Oxidation and Reduction Some Definitions

2 H20(g) (hydrogen-oxygen fuel-cell reaction) 2 Fe203(s) (rusting of iron) 

Rust is produced by the oxidation of iron to form iron oxide. 

The flame on a gas stove results from the oxidation of carbon in natural gas. 

These definitions of oxidation and reduction are useful because they show the origin of the term oxidation, and they allow us to quickly identify reactions involving elemental oxygen as oxidation and reduction reactions. However, as you will see, these definitions are not the most fundamental. 

Each of these reactions involves loss of oxygen. In the first reaction, hydrogen loses oxygen in the second reaction, iron loses oxygen and in the third reaction, carbon loses oxygen. In each case, the substance that loses oxygen is reduced in the reaction. One definition of reduction is simply the loss cf oxygen. 

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The vast number of thermodynamically possible reactions obtained by permuting oxidants and reductants within the scope of this review present major problems of classification and selection. To only a limited extent is the modernity or detail of a paper indicative of its relevance, some of the definitive papers having been published before 1950. Discussion has been concentrated, therefore, at points where a kinetic investigation of a reaction has resulted in a real advance in our understanding both of its mechanism and of those of related reactions, and work which has been more of a confirmatory nature will not receive comparable consideration. Detailed reference to products, spectra, etc. will be made only when the kinetics produce real ambiguities. 

Table 9-1. Some useful definitions of oxidation and reduction which are of use in different circumstances.

Palladium(II) complexes provide convenient access into this class of catalysts. Some examples of complexes which have been found to be successful catalysts are shown in Scheme 11. They were able to get reasonable turnover numbers in the Heck reaction of aryl bromides and even aryl chlorides [22,190-195]. Mechanistic studies concentrated on the Heck reaction [195] or separated steps like the oxidative addition and reductive elimination [196-199]. Computational studies by DFT calculations indicated that the mechanism for NHC complexes is most likely the same as that for phosphine ligands [169], but also in this case there is a need for more data before a definitive answer can be given on the mechanism. 

Early work with sulfur dioxide showed a linear relationship between visible injury and reduction in yield for many crop species. The assessment was made that no reduction in yield would be found unless visible injury were noted. Definitive research with ozone, other oxidants, or mixtures of these pollutants with other gases has not been done. Thus, we do not know whether such relationships between visible injury and yield hold for the oxidants, but data in Table 11-3 suggest that for acute exposures there may be good correlations between injury and yield reductions. Many researchers have hypothesized that the oxidants may have an effect on plants that will produce a yield reduction with little or no visible injury. Such studies need to be designed in a more defmitive manner before it is concluded that yield reductions without visible symptoms are clearly acceptable. Projections of yield losses have made use of some of the data reported earlier. ... 

Although the number of valence electrons present on an atom places definite restrictions on the maximum formal oxidation state possible for a given transition element in chemical combination, in condensed phases, at least, there seem to be no a priori restrictions on minimum formal oxidation states. In future studies we hope to arrive at some definitive conclusions on how much negative charge can be added to a metal center before reduction and/or loss of coordinated ligands occur. Answers to these questions will ultimately define the boundaries of superreduced transition metal chemistry and also provide insight on the relative susceptibility of coordinated ligands to reduction, an area that has attracted substantial interest (98,117-119). 

Ronald Breslow and his collaborators have given some attention to the problem of estimating the degree of destabilization of cyclobutadiene with respect to nonconjugated models. They have concluded from electrochemical measurements of oxidation-reduction potentials of the system 37 38, of which only the quinone 38 has the cyclobutadiene fragment, that the C4H4 ring is destabilized by some 12-16 kcal mole-1 and so is definitely antiaromatic.15... 

The high formal oxidation states of metals in some of these adducts is noteworthy, e.g., Fe(IV) (entries 17 and 18), Ru(IV) (entries 21 and 22), and Pt(IV) (entries 55 and 56). Such adducts are important because they provide definite examples of species often postulated as intermediates in oxidative addition-reductive elimination processes (compare Section II,G,1) and in homogeneous catalysis (134,220a, 410a). In the case of germanium, a tris(germyl) adduct of Pt(IV) has been described (57), but no more than two silyl groups per metal atom are known to result from oxidative addition. 

The transition states of the latter are therefore more sensitive to stereochemical and electronic influences, which also leads to a higher selectivity than in analogous electrochemical conversions. At some oxide electrodes, such as the Ni(OH)2 electrode [13], the oxidation occurs as inner sphere electron transfer by hydrogen atom transfer. Also at doped titanium anodes this seems to be partially the case [14]. It cannot be definitely excluded that also in some oxidations at platinum anodes, higher valency oxides at the surface act as inner sphere electron transfer agents. Electrochemical" inner sphere electron transfers are intentionally used in indirect electrochemical conversions where selective chemical oxidants or reductants are regenerated by electron transfer from the electrode [15]. They are also immobilized by attaching polymer-bound electrocatalysts as mediators to the electrode surface [16]. 

Digestion solutions generally contain arsenic in both valencies. Oxidation or reduction of As during combustion is possible. This alters the original distribution ratio of the valencies. Some of the separation and determination methods demand a definite valency status. Usually As(IU) is oxidized by KMn04—a fast reaction— and As(V) reduced by KI, but not under all conditions sufficiently fast and complete. Some methods of digestion and arsenic isolation are described in more detail below ... 

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