Initiation oxidation chemistry

The third primary intermediate in the oxidation chemistry of a-tocopherol, and the central species in this chapter, is the orr/zo-quinone methide 3. In contrast to the other two primary intermediates 2 and 4, it can be formed by quite different ways (Fig. 6.4), which already might be taken as an indication of the importance of this intermediate in vitamin E chemistry. o-QM 3 is formed, as mentioned above, from chromanoxylium cation 4 by proton loss at C-5a, or by a further single-electron oxidation step from radical 2 with concomitant proton loss from C-5a. Its most prominent and most frequently employed formation way is the direct generation from a-tocopherol by two-electron oxidation in inert media. Although in aqueous or protic media, initial... 

Rosenau, T. Kloser, E. Gille, L. Mazzini, F. Netscher, T. Vitamin E. Chemistry studies into initial oxidation intermediates of a-tocopherol disproving the involvement of 5a-C-centered Chromanol Methide radicals.. /. Org. Chem. 2007, 72(9), 3268-3281. 

The entire group of these compounds dates back to the very beginnings of organic dye chemistry. In 1858, E. Verguin in France oxidized a material which he named aniline but which was in actual fact a mixture of aniline, o-toluidine, and p-toluidine. He performed the reaction in nitrobenzene in the presence of tin(IV)chloride or iron(III)chloride and received bluish red fuchsin (120). The process has been industrially exploited since 1859. The central carbon atom is furnished by the CH3 group of p-toluidine, which is initially oxidized to its aldehyde. 

Essentially, the oxidation chemistry of the aliphatics higher than C2 has already been discussed since the initiation step is mainly CC bond cleavage with some CH bond cleavage. But the initiation steps for pure ethene or acetylene oxidation are somewhat different. For ethene the major initiation steps are [4, 39a]... 

RuCljCPPhj) After [RuO ]" (n = 0-2), RuO and RuQj this is probably the most frequently used Ru oxidation catalyst, particularly for alcohol oxidations. Whereas the catalytic action of RUCI3 or RuO in most cases studied probably derives from their initial oxidation to [RuO ]", it is often not clear how RuCl CPPhj) operates as a catalyst. There are reviews on its oxidation chemistry [56, 58], Its CAS number is 15529-49-4. 

Again, the chemistry is analogous to the OH-initiated oxidation of chloride and bromide in solution. The subsequent aqueous-phase chemistry for the halogens is summarized in the following chapter. 

Other species that can initiate this sulfur oxidation chemistry are N03 (discussed in Chapter 7.D.1) and ClJ. The latter radical anion is formed in sea salt particles when atomic chlorine is generated and reacts with chloride ion. In addition, Vogt et al. (1996) have proposed that oxidation of SO2- by HOC1 and HOBr in sea salt particles may be quite important. Table 8.13 summarizes the aqueous-phase chlorine chemistry that occurs in sea salt particles and Table 8.14 the oxidation of S(IV) by reactive chlorine and bromine species in solution. 

Reversible redox reactions can initiate radical chemistry without a follow-up reduction or oxidation reaction. In successful reactions of this type, the redox step that produces the radical is thermodynamically disfavored. For example, Cu(I) complexes react reversibly with alkyl hahdes to give Cu(II) hahde complexes and an alkyl radical. The alkyl radical can react in, for example, an addition reaction, and the product radical will react with the Cu(II) hahde to give a new alkyl halide. This type of reaction sequence, which has been apphed in living radical polymerizations, is in the general family of nonchain radical reactions discussed earlier. ... 

Our radiolysis studies also indicate that phosphonates react quite slowly with the superoxide anion radical. Although our studies do not support the formation of radical cations as an initial oxidation step, we cannot rule out the possibility that radical cations are not involved in the oxidation of the C—P bond, as previously proposed [44], It is also possible that more electron-rich organphosphorus compounds or organophosphorus compounds in the adsorbed state may exhibit different redox and hydroxyl radical chemistries than what is observed under pulse radiolysis employing homogeneous conditions. 

The chemistry of sulfur in flames has received considerable attention [79]. Results from various combustion systems indicate that the initial speciation of the gas phase sulfur is of minor importance, both for its interaction with fuel oxidation and nitrogen chemistry and for the SO3/SO2 ratio in the flue gas. This is fortuitous, since even for the simplest sulfur compounds, such as H2S, knowledge of its oxidation chemistry is incomplete. However, in flames the oxidation is believed to proceed by the overall sequence... 

Many compounds will undergo dimerization reactions those containing thiols (e.g., disulfide formation) olefins, alcohols, and carboxylic acids (or other carbonyl chemistry e.g., aldol condensation reactions). Indoles have been shown to dimerize under acidic conditions. The dimerization is presumed to occur as shown in Figure 120 via protonation at C3 and nucleophilic attack of a second indole on C2. Phenols have been shown to dimerize under free radical initiated oxidative conditions, usually to ortho phenols. Nalidixic acid API undergoes dimerization under thermolysis conditions to decarboxylate and produce a dimeric structure (Fig. 121) (172). 

The use of hypervalent iodine reagents for heteroatom-heteroatom bond forming reactions is well established in the context of classical oxidation chemistry [1-11]. For example, oxidations of anilines to azobenzenes, thiols to disulfides, and sulfides to sulfoxides with aryl-A3-iodanes were documented decades ago [1-5]. During the last ten years, particular attention has also been given to oxidative transformations of compounds derived from heavier elements, including the interception of reaction intermediates or initially formed products with external nucleophiles. A second important development is the utilization of sulfonyliminoiodanes, ArI = NS02R, for heteroatom-nitrogen bond formation, especially for imidations of sulfur, selenium, phosphorus and arsenic com-... 

Benzene has been observed as a product of both the OH- and NQz-initiated oxidation of cyclohexa-1,3-diene, indicating a hydrogen atom abstraction in both reactions. In the presence of NO and molecular oxygen, the N02-initiated reaction leads to removal of cyclohexa-1,3-diene by reaction with both NO2 and OH. Formic acid was detected as a product in this system, providing evidence for significant formation of stabilized C6o -hydroxyperoxy radicals from the OH-initiated chemistry, and their subsequent reaction with NO. Mechanisms consistent with the observations have been proposed.80... 

The mechanism of the hydroxyl radical-initiated oxidation of /i-pincnc in the presence of NO has been investigated using a discharge-flow system. Propagation of hydroxyl radicals was observed after the addition of O2 and NO, and the measured concentration profiles were compared with simulations based on both the master chemical mechanism and the regional atmospheric chemistry mechanism for /i-pinene oxidation.228... 

With the involvement of volatile organic compounds (VOCs) in the oxidation chemistry. Figure 11 represents a slice through an n-dimen-sional surface where there should be a third axis to represent the concentration of VOCs. The peak initial concentrations of ozone generated from various initial concentrations of NO , and VOCs are usually represented as an O3 isopleth diagram , an example of which is shown... 

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