration with NO, deprotonation is fast and occurs with no kinetic (deuterium) isotope effect. However, the nitrosoarenes (unlike their nitro counterparts) are excellent electron donors as judged by their low oxidation potentials as compared to parent arene.246 As a result, nitrosoarenes are also much better Bronsted bases249 than the corresponding nitro derivatives, and this marked distinction readily accounts for the large differentiation in the deprotonation rates of their respective conjugate acids (i.e., Wheland intermediates). 

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