Oxygen ionic peroxides

Of the known elements the inert gases alone arc without oxides. Here are considered only what may be termed the normal oxides, namely those with separate oxygen atoms or ions attached directly and only to the atom or ion of another element. Besides these there are the peroxides (p. 383) and superoxides (p. 384) in which the oxygen atoms are attached in pairs. In hydrogen peroxide (p. 381) they are connected by a single bond H 0—O—H in ionic peroxides, such as Na202, they appear as 0 ions and in superoxides, such as KOg, as Og ions. 

The Group HA metals react with oxygen to form normal ionic oxides, MO, but at high pressures of oxygen the heavier ones form ionic peroxides, MO2 (Table 6-4). 

Hemoglobin and myoglobin in their ferric forms show rudimentary peroxidatic and catalatic activity, but ferrous peroxidase does not combine reversibly with molecular oxygen. Ionic iron also gives the hydrogen peroxide reactions but not the combination with oxygen. 

While these results support the ionic orthoester mechanism, it was originally suggested that an oxygen radical may participate since it was claimed that the reaction proceeds in the presence of dibenzoyl peroxide instead of zinc, and that the presence of hydroquinone or exclusion of oxygen completely inhibits the reaction. Later work, however, could not confirm the previously observed influence of hydroquinone or oxygen. 

E++02, etc where E is an element. The metallic atoms are bonded to the oxygen bridge with ionic bonding. For convenience, differentiation is made between simple and complex inorganic peroxide compds. According to VoPnov (Ref 5) simple peroxide compds also... 

The principal product of the reaction of the alkali metals with oxygen varies systematically down the group (Fig. 14.15). Ionic compounds formed from cations and anions of similar radius are commonly found to he more stable than those formed from ions with markedly different radii. Such is the case here. Lithium forms mainly the oxide, Li20. Sodium, which has a larger cation, forms predominantly the very pale yellow sodium peroxide, Na202. Potassium, with an even bigger cation, forms mainly the superoxide, K02, which contains the superoxide ion, O,. ... 

A peroxide can be a binary ionic compound containing the 02 2 ion, such as Na202, or a covalent compound, such as H202, with oxygen in the -1 oxidation state. 

The use of gaseous oxygen as an oxidant in ionic liquids also appears to be limited by its low solubility, for example, in [BMIM]PFg for the oxidation of aromatic aldehydes to give carboxylic acids 219). Hydrogen peroxide and organo-peroxide, with their higher solubilities, have been used efficiently for enzymatic oxidation 220). 

Electron-donating substituents make the aromatic subsU ate more reactive than benzene and lead to o,/ -orientation, while electron-withdrawing substituents decrease the reactivity and give mostly m-orientation products. The detailed mechanism of the formation of the a complex has been studied by oxygen-18 labeling of the sulfonyl oxygen in p-nitrobenzenesulfonyl peroxide. The ionic mechanism for aromatic substitution by sulfonyl peroxides has been confirmed by carrying out the substitution reaction in the presence of redox catalysts such as copper and cobalt salts and aluminum chloride. Small differences in the rate of the products can be found in the presence or absence of these additives, and it has been concluded that the ionic mechanism accounts satisfactorily for these results. ... 

The discussion of the hyperfine tensor has indicated clearly that the analysis of the hyperfine pattern has been very valuable in developing an understanding of the adsorbed oxygen ion. On balance, in the oxide systems it would seem preferable to use the term superoxide rather than peroxide or peroxy for 02. Overall, the picture is largely consistent with an ionic model for 02 on surfaces. 

Three types of reactive species are formed under irradiation and may become trapped in polymers ionic species, radicals, and peroxides. Little is known about the role of ions in the chemical transformations in irradiated polymers. Long-lasting ions arise, as demonstrated by radiation-induced conductivity, and may become involved in postirradiation effects. The presence of trapped radicals is well-established in irradiated polymers, but certain problems remain unsolved concerning their fate and particularly the migration of free valencies. Stable peroxides are produced whenever polymers are irradiated in the presence of oxygen. Both radicals and peroxides can initiate postirradiation grafting, and the various active centers can lead to different kinetic features. 

The addition of hydrogen halides to simple olefins, in the absence of peroxides, takes place by an electrophilic mechanism, and the orientation is in accord with Markovnikov s rule.116 When peroxides are added, the addition of HBr occurs by a free-radical mechanism and the orientation is anti-Markovnikov (p. 751).137 It must be emphasized that this is true only for HBr. Free-radical addition of HF and HI has never been observed, even in the presence of peroxides, and of HCI only rarely. In the rare cases where free-radical addition of HCI was noted, the orientation was still Markovnikov, presumably because the more stable product was formed.,3B Free-radical addition of HF, HI, and HCI is energetically unfavorable (see the discussions on pp. 683, 693). It has often been found that anti-Markovnikov addition of HBr takes place even when peroxides have not been added. This happens because the substrate alkenes absorb oxygen from the air, forming small amounts of peroxides (4-9). Markovnikov addition can be ensured by rigorous purification of the substrate, but in practice this is not easy to achieve, and it is more common to add inhibitors, e.g., phenols or quinones, which suppress the free-radical pathway. The presence of free-radical precursors such as peroxides does not inhibit the ionic mechanism, but the radical reaction, being a chain process, is much more rapid than the electrophilic reaction. In most cases it is possible to control the mechanism (and hence the orientation) by adding peroxides... 

In view of the data, this novel reaction is unlikely to proceed by any mechanism known for other epoxidation reactions. There are several facts that argue strongly against a free radical mechanism. Our data show that the reaction is not affected by the presence of oxygen in contrast to the work of Indictor and Brill (6). Complexes of cobalt, manganese, and iron, which are most effective in converting peroxides to radicals (5,8), are not catalysts for this reaction. This reaction is stereospecific, and therefore must proceed by an ionic mechanism. 

The addition of thiols to C—C multiple bonds may proceed via an electrophilic pathway involving ionic processes or a free radical chain pathway. The main emphasis in the literature has been on the free radical pathway, and little work exists on electrophilic processes.534-537 The normal mode of addition of the relatively weakly acidic thiols is by the electrophilic pathway in accordance with Markovnikov s rule (equation 299). However, it is established that even the smallest traces of peroxide impurities, oxygen or the presence of light will initiate the free radical mode of addition leading to anti-Markovnikov products. Fortunately, the electrophilic addition of thiols is catalyzed by protic acids, such as sulfuric acid538 and p-toluenesulfonic acid,539 and Lewis acids, such as aluminum chloride,540 boron trifluoride,536 titanium tetrachloride,540 tin(IV) chloride,536 540 zinc chloride536 and sulfur dioxide.541... 

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