Oxidation-reduction disproportionation

KEY TERMS oxidation-reduction disproportionation Hess Law exothermic reaction... 

Chemically, nonmetals are usually the opposite of metals. The nonmetallic nature will increase towards the top of any column and toward the right in any row on the periodic table. Most nonmetal oxides are acid anhydrides. When added to water, they will form acids. A few nonmetals oxides, most notably CO and NO, do not react. Nonmetal oxides that do not react are neutral oxides. The reaction of a nonmetal oxide with water is not an oxidation-reduction reaction. The acid that forms will have the nonmetal in the same oxidation state as in the reacting oxide. The main exception to this is N02, which undergoes an oxidation-reduction (disproportionation) reaction to produce HN03 and NO. When a nonmetal can form more than one oxide, the higher the oxidation number of the nonmetal, the stronger the acid it forms. 

Oxidative-reductive disproportionation is a rather typical property of some pseudo bases. Thus, l,3-dimethyl-2-hydroxybenzimidazoline (229), which exists in the solid state in the open-chain form (228), on heating at 165-185°C, is converted to mixture of 1,3-dimethylbenzimidazolone (230) (49%) and 1,3-dimethylbenzimidazoline (231) (46%) (85KGS1694). Evidently, the process proceeds via an equilibrium amount of (229) undergoing hydride transfer. 

High temperature thermodynamic data are available only for three sulfites calcium, potassium, and sodium. Most sulfites are fairly unstable, decomposing at relatively low temperatures. The decomposition reactions are not always exactly known, with diverse decomposition products, including sulfur, being reported. There are two major decomposition reactions (1) decomposition to the oxide and S02, and (2) oxidation-reduction (disproportionation) to the sulfate and oxide and S02, i.e.,... 

Table 6 presents a summary of the oxidation—reduction characteristics of actinide ions (12—14,17,20). The disproportionation reactions of UO2, Pu , PUO2, and AmO are very compHcated and have been studied extensively. In the case of plutonium, the situation is especially complex four oxidation states of plutonium [(111), (IV), (V), and (VI) ] can exist together ia aqueous solution ia equiUbrium with each other at appreciable concentrations. 

The modes of thermal decomposition of the halates and their complex oxidation-reduction chemistry reflect the interplay of both thermodynamic and kinetic factors. On the one hand, thermodynamically feasible reactions may be sluggish, whilst, on the other, traces of catalyst may radically alter the course of the reaction. In general, for a given cation, thermal stability decreases in the sequence iodate > chlorate > bromate, but the mode and ease of decomposition can be substantially modified. For example, alkali metal chlorates decompose by disproportionation when fused ... 

The reaction was considered as an oxidation-reduction process, where the phosphite and TeCl4 are converted into phosphorochloridate and tellurium dichloride, respectively. TeCl2 suffers a disproportionation into Te and TeCl4 which can participate again in the reaction. 

The rate of disproportionation increases with decreasing pH and rising temperature and these conditions also favour oxide reduction. To achieve a reasonable rate of dissolution, one has to compromise on the pH. A pH of 3 is used in kaolin bleaching (Jepson, 1988), whereas in soil analysis the system is usually buffered with citrate and bicarbonate at ca. pH 7 (Mehra Jackson, 1960). Citrate also complexes the dissolved Fe " and prevents its precipitation as Fe" sulphide. For the dithionite/EDTA system, Rueda et al. (1992) found a maximum efficiency at pH 5-6 and an activation energy for goethite dissolution of 70 kj mol". They stressed the importance of adsorption of S2O4 on the surface to ensure reduction of Fe ". 

Diazinium salts resemble pyridinium salts in their behavior. They form pseudo-bases with hydroxide ions which can disproportionate (e.g. 2-methylphthalazinium ion (199) — 2-methylphthalaz-l-one + 2-methyl-l,2-dihydrophthalazine) or undergo ring fission (e.g. 3-methylquinazolinium ion — (200). Aqueous acid converts (201) into (202), presumably by attack of a water molecule on a protonated species with subsequent intramolecular oxidative-reductive rearrangement of an intermediate carbinol base (201a) as shown. 

In Sections IV.A-D, reactions which involved formyl oxidation or disproportionation were described. Reactions of formyl complexes with reducing agents will now be examined. At the outset, it can be stated that no well-defined reductions utilizing H2 have been found (47, 62, 66). This is disappointing, since such reactions would probably have relevancy to homogeneous CO reduction catalyst pathways. 

For elements of several possible oxidation states (e.g., sulfur, the transition metals) all relevant disproportionation equilibria need to be examined. Pseudo-equilibrium conditions may be maintained for some time, where competing oxidation-reduction systems can be involved. For example, the following kinds of reactions... 

The remainder of this chapter is concerned with the stabilities of ions (mainly cations) in aqueous solution, with respect to oxidation, reduction and disproportionation. Ions in solution are surrounded by solvent molecules, oriented so as to maximise ion-dipole attraction (although there may be appreciable covalency as well). The hydration number of an ion in aqueous solution is not always easy to determine experimentally it is known to be six for most cations, but may be as low as four for small cations of low charge (e.g. Li+) or as high as eight or nine for larger cations (e.g. La3+). 

Protonation of dinitrogen or hydrazido(2-) ligands to yield ammonia [reactions (44)—(50)] is not coupled to electron transfer from an external reductant. Electrons for (new) N—H electron pair bonds must therefore come from the metal as is the case in reaction (46), or from ligand-centered oxidation or disproportionation reactions, as appears to be the case in reaction (49). 

The possibility of several cationic species introduces complexity into the aqueous chemistries, particularly of U, Np, Pu, and Am. Thus all four oxidation states of Pu can coexist in appreciable concentrations in a solution. The solution chemistries and the oxidation-reduction potentials are further complicated by the formation in the presence of ions other than perchlorate, of cationic, neutral, or anionic complexes. Furthermore, even in solutions of low pH, hydrolysis and the formation of polymeric ions occurs. Third, there is the additional complication of disproportionation of certain ions, which is particularly dependent on the pH. 

Radical ions are, in the main, not very important as active centres of polymerizations. In media suitable for the existence both of radicals and of ions, the latter are usually more reactive. Moreover, the radicals decay by combination their contribution to chain propagation is usually negligible. Radical ions are more important as precursors of active centres, as intermediates generated from initiators and monomers through their radical ends they can combine (disproportionate) yielding active centres, frequently diions. Studies of radical ion behaviour contribute to our knowledge of the processes connected with electron transfer from molecule to molecule. These oxidation-reduction processes are very important in macromolecular chemistry. 

Ligand replacement by CO followed by disproportionation leads to oxidation-reduction because the exchange is followed by ligand-bridged electron transfer between the two species first formed. This behavior has been observed for CN replacement in [Co(CN)5] to give [Co(CN)3(CO)2] and [Co(CN)6] whereas only replacement without disproportionation takes place for K[Pd(CNXCO)] formed from K2[Pd(CN)2] . ... 

Deposition reactions generally involve complicated chemical reaction schemes, however overall CVD reactions can be classified to include pyrolysis, reduction, oxidation, hydrolysis, disproportionation, or combinations of these. Additionally, in certain cases the substrate may be a part of the reaction or may act as a catalyst. Coatings are generally grown at sub-atmospheric pressures, although high growth rate depositions have been done at (or close to) atmospheric pressures. 

The oxidation-reduction behavior of plutonium is described by the redox potentials shown in Table I. (For the purposes of this paper, the unstable and environmentally unimportant heptavalent oxidation state will be ignored.) These values are of a high degree of accuracy, but are valid only for the media in which they are measured. In more strongly complexing media, the potentials will change. In weakly complexing media such as 1 M HClOq, all of the couples have potentials very nearly the same as a result, ionic plutonium in such solutions tends to disproportionate. Plutonium is unique in its ability to exist in all four oxidation states simultaneously in the same solution. Its behavior is in contrast to that of uranium, which is commonly present in aqueous media as the uranyl(VI) ion, and the transplutonium actinide elements, which normally occur in solution as trlvalent... 

Disproportionation— An oxidation-reduction reaction in which the same chemical species is oxidized and reduced. 

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