Competitive oxidation

In reahty the chemistry of breakpoint chlorination is much more complex and has been modeled by computer (21). Conversion of NH/ to monochloramine is rapid and causes an essentially linear increase in CAC with chlorine dosage. Further addition of chlorine results in formation of unstable dichloramine which decomposes to N2 thereby causing a reduction in CAC (22). At breakpoint, the process is essentially complete, and further addition of chlorine causes an equivalent linear increase in free available chlorine. Small concentrations of combined chlorine remaining beyond breakpoint are due primarily to organic chloramines. Breakpoint occurs slightly above the theoretical C1 N ratio (1.75 vs 1.5) because of competitive oxidation of NH/ to nitrate ion. Organic matter consumes chlorine and its oxidation also increases the breakpoint chlorine demand. Cyanuric acid does not interfere with breakpoint chlorination (23). 

Oxidation of complex sulphide ores — competitive oxidation of cations... 

Cations forming insoluble chromates, such as those of silver, barium, mercury (I), mercury(II), and bismuth, do not interfere because the acidity is sufficiently high to prevent their precipitation. Bromide ion from the generation may be expected to form insoluble silver bromide, and so it is preferable to separate silver prior to the precipitation. Ammonium salts interfere, owing to competitive oxidation by bromate, and should be removed by treatment with sodium hydroxide. 

From the results discussed above as well as from the literature data [5-10,12-14] it follows that an important role of O2 in the SCR process is to convert NO into NOj. The latter then initiates methane oxidation into CO, and is itself reduced into NO and N2. Both NO, and O2 may participate in CH4 oxidation (Fig. 1B) and the ratio between the rates of these competitive oxidation reactions will be critical for the selectivity of the SCR process. Hence, the absolute rates of CH4 oxidation by Oj were compared with those occurring in the SCR process. The rates of these reactions were determined under different reaction conditions (using the... 

Competitive oxidation of equimolar mixtures ofn-hexane and another alkane (alkane II) over TS-2 using lIT) ets oxidant... 

COMPETITIVE OXIDATION OF CARBON MONOXIDE IN THE PRESENCE OF HYDROGEN... 

MCBA enhances the solubility of the cobalt salts in MeCN solution, thereby ensuring better efficiency to a needed redox decomposition of the hydroperoxide intermediate of the substrate en route to the products". By using the HPI/Co(II)/MCBA/02 system in MeCN solution at 25 °C, competitive oxidations of p-X-substituted benzyl alcohols were run pairwise (Scheme 8). From the amount of the aldehydes produced, the relative reactivity (kx/ h) could be reckoned, and the acquired data provided a p = —0.68 in a Hammett plot vs. <7+. ... 

To estimate the relative reactivity of allylic, benzylic, and nonconjugative aliphatic alcohols toward the Ar3BiCl2/DBU system, intermolecular competitive oxidations were examined. As summarized in Scheme 20, cinnamyl and benzylic alcohols were preferentially oxidized in the presence of ethyl alcohol. The chemos-electivities observed for the Ar3BiCl2/DBU oxidant (Ar = o-tolyl) are considerably higher than those achieved by Dess-Martin periodinane [83, 84]. 

Scheme 23 Competitive oxidations between primary and secondary benzyl alcohols by using the bulky bismuthonium salts [90]...

Figure 10. Competitive oxidation of cC6 and cC12 and of n-C5 and n-C8 by PhIO on FePcY [63].

A number of substrates having a benzylic ether moiety were reacted with 51 to afford the corresponding benzylic esters in good yields (equation 84). For evaluating the effects of p-substiments on the oxidation of a series of benzylic ethers, a competitive oxidation of p-substimted benzylic propyl ethers with 51 was carried out. The Hammett correlation plot for the oxidation reaction gave a better correlation of the relative ratio factors with the a rather than with the a+ substituent constants and afforded a reaction constant p+ = —0.57 (r = 0.99). This p+ value shows that 51 is an electrophilic species and appears to be comparable to the p+ value of —0.65 for benzylic hydrogen abstraction from dibenzyl ethers by the benzoyloxy radicaP . 

It was reported earher that the oxidation of a sulfoxide to a sulfone involves either an initial nucleophihc attack of the nucleophilic oxidant or an electrophihc attack by an electrophilic oxidant. It is noteworthy that the oxidation of p-tolyl methyl, phenyl methyl and p-chlorophenyl methyl sulfoxides to the sulfones using the sulfonylperoxy intermediate 51 appears to be electrophihc, namely the relative reactivity order was p-tolyl methyl > phenyl methyl > p-chlorophenyl methyl sulfoxide based on competitive oxidations. 

The unfortunately large difference in the reactivity of 1-butene and 2-butene to photosensitized oxidation is not surprising in view of the previous observed structural effects in such oxidations (10). Competitive oxidation experiments are underway to determine the exact magnitude of the rate difference involved and to evaluate the effect of impurities which might quench the reaction. 

Determination of the antioxidant activity in the system comprising p-carotene and linoleic acid is based on competitive oxidation of p-carotene during heat-induced auto-oxidation of hnoleic... 

Kinetic and mechanistic investigations on the o-xylene oxidation over V205—Ti02 catalysts were carried out by Vanhove and Blanchard [335, 336] using a flow reactor at 450°C. Possible intermediates like o-methyl-benzyl alcohol, o-xylene-a,a -diol, toluic acid and phthalaldehyde were studied by comparing their oxidation product distribution with that of toluene. Moreover, a competitive oxidation of o-methylbenzyl alcohol and l4C-labelled o-xylene was carried out. The compounds investigated are all very rapidly oxidized, compared with o-xylene, and essentially yield the same products. It is concluded, therefore, that these compounds, or their adsorbed forms may very well be intermediates in the oxidation of o-xylene to phthalic anhydride. The ratio in which the partial oxidation products are formed appears to depend on the nature of the oxidized compounds, i.e. o-methylbenzyl alcohol yields relatively more phthalide, whereas o-xylene-diol produces detectable amounts of phthalan. This... 

Select the most appropriate type of chemical agent, checking whether there are similar/ competitive oxidation processes available (e. g. application of AOPs etc.) which might be more efficient or economical. 

Inoue, T., Watanabe, T., Fujishima, A., and Honda, K., Competitive Oxidation at Semiconductor Photoanodes, in Semiconductor Liquid--Junction Solar Cells, Heller, A., Ed., The Electrochemical Society, Princeton, N3, 1977, 210. 

Evidence of variables that influence the relative rates of reaction of olefins and alcohols was obtained from experiments with compounds that have both olefinic and alcoholic functions and by the competitive oxidation of mixtures of olefins and alcohols. The data of Table VI show that when the double bond has no substituents, as in allyl alcohol, but-3-en-l-ol, or 2-methylbut-3-en-l-ol, only the epoxide is formed but when the double bond has substituents, the epoxida-tion rate is decreased and ketone and aldehyde products from the oxidation of the OH group are formed. This effect is more pronounced with a greater degree of substitution. Since the double bond and the OH group are part of the same molecule, the difference must arise from the different abilities of the reactants to coordinate and react at the titanium center restricted transition-state shape selectivity is a possibility. The terminal double bond, sterically less hindered, interacts strongly with titanium, preventing coordination of the competing OH... 

The competitive oxidation of olefins has also been investigated in the presence of alkanes. As discussed below, alkanes are oxidized by H202 in the... 

As was mentioned in Section V.C.3.b, when competitive oxidation of 1-octene and -hexane is carried out, the alkene is preferentially oxidized. Correspondingly, alkenes react at lower temperatures than alkanes. It is therefore surprising that under noncompetitive reaction conditions, the rate of oxidation of n-hexane is higher than that of 1-octene (Huybrechts et al., 1992). One possible explanation for this observation is that the reaction conditions were different (Clerici et al., 1993b). At 373 K titanium peroxo compounds decompose, thereby giving rise to radical chain reactions that are negligible at lower temperatures. Thus there could be a different mechanism for low- and high-temperature oxidations made more complex by secondary uncatalyzed oxidation of initial products (Spinace et al., 1995). 

Whereas diallyl ethers do not readily undergo Zr-promoted cyclization due to competitive oxidative addition [23a], diallylamines have been shown to readily undergo Zr-promoted cyclization, even in cases where the corresponding all-C dienes fail to do so [231. Since the development of the Zr-catalyzed diene cyclization with n-butylmagnesiums. such as n-BuMgBr... 

For successful operation a selective CO oxidation catalyst in a reformer-PEFC system must be operated at ca. 353-373 Kin a complex feed consisting of CO, 02, H2, C02, H20 and N2, and be capable of reducing CO concentrations from about 1% to below 50 ppm - this is equivalent to a CO conversion of at least 99.5% [4, 54, 60], In addition, this conversion must be achieved with the addition of equimolar 02 (twice the stoichiometric amount) and the competitive oxidation of H2 must be minimized. This is expressed as selectivity, which is defined as the percentage of the oxygen fed consumed in the oxidation of CO for commercial operation a selectivity of 50% is acceptable, since at this selectivity minimal H2 is oxidized to water. 

Nevertheless, dehydrogenation of benzyl alcohols can help to elucidate the reaction mechanism. Competitive oxidation of differently substituted benzyl alcohols produced a linear plot of log(kx/kH) versus the Brown-Okamoto o+, with a slope corresponding to a Hammett p+ value of-0.75 (Figure 13.2). The moderate negative value of p+ can be interpreted in terms of a positively charged transition state. The existence of an incipient carbenium ion intermediate in the reaction pathway can account for the experimental evidence and in particular for the faster oxidation of 3-octanol respect to 2-octanol (entry 5 in Table 13.2 vs. entry 1 in Table 13.3),... 

Moreover, it is possible to selectively oxidize secondary alcohols in the presence of primary alcohols without using any protecting group, as shown in the case of competitive oxidation of cyclooctanol and 1-octanol (Figure 13.3). 

---