Oxidation chemistry

Incorporation of OH is another critical aspect of the oxidation chemistry. Reduction to the ppb level is necessary for the manufacture of low loss optical fiber. Hydrogen is iacorporated iato the glass according to the reaction... 

The scope of oxidation chemistry is enormous and embraces a wide range of reactions and processes. This article provides a brief introduction to the homogeneous free-radical oxidations of paraffinic and alkylaromatic hydrocarbons. Heterogeneous catalysis, biochemical and hiomimetic oxidations, oxidations of unsaturates, anodic oxidations, etc, even if used to illustrate specific points, are arbitrarily outside the purview of this article. There are, even so, many unifying features among these areas. 

Numerous oxides of sulfur have been reported and those that have been characterized are SO [13827-32-2] S2O [20901 -21 -7] S O (n = 6-10), SO2, SO, and SO4 [12772-98-4]. Among these, SO2 and SO ate of principal importance. Sulfur oxide chemistry has been reviewed (210—212). Sulfur trioxide, SO, is discussed elsewhere (see Sulfuric acid and sulfur trioxide). 

Most talc sold to paper, ceramics, and other industrial customers is manufactured to specifications agreed to between the producer and consumer. In paper, properties such as color, abrasion, surface area, and tint ate most important, whereas in ceramics, oxide chemistry, fired color, pressing characteristics, and alkaH metal content ate mote important. There ate some military specifications for talc used in corrosive coatings (6) and for cosmetic talc products used for cleaning of personnel in chemical warfare zones (7). 

Gas-phase oxidation of thiols has been discussed in some depth (33). This review mainly emphasi2es atmospheric processes, but a section on nitrogen oxides and thiols appears to be broadly appHcable. The atmospheric oxidation chemistry of thiols is quite different from that of alcohols. 

Major unknowns in the mechanism by which a hydrocarbon fuel bums concern the pyrosynthesis reactions that lead to the formation of polycyclic aromatic hydrocarbons (PAHs) and soot and the oxidation chemistry of atoms other than carbon and hydrogen (heteroatoms) in the fuel, particularly nitrogen, sulfur, and halogens. 

The bis( U-oxo)dimetal [M2( tt-0)2] " core (Scheme 12d) has been proposed as a common motif for oxidation chemistry mediated by manganese, iron, and copper... 

Fenton chemistry comprises reactions of H2O2 in the presence of iron species to generate reactive species such as the hydroxyl radical OH. These radicals ( = 2.73 V) lead to a more eflident oxidation chemistry than H2O2 itself (E° = 1.80 V). 

Table 6.2 Rationalization of the oxidation chemistry applied in the synthesis of heterogeneous catalysts and related functional materials.

For nearly two decades TACN-type ligands are of continuing interest for oxidation chemistry. A more recent example is described by ShuFpin and cow-orkers, who prepared novel di- and tetranuclear complexes with TACN ligands 25 bearing pendant acetato arms bridging the iron(lll) centers (Scheme 18) [111]. The... 

The results of the experiments we carried out and the comparison with the data arising from homogeneous oxidation chemistry allow us to propose and generalize the following conclusions that in part contrast previous literature findings ... 

Jolivet J-P (2000) Metal Oxide Chemistry and Synthesis. From Solution to Solid State. Wiley, Chichester... 

Metal Oxides Chemistry and Applications, edited by J. L. G. Fierro... 

Despite the importance of the oxidative polymerization of 5,6-dihydroxyin-dole, in the biosynthesis of pigments, little experimental data are known on the oxidation chemistry of the oligomers of 1. For such reasons, three major dimers of 1, such as 2-4 (Scheme 2.9), have been computationally investigated at PBEO/ 6-31+G(d,p) level of theory both in gas and in aqueous solution (by PCM solvation model) to clarify the quinone methide/o-quinone tautomeric distribution. 

Oxidation chemistry of a-tocopherol (1) generally involves three primary intermediates (2-4), which are formed according to the respective reaction conditions used, their... 

FIGURE 6.2 Primary key intermediates in the oxidation chemistry of a-tocopherol. 

Chromanoxylium cation 4 preferably adds nucleophiles in 8a-position producing 8a-substituted tocopherones 6, similar in structure to those obtained by radical recombination between C-8a of chromanoxyl 2 and coreacting radicals (Fig. 6.4). Addition of a hydroxyl ion to 4, for instance, results in a 8a-hydroxy-tocopherone, which in a subsequent step gives the /zara-tocopherylquinone (7), the main (and in most cases, the only) product of two-electron oxidation of tocopherol in aqueous media. A second interesting reaction of chromanoxylium cation 4 is the loss of aproton at C-5a, producing the o-QM 3. This reaction is mostly carried out starting from tocopherones 6 or /zora-tocopherylquinone (7) under acidic catalysis, so that chromanoxylium 4 is produced in the first step, followed by proton elimination from C-5a. In the overall reaction of a tocopherone 6, a [ 1,4] -elimination has occurred. The central species in the oxidation chemistry of a-tocopherol is the o-QM 3, which is discussed in detail subsequently. 

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... 

The questions whether 5a-C-centered radicals exist in oxidation chemistry of a-tocopherol and whether mechanisms proposed in early days of vitamin E research are correct might appear academic at a first glance, but as soon as one recalls the immense medical, physiological, and economic importance of a-tocopherol and its... 

FIGURE 6.46 Oxidation chemistry of 5-(4-methylphenyl)- y-tocopherol (76), establishing a reaction system phenylogous to a-tocopherol (1), with quinone methide 77 and benzyl bromide 78 being the conjugatively stabilized, phenylogous counterparts of o-QM 3 and 5a-bromo-a-tocopherol (46), respectively. 

The oxidation chemistry of a-tocopherol and the regioselectivity of the o-QM formation are well investigated (Section 6.3.2). This is in complete contrast to [3-tocopherol. At a first glance, the [3-tocopherol case would appear much simpler as the free aromatic position at C-7 offers no alternative methyl group to be involved in o-QM formation. However, preliminary results show that the oxidation chemistry of [3-tocopherol is rather complex, the free position undergoing alkylation by the o-QM... 

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