Oxidation by dioxygen

The results of many studies carried out with zeolites, mostly in the 1970s and reviewed in ref [52], did not meet these expectations. ZeoUtes cannot compete with conventional catalytic systems both in the selective and complete oxidations. The main reason is that the transition metals introduced into a zeoUte matrix lose their ability to activate dioxygen, as was evidenced by isotopic O2 exchange [53, 54]. The 

But the situation is dramatically changed if monooxygen donors are used instead of dioxygen. The two best known examples are titanosilicalites TS-1, which proved to be excellent catalysts for liquid-phase oxidation by H2O2, and FeZSM-5 zeolites, which are efficient catalysts for gas-phase oxidation by N2O. Numerous works with H2O2 are well known and discussed in several reviews [55, 56]. We shall consider the oxidation by N2O. 

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Regulation of the total oxidation of reductants (HC, CxIIvO.) by 02, taking into account the difference of temperature between their mild oxidation by NOz and their total oxidation by dioxygen. This point requires the choice of a total oxidation function, but not too much active. 

The reaction CO+NO may be seen as an oxidation of CO by NO, which competes with the oxidation of CO by 02. On Rh, CO oxidation by dioxygen is almost two orders of magnitude faster than CO oxidation by NO. Nevertheless, if the three reactants are present together, the rate of CO oxidation by 02 dramatically decreases while that of CO by NO increases. As a consequence, both reactions occur practically at the same rate [76]. 

MeCN, the divalent analog Co2Cl4(eHTP) undergoes P oxidation (by dioxygen) at each donor atom to produce the corresponding hexaphosphine oxide.158... 

Catalysis by Transition Metal Ions and Complexes in Hydrocarbon Oxidation by Dioxygen... 

The phenomenon of chemical induction was intensively studied by Jorissen [33-37]. He discovered that indigo was not oxidized by dioxygen but was simultaneously oxidized in the presence of oxidized triethylphosphine or benzaldehyde. He measured the factor of chemical induction in these reactions as equal to unity. Later, he proved that the oxidation product of benzaldehyde, benzoic peracid, did not oxidize indigo under conditions of experiment. This shows that a very active intermediate was formed during the oxidation of benzaldehyde and that it was not perbenzoic acid. Engler assumed peroxide to be in two forms, namely, an active moloxide A02 and a more stable peroxide. A new correct interpretation of chemical induction in oxidation reactions was provided later by the chain theory of oxidation of organic compounds (see later). 

Main Discoveries and Concepts in the Field of Oxidation by Dioxygen, Which Appeared before the Chain Theory [43-46]... 

Paraffins were oxidized by dioxygen to carbon acids... 

The peroxide theory of Bach [20] and Engler [23] fixed the phenomenon of peroxide formation as the primary product of hydrocarbon oxidation by dioxygen. However, the problem of the mechanism of peroxide formation remained unsolved. The new stage of successful study of organic compound oxidation began after the discovery of free radicals as active intermediates of many chemical processes. 

A very serious problem was to clear up the formation of hydroperoxides as the primary product of the oxidation of a linear aliphatic hydrocarbon. Paraffins can be oxidized by dioxygen at an elevated temperature (more than 400 K). In addition, the formed secondary hydroperoxides are easily decomposed. As a result, the products of hydroperoxide decomposition are formed at low conversion of hydrocarbon. The question of the role of hydroperoxide among the products of hydrocarbon oxidation has been specially studied on the basis of decane oxidation [82]. The kinetics of the formation of hydroperoxide and other products of oxidation in oxidized decane at 413 K was studied. In addition, the kinetics of hydroperoxide decomposition in the oxidized decane was also studied. The comparison of the rates of hydroperoxide decomposition and formation other products (alcohol, ketones, and acids) proved that practically all these products were formed due to hydroperoxide decomposition. Small amounts of alcohols and ketones were found to be formed in parallel with ROOH. Their formation was explained on the basis of the disproportionation of peroxide radicals in parallel with the reaction R02 + RH. 

Rust [55] studied the oxidation of branched alkanes and was the first to observe the formation of dihydroperoxides as primary products of the hydrocarbon oxidation [55], Dihydroperoxide was found to be the main product of 2,4-dimethylpentane oxidation by dioxygen at 388 K ... 

The first three conditions of chain reaction assume that a chemical system contains free radicals. Therefore, a mechanism providing a continuous generation of radicals must exist. For instance, vinyl monomers CH2=CHX are oxidized by dioxygen only in the presence... 

The oxidation of ethers by dioxygen is very similar to hydrocarbon oxidation. The following three peculiarities of ethers can be mentioned since they influence the mechanism of their oxidation by dioxygen. 

The BDE of the a-C—H bonds of ethers R1R2C HOC HR1R2 are weaker in comparison with the C—H bonds of the parent hydrocarbons [2], When a-C—H of ether is cleaved, the formed a-alkoxyalkyl radical is stabilized by the interaction of an unpaired electron with p-electrons of the oxygen atom. Therefore, the attack of the peroxyl radical on the ether molecule occurs more rapidly than on hydrocarbon. Most of the ethers are easily oxidized by dioxygen at moderate temperatures. Table 7.11 contains a list of BDEs of ethers, as well as BDEs of hydrocarbons, and values of enthalpies of peroxyl radical abstraction with ether. 

Like hydrocarbons, ethers are oxidized by dioxygen via the chain mechanism. The mechanism of ether oxidation with initiator I includes the following elementary steps [8,9] ... 

Aldehydes are oxidized by dioxygen by the chain mechanism in reactions brought about in different ways initiated, thermal, photochemical, and induced by radiation as well as in the presence of transition metal compounds [4-8]. Oxidation chains are usually very long from 200 to 50,000 units [4], Acyl radicals add dioxygen very rapidly with a rate constant of 10s—109 Lmol V1 [4], Therefore, the initiated chain oxidation of aldehyde includes the following elementary steps at high dioxygen pressures [4-7] ... 

Ketones, like hydrocarbons and other organic compounds, are oxidized by dioxygen via the chain mechanism [4,62]. The carbonyl group weakens the adjacent C—H bond. Therefore, a peroxyl radical attacks the a-C—H bond as this bond is the most reactive in a ketone. The pecularities of ketone oxidation are the same as aldehyde oxidation. 

Amides, as amines and hydrocarbons, are oxidized by dioxygen according to the chain mechanism [1], The initiated oxidation of amides proceeds according to the classical scheme... 

Aliphatic esters are oxidized by dioxygen through the chain mechanism similar to the mechanism of hydrocarbon oxidation. In the presence of initiator I at such dioxygen pressure when [O2] > 10 4 mol L 1 in an ester solution, ester AcOCH2R oxidation includes the following elementary steps [39,40]. 

CATALYSIS BY TRANSITION METAL IONS AND COMPLEXES IN HYDROCARBON OXIDATION BY DIOXYGEN... 

Salts of transition metals are widely used in technological processes for the preparation of various oxygen-containing compounds from hydrocarbon raw materials. The principal mechanism of acceleration of RH oxidation by dioxygen in the presence of salts of heavy metals was discovered by Bawn [46 19] for benzaldehyde oxidation (see Chapter 1). Benzaldehyde was oxidized with dioxygen in a solution of acetic acid, with cobalt acetate as the catalyst. The oxidation rate was found to be [50] ... 

Tin(II) was found to be oxidized by dioxygen via the chain branching mechanism [156-162]. The oxidation rate is v = k[02]2 in organic solvents and v = [Sn(II)]1/2[02]1/2 in aqueous solutions. The reaction, under certain conditions, has an induction period. Free radical acceptors retard this reaction. The following kinetic scheme was proposed for tin(II) oxidation by dioxygen. 

Ketones are resistant to oxidation by dioxygen in aqueous solutions at T= 300-350 K. Transition metal ions and complexes catalyze their oxidation under mild conditions. The detailed kinetic study of butanone-2 oxidation catalyzed by ferric, cupric, and manganese complexes proved the important role of ketone enolization and one-electron transfer reactions with metal ions in the catalytic oxidation of ketones [190-194],... 

Phosphites can react not only with hydroperoxides but also with alkoxyl and peroxyl radicals [9,14,17,23,24], which explains their susceptibility to a chain-like autoxidation and, on the other hand, their ability to terminate chains. In neutral solvents, alkyl phosphites can be oxidized by dioxygen in the presence of an initiator (e.g., light) by the chain mechanism. Chains may reach 104 in length. The rate of oxygen consumption is proportional to v 1/2, thus indicating a bimolecular mechanism of chain termination. The scheme of the reaction... 

The effectiveness of the antioxidant depends not only on its reactivity, but also on its molecular weight that affects the rate of the antioxidant loss due to evaporation. The following example illustrates this dependence. Antioxidants of the structure 2,6-bis (1, l-dimethylethyl)phenols with para-substituents of the general structure ROCOCH2CH2 were introduced into decalin and polypropylene films that were oxidized by dioxygen at... 

In addition to peroxyl radicals and hydroperoxide, amines are oxidized by dioxygen and this reaction was found to be catalyzed by the copper surface also. This reaction was studied in chlorobenzene and occurs with the rate ... 

Another type of spin traps, which have been recommended for the detection of superoxide, are the derivatives of hydroxylamine. In 1982, Rosen et al. [25] showed that superoxide is able to oxidize the hydroxylamine derivative 2-ethyl-1-hydroxy-2,5,5-trimethyl-3-oxazoli-dine (OXANOH) to corresponding free radical 2-ethyl-1-hydroxy-2,5,5-trimethyl-3-oxazolidinoxyl (OXANO). Although this radical is very stable and easily identified by its ESR spectrum, it is also easily reduced by ascorbic acid and other reductants. Furthermore, OXANOH and other hydroxylamines are oxidized by dioxygen in the presence of transition metal ions to form superoxide, and therefore, superoxide detection must be carried out in the presence of chelators. 

The kinetic results reported by Jameson and Blackburn (11,12) for the copper catalyzed autoxidation of ascorbic acid are substantially different from those of Taqui Khan and Martell (6). The former could not reproduce the spontaneous oxidation in the absence of added catalysts when they used extremely pure reagents. These results imply that ascorbic acid is inert toward oxidation by dioxygen and earlier reports on spontaneous oxidation are artifacts due to catalytic impurities. In support of these considerations, it is worthwhile noting that trace amounts of transition metal ions, in particular Cu(II), may cause irreproducibilities in experimental work with ascorbic acid (13). While this problem can be eliminated by masking the metal ion(s), the masking agent needs to be selected carefully since it could become involved in side reactions in a given system. 

Coulter ED, Shenvi NV, Beharry Z, et al. 2000b. Rubrerythrin-catalyzed substrate oxidation by dioxygen and hydrogen peroxide. Inorg Chim Acta 297 231. ... 

In addition to enzymatic oxidation, flavonoid oxidation can take place via autoxidation (metal-catalyzed oxidation by dioxygen) and ROS scavenging. The former process can be related to flavonoid cytotoxicity (ROS production) while the latter is one of the main antioxidant mechanisms. Both processes may be modulated by flavonoid-protein binding. Although poorly documented so far, these points could be important and, for instance, albumin-flavonoid complexes with an affinity for LDL could act as the true plasma antioxidants participating in the regeneration of a-tocopherol from the a-tocopheryl radical formed... 

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