Superoxide ion, formation

The reaction of 1-hydroxy- or 1-aminonaphthoquinone with O2 shows a significant feature of the superoxide ion formation. The superoxide ion forms a van der Waals complex with another product of this reaction, a semiquinone. Hydrogen bonds are formed between Oj" and the OH and NH2 groups of the corresponding semiquinone. As a result, the reaction equilibrium is shifted to the right (Liwo et al. 1997). 

Although this reaction shows the formation of 02 +, it is also possible to add one electron to the 02 molecule to produce (),, the superoxide ion, or two electrons to form O/, the peroxide ion. In each case, the electrons are added to the antibonding 7r orbitals, which reduces the bond order from the value of 2 in the 02 molecule. For ()2 the bond order is 1.5, and it is only 1 for 022-, the peroxide ion. The 0-0 bond energy in the peroxide ion has a strength of only 142k) moT1 and, as expected, most peroxides are very reactive compounds. The superoxide ion is produced by the reaction... 

In order to rationalize the complex reaction mixtures in these slurry reactions the authors suggested that irradiations of the oxygen CT complexes resulted in simultaneous formation of an epoxide and dioxetane36 (Fig. 34). The epoxide products were isolated only when pyridine was co-included in the zeolite during the reaction. Collapse of the 1,1-diarylethylene radical cation superoxide ion pair provides a reasonable explanation for the formation of the dioxetane, however, epoxide formation is more difficult to rationalize. However, we do point out that photochemical formation of oxygen atoms has previously been observed in other systems.141 All the other products were formed either thermally or photochemically from these two primary photoproducts (Fig. 34). The thermal (acid catalyzed) formation of 1,1-diphenylacetaldehyde from the epoxide during photooxygenation of 30 (Fig. 34) was independently verified by addition of an authentic sample of the epoxide to NaY. The formation of diphenylmethane in the reaction of 30 but not 31 is also consistent with the well-established facile (at 254 nm but not 366 or 420 nm) Norrish Type I... 

Trace amounts of Cu(II) were reported to catalyze the oxidation of I-to I2 (156) and the phosphinate ion (H2P02) to peroxodiphosphate ion (PDP), which could be present as P20g, HP20 or H2P20f (757). Individual kinetic traces showed some unusual patterns in these reactions, such as the variation between first- and zeroth-order kinetics with respect to the formation of I2 under very similar conditions, or an autocatalytic feature in the concentration profiles of PDP, but these events were not studied in detail. The catalytic effect was interpreted in terms of a Cu(II) / Cu(I) redox cycle and the superoxide ion radical,... 

The reaction of superoxide ion with carbon tetrachloride is important for olefin epoxidations. This reaction includes the formation of the trichloromethyl peroxide radical Oj" + CCI4 —> Cl + CI3COO. The trichloromethyl peroxide radicals formed oxidize electron-rich olefins. The latter gives the corresponding epoxides. This peroxide radical is a stronger oxidizing agent than the superoxide ion itself (Yamamoto et al. 1986). 

The first of the reaction steps in the amine-ozone interaction also consists of one-electron transfer from the amine to ozone, with the formation of the corresponding cation and anion-radicals. The ozone anion-radical has been revealed at low temperatures. Formation of the superoxide ion and the amine nitroxide are the understandable results of the reaction (Razumovskii and Zaikov 1984, reference therein). 

In this type of spin traps, 5,5-dimethyl-l-pyrroline-Af-oxide (DMPO) deserves particular mention. DMPO is widely employed as a spin trap in the detection of transient radicals or ion-radicals in chemical and biological systems (see, e.g., Siraki et al. 2007). Characteristic ESR spectra arising from the formation of spin adducts are used for identification of specific spin species. In common opinion, such identification is unambiguous. However, in reactions with superoxide ion (Villamena et al. 2004, 2007b), carbon dioxide anion-radical (Villamena et al. 2006), or carbonate anion-radical (Villamena et al. 2007a), this spin trap gives rise to two adducts. Let us consider the case of carbonate anion-radical. The first trapped product arises from direct addition of carbonate anion-radical, second adduct arises from partial decarboxylation of the first one. Scheme 4.25 illustrates such reactions based on the example of carbonate anion-radical. 

In 1993, Blatter and Frei [34] extended the Aronovitch and Mazur [28] photo-oxidation into zeolitic media, which resulted in several distinctive advantages as described below. Irradiation in the visible region (633 nm) of zeolite NaY loaded with 2,3-dimethyl-2-butene, 16, and oxygen resulted in formation of allylic hydroperoxide, 17, and a small amount of acetone. The reaction was followed by in situ Fourier-transform infrared (FTlR) spectroscopy and the products were identified by comparison to authentic samples. The allylic hydroperoxide was stable at - 50°C but decomposed when the zeolite sample was warmed to 20°C [35]. In order to rationalize these observations, it was suggested that absorption of light by an alkene/Oi charge-transfer complex resulted in electron transfer to give an alkene radical cation-superoxide ion pair which collapses... 

Whether the ion pair [(C6H5)2CO 02 "] separates to yield alkali metal superoxide (M02) or collapses to yield alkali metal peroxide (M202) depends upon the stability of the alkali metal superoxide. Thus, in general the yield of superoxide increases as the alkali metal is changed from lithium to sodium to potassium to rubidium, a sequence that parallels the stabilities of the superoxides. Superior yields of superoxides are observed in pyridine solution (Table XI). This is apparently connected with the ability of pyridine to stabilize the superoxide ion by complex formation (25). 

Of the reactions listed in Table II, the only process that leads to a decrease of the energy of molecular oxygen is the formation of the free superoxide ion, Oj ( — 10.15 kcal/mol). The superoxide ion would therefore be expected to be the dioxygen species most commonly formed on oxide surfaces and in fact it is the species most studied, both in the bulk of various matrices and on surfaces. The other species (Oj and Oj ) are not stable in the gas phase, although they can be stabilized in the solid state (Table I) due to the additional coulombic stabilization from the lattice. 

Oxidation of cobalt(ll) to cobalt(lll) by oxygen in the presence of N-hydroxyethylethylenediamine and carbon produces large amounts of ethylenediamine. Other products are formaldehyde, formic acid, and ammonia. The sum of the moles of ethylenediamine and ammonia produced is equal to the total number of moles of cobalt(ll) oxidized. A steady-state concentration of Co(ll)-Co(lll) is established in which the ratio Co(lll)/ Co(ll) = 1.207. Thus cobalt ion behaves as a true catalyst for cleavage of the N-hydroxyethyl-ethylenediamine. The total amount of cobalt(ll) oxidized per unit time, X, was calculated from the derived equation X = 3.8 + 7.0 k2 T — 3.8e-2-2k 1, where k2 = 0.65 hr.—1 The observed rate of formation of ethylenediamine plus ammonia also follows this equation. It is proposed that the cobalt ion serves as a center where a superoxide ion [derived from oxidation of cobalt-(II) by oxygen] and the ligand are brought together for reaction. 

The reversible formation of a low-spin [Co (III) (NHS) n02 ]2+ complex within a Co (II) Y zeolite has been demonstrated by EPR spectroscopy. In this complex n is probably equal to five. A maximum of one cobalt complex per large cavity was farmed. The cobalt hyperfine structure shows that the unpaired electron is only 8% on the metal ion. Experiments utilizing 170 indicate that 02 enters the coordination sphere of the Co2+ ions and that the unpaired electron is largely associated with the oxygen molecule. The oxygen-17 hyperfine structure reveals that the two oxygen atoms are not equivalent hence, it is concluded that the oxygen is bonded as a peroxy-type superoxide ion. 

The major in situ process that results in the formation of H202 is undoubtedly photochemical (e.g., 12, 15, 49, 50). Photochemical formation of H202 in fresh and salt waters probably results from the disproportionation of the superoxide ion radical, 02 (8, 9, 15, 51, 52). The kinetics of superoxide disproportionation are well established (53), and its steady-state concentration can be calculated. Because of the known effects of superoxide ion in cells (47), its presence in surface waters may be important in biologically mediated processes. However, other sources, such as biological formation (e.g., 45, 54), redox chemistry (21, 24, 29, 31, 32), wet (e.g., 55) and dry (50, 56, 57) deposition, and surfaces (e.g., 58) may also be important. 

Most likely, the precursor for the formation of H202 in natural waters is the superoxide ion (02 -), which may have an even greater potential than H202 to affect geochemical and biological processes in the ecosystem. Therefore estimates of its lifetime and steady-state concentration are important. The aqueous chemistry of 02 was extensively reviewed by Bielski and coworkers (53). In aqueous solution 02 is in equilibrium with the conjugate acid ... 

The treatment of benzaldehyde with potassium hydroxide in acetonitrile results in the formation of the same product, i.e., cynnamyl nitrile (Sawyer Gibian 1979). Thus, in the presence of water or other proton sources, the 02 ion forms strong bases as well as oxygen and the peroxide anion. Therefore, many reactions that are ascribed to the superoxide ion are actually reactions with proton donors. These reactions produce effective oxidants (02 and HOO-) and strong bases (OH- or B-). 

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