Peroxo radical

It is possible that steric factors prevent the formation of the bridging peroxide species in the case of vitamin Bj2,. (55), however, other cases have been reported (59) in which oxygenation proceeds no further than the formation of mononuclear peroxo radicals (i.e. first stage above). 

From the evidence described above concerning the uncommon behavior of radical reactions in hydrocarbon oxidation at a titanium center inside a zeolite, the possibility that peroxo radicals are also involved in oxidation reactions of other compounds should be considered. In contrast to the mechanism discussed above for the hydrocarbons, in this case both the peroxo and the reactant molecule would be coordinated at the Tilv center, and the reaction would therefore take place between two coordinated species. 

A crucial point in this mechanism is the formation of the peroxo radical, which requires the reduction of the Ti4+ to Ti3+—a process that is easier with highly dispersed Tiiv and should therefore be favored in titanium silicates in which the dispersion of Tilv is the highest ever observed. 

Ozone is commonly prepared by silent electric discharge in oxygen, which gives up to 10% 03. The gas is blue. Pure ozone can be obtained by fractional liquefaction of 02—03 mixtures. There is a two-phase liquid system the one with 25% of 03 is stable, but a deep purple phase with 70% of 03 is explosive, as is the deep blue pure liquid (bp -112°C). The solid (mp -193°C) is black violet. Small quantities of 03 are formed in electrolysis of dilute sulfuric acid, in some chemical reactions producing 02, and by the action of uv light on 02 the combination of certain peroxo radicals and some chemical reactions may also result in 03.19... 

The oxidation of a branched C4 alkane, isobutane, was carried out to probe the mechanism of the C-H bond activation of alkanes on the VPO catalysts [103]. Maleic anhydride was among the products of oxidation of this branched alkane. In the case of isobutane 29 (Figure 15), the surface-bound peroxo radical would show discrimination in activating first the weaker tertiary C-H bond. The... 

The ORR is a multielectron reaction that may include a number of elementary steps involving different reaction intermediates. The detailed mechanism is still not known, since neither ex situ nor in situ techniques are capable of identifying all reaction intermediates formed under genuine reaction conditions exist. Of the various possibilities [13], it has been proposed that on metal surfaces the most plausible reaction pathway for the ORR in both alkaline and acidic electrolytes can be described by the so-called serial reaction pathway (Scheme 3.1) where after the transfer of two electrons and (simultaneous) fast protonation of superoxo/peroxo radicals (not included in the reaction scheme), O is reduced to H O (with rate constant k. ... 

Oxygen may substantially reduce the catalytic activity of porphyrin CCT catalyst.75 First, the peroxo radical, Rn02 forms as in eq 32. 

The peroxo radical then oxidizes the cobalt porphyrin with the formation of several products with a rate constant of 7000 M 1 s 1, typical for peroxo radicals in their reaction with cobalt11 complexes. In addition to the alkylperoxoCo111—porphyrin, 92, in eq 36, there is concomitant formation of a Por Co species in which the porphyrin ligand has been modified. 

Reaction of the porphyrin ligand leads to degradation of the Soret band and reduction of ring current in NMR spectra after prolonged polymerization in the presence of air.75 The concentration of Por Co species increases with time during a polymerization, possibly because of multiple reactions of peroxo radicals with the porphyrin macrocycle. 

Mechanisms of the photooxygenation reaction in the cases of other than iron metal scans to be different from that postulated for the FeCIs-catalyzed process [60h]. Low-valent species are apparently involved in the oxidation. These species can possibly add an oxygen molecule to produce metal peroxo radicals and peroxo... 

It has been proposed that the crucial step of the oxidation by the reagent O2 - H2O2 - VOj"- pyrazine-2-carboxylic acid is the very efficient generation of HO radicals [15]. These radicals abstract a hydrogen atom from the alkane, RH, to generate the alkyl radical, R. The latter reacts rapidly with an O2 molecule affording the peroxo radical, ROO. This radical is then transformed simultaneously into three products alkyl hydroperoxide, ketone, and alcohol. The relative content of the last two products is increased if the reaction temperature is higher. 

The peroxo radical thus formed initiates the oxidation of cyclohexane. 

Once an aldehyde radical is formed through the intervention of an OH group, the subsequent step proceeds via a peroxo radical, producing acetic acid. This peroxo radical is consumed and can be regenerated via the direct interaction of aldehyde radicals with oxygen. This chain reaction mechanism explains the findings than one photon can contribute to the photoconversion of several pollutant molecules and can give as a result, quantum yields in excess of 100%. 

Possible pathways of further reactions of Q are depicted in Scheme 9 (Eqs. 14-16 similar reactions are expected for DNA ). Reactions of 4 -DNA radicals with O2 (Eq. 14 in Scheme 9) are well characterized (405, 406) and lead to peroxo radicals R, which are converted after a series of rearrangements to... 

Interestingly, it has been found that the formation of SOA is accelerated in the presence of SO2 and sulfuric acid (Jang et al. 2002, Edney et al. 2005, Surrat et al. 2007). The first step in atmospheric SO2 oxidation is OH addition (Chapter 5.5.2.1) and this radical can react with alkoxy radicals (RO) to form sulfonic acid and further with organic peroxo radicals to form dialkyl sulfates ... 

As mentioned, O2 cannot be photochemically destructed in the troposphere because of its missing short wavelength (below and Chapter 5.3.7). The main fate of O2 in the troposphere is the addition of several radicals X to produce peroxo radicals (reaction 5.1 and below), an important step in the oxidation of trace gases ... 

The fate of the alkyl radical R is combined with O2 to form the alkyl peroxo radical RO2 (R—O—O ) ... 

There are a number of reactions competing with reaction (5.59), mostly with other peroxo radicals. Combining with HO2 leads to organic peroxides the simplest is methylhydroperoxide CH3OOH ... 

Two organic peroxo radicals combine to give an alcohol and aldehyde ... 

Aldehydes are very reactive substances. They react with OH by the abstraction of H (almost from the chain) to produce finally bicarbonyls, e. g. dialdehydes and ketoaldehydes (the carbonyl group denoted by C=0). Nevertheless, more important is the photolysis of aldehydes and ketones that initiates radical chains. We have seen that the simplest aldehyde HCHO is photolyzed (reaction 5.46) and gains H (which turns into HO2) and the formyl radical HCO (a carbon radical), which reacts with O2 to HO2 and CO. Therefore, HCHO is an efficient radical source. In analogy, higher aldehydes are photolyzed and produce H atoms and acyl radicals RCO, for example acetyl CH3CO, which do not decompose like the formyl radical HCO and add O2, thereby giving acyl peroxo radicals R—C(0)02i... 

Fig. 5.9 Scheme of organic radical chemistry and fate of characteristic organic groups. RH hydrocarbon, R organic radical, RO2 organic peroxo radical, ROOH organic peroxide, RO organic 0x0 radical, RCHO aldehyde, RCOOH carbonic add, RC(OC)R keton, R=R olefin. Reaction between RO or RO2 and NO or NO2 are not included in this scheme. 

Reaction (5.142) is valid for the aqueous and gaseous phases in all environmental media. The 0x0 and peroxo radicals provide the intermediates. In Chapter 2.8.2.8, we summarized the characteristic behavior of atmospheric H2O2 resulting from its... 

Equation (5.172) does not represent an elementary reaction it is a multistep mechanism involving NO3 or the dimer (NO)2. This NO3 (0=N00) is an isomer to the nitrate radical 0=N=0 (O) and the first step in NO oxidation. It is clear that this very instable peroxo radical will almost decompose by quenching to NO + O2, which results in a slow reaction probability (we will meet this reaction later in biological systems) ... 

Organic peroxo radicals also add NO, where the intermediate CH3OONO rearranges to CH3ONO (O) ... 

In aqueous solution, peroxo radicals can also react with bisulfite, gaining the sulfite radical ... 

As for other peroxo radicals, it can transfer O onto other dissolved species (NO NO2) and then decompose them (finally, the methylperoxo radical forms HCHO). 

Another class of atmospherically important compounds is ketones, which are almost of biogenic origin. The most simple is acetone, which is ubiquitous in air. With the exception of acetone, higher ketones react preferably with OH at any carbon atom by H abstraction. With subsequent O2 addition the fate of RO2 is known, either forming aldehyde (HCO) at terminal carbon or ketone (C=0) in the middle of the chain. The photolysis of acetone (Emrich and Warneck 2000) provides the known radicals, which finally convert into HCHO and form the acetyl-peroxo radical (see 5.46 and 5.47). 

It is assumed, what recombination of cumyl peroxo radicals ROO occurs through intermediate formation of tetroxide RO R, decomposition of which is realized on Russell mechanism [34], (through cyclic six-member transient state) and results to formation of decay products CH, a-methyl styrene and 02,on scheme ... 

In [43] mechanism of AP formation on reaction of quadratic chain termination of cumyl peroxo radicals ROO is offered ... 

As mentioned in the introduction, the Gassman oxindole synthesis is a good way to gain access to isatins. Historically, the isolated oxindoles were oxidized to the corresponding isatin using red mercuric oxide and boron trifluoride.These harsh and environmentally unfriendly conditions have been replaced by an air oxidation approach in which an in situ generated anion reacts with oxygen to make an intermediate peroxo radical that... 

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