Metal ion-oxygen systems

Table I. Hydroxylation of Toluene by Metal Ion—Oxygen Systems...

Metal Ion-Oxygen Systems. Hydroxylation of aromatic compounds occurs during the autoxidation not only of ferrous ion but also of other metal ions. The ions we have investigated are all capable of undergoing one-electron oxidation. [This is well known for ferrous, titanous, and cuprous ions and has recently been demonstrated also for stannous ion (43).] The main problem in elucidating the course of the hydroxylations is determining the nature of the hydroxylating species. 

In part of a previous publication (3J), we reported some preliminary conclusions on the hydroxylation of aromatic compounds by the metal ion-oxygen systems. This work has been extended, and it is clear that the most interesting feature is the strong dependence of the distribution of phenolic products on the initial concentration of the metal ions. In general, the proportion of the meta derivative formed from each of the four monosubstituted aromatics increases and the para isomer decreases with increase in the metal ion concentration. 

Of possible hydroxylating species, the two most obvious for consideration are the hydroxyl and perhydroxyl radicals. Nofre et al. (29) suggest that the former is the active species in the metal ion-oxygen systems, while Staudinger and his colleagues (37) propose that these systems involve a combination of both species. The hydroxyl radical is involved in hydroxylations by Fenton s reagent and the closely related... 

At present it is not possible to make a final decision as to the mechanism of hydroxylation by the metal ion-oxygen systems. Work is, however, in progress which, it is hoped, will resolve this interesting problem. 

The active hydroxylating species in this system is unlikely to be either the hydroxyl or perhydroxyl radical for the same reasons as discussed above for the metal ion-oxygen systems—namely, the lack of dimeric aromatic products and the difference in phenolic isomer ratios from those obtained using Fenton s reagent. 

Catalysts include oxides, mixed oxides (perovskites) and zeolites [3]. The latter, transition metal ion-exchanged systems, have been shown to exhibit high activities for the decomposition reaction [4-9]. Most studies deal with Fe-zeolites [5-8,10,11], but also Co- and Cu-systems exhibit high activities [4,5]. Especially ZSM-5 catalysts are quite active [3]. Detailed kinetic studies, and those accounting for the influence of other components that may be present, like O2, H2O, NO and SO2, have hardly been reported. For Fe-zeolites mainly a first order in N2O and a zero order in O2 is reported [7,8], although also a positive influence of O2 has been found [11]. Mechanistic studies mainly concern Fe-systems, too [5,7,8,10]. Generally, the reaction can be described by an oxidation of active sites, followed by a removal of the deposited oxygen, either by N2O itself or by recombination, eqs. (2)-(4). 

In the previous discussion, the electron-nucleus spin system was assumed to be rigidly held within a molecule isotropically rotating in solution. If the molecule cannot be treated as a rigid sphere, its motion is in general anisotropic, and three or five different reorientational correlation times have to be considered 79). Furthermore, it was calculated that free rotation of water protons about the metal ion-oxygen bond decreases the proton relaxation time in aqua ions of about 20% 79). A general treatment for considering the presence of internal motions faster than the reorientational correlation time of the whole molecule is the Lipari Szabo model free treatment 80). Relaxation is calculated as the sum of two terms 8J), of the type... 

Hydroxylation by the Metal lon—Oxygen Systems. A monosubsti-tuted benzene was suspended in aqueous solution of a metal salt through which oxygen was bubbled. Two aromatic compounds (toluene and anisole) were treated this way with each of four metal salts (ferrous sulfate in the presence of EDTA, titanous chloride, cuprous chloride and stannous pyrophosphate) a third compound (fluorobenzene) was oxidized with the ferrous, titanous, and cuprous systems, and a fourth aromatic compound (nitiobenzene) was treated with ferrous ion with EDTA. The initial concentration of the metal ion was varied. 

When toluene was hydroxylated with the titanous ion-oxygen system in a constant volume of water, the yield of phenolic products depended on the amount of metal salt. The yield increased proportionately with the concentration of the metal ion. Under these conditions the phenolic isomer ratios changed continuously, the proportion of the meta isomer increasing with the metal ion concentration (Table i). 

To achieve an effective destruction of tumor cells, a high quantum yield of singlet oxygen is required. Even in the absence of heavy atom substitution(s) and coordination of transition-metal ions, porphyrin systems generally satisfy this criterion. That is why most of the sensitizers currently under clinical evaluation for PDT are porphyrins or porphyrin-based molecules. 

The treatment units used for color removal are the same as those used for turbidity removal. However, the pH must be increased prior to filtration so that the metal hydroxides are removed by the filters. At low pH values, metal ions or their soluble complexes readily pass through the filters and form insoluble species in storage tanks and in the distribution system. For iron salts, it is important that the pH be greater than 6 as the oxidation of iron(II) to iron(III) occurs rapidly above this pH in the presence of dissolved oxygen or other strong oxidants (18). 

Ascorbic acid is a reasonably strong reducing agent. The biochemical and physiological functions of ascorbic acid most likely derive from its reducing properties—it functions as an electron carrier. Loss of one electron due to interactions with oxygen or metal ions leads to semidehydro-L-ascorbate, a reactive free radical (Figure 18.30) that can be reduced back to L-ascorbic acid by various enzymes in animals and plants. A characteristic reaction of ascorbic acid is its oxidation to dehydro-L-aseorbie add. Ascorbic acid and dehydroascor-bic acid form an effective redox system. 

Crevice corrosion of copper alloys is similar in principle to that of stainless steels, but a differential metal ion concentration cell (Figure 53.4(b)) is set up in place of the differential oxygen concentration cell. The copper in the crevice is corroded, forming Cu ions. These diffuse out of the crevice, to maintain overall electrical neutrality, and are oxidized to Cu ions. These are strongly oxidizing and constitute the cathodic agent, being reduced to Cu ions at the cathodic site outside the crevice. Acidification of the crevice solution does not occur in this system. 

Although this reaction may cause slight problems, the primary issue concerning ammonia is ammoniacal corrosion of CR system metals where oxygen is present and the pH is over 8.3. Under these circumstances, copper and its alloys and other nonferrous metals are attacked, and severe damage results due to the formation of a stable cupric ammonium complex ion. 

---