Oxidation-induced reactions

P. Wenger and C. Urfer. N. Smith found that in the oxidation of tin or copper in air, ammonia is simultaneously oxidized—induced reactions zinc has no action, but in glass dishes positive results were sometimes obtained—while only a slight action -was observed in the oxidation of ferrous and manganous hydroxides. 

S.9.4.5. Oxidation-induced reactions of TigCii and other metal-carbide clusters... 

Fig. 8.2 Oxidation potential of model electrolyte clusters vs. the rate estimated from the Marcus electron transfer theory. EC(—H) denotes a neutnil EC radical with one proton abstracted from it in the process of the oxidation-induced reaction. Oxidation potentials are from M05-2X/6-31+G calculations with PCM(e=20) [16]. Rates were ceilculated in this work...

Oxidation of the platinum(II) center by sUver(I) salts has occasionally been observed when attempting to prepare adducts containing platinum-silver bonds (for example, see Sect. 4.2.1). One can speculate that these oxidations could proceed via an inner-sphere process involving the transient formation of a Pt -Ag mixed species. Two examples have recently appeared in which the bimetallic species can be observed prior to oxidatively induced reactions of dimethylplati-num(II) and dime thy Ipalladium(II) species. 

The term electrochromism was apparently coined to describe absorption line shifts induced in dyes by strong electric fields (1). This definition of electrocbromism does not, however, fit within the modem sense of the word. Electrochromism is a reversible and visible change in transmittance and/or reflectance that is associated with an electrochemicaHy induced oxidation—reduction reaction. This optical change is effected by a small electric current at low d-c potential. The potential is usually on the order of 1 V, and the electrochromic material sometimes exhibits good open-circuit memory. Unlike the well-known electrolytic coloration in alkaU haUde crystals, the electrochromic optical density change is often appreciable at ordinary temperatures. 

Fig. 15. Free radical induced oxidative degradation reactions.

An excess of a standard solution of iron(II) must therefore be added and the excess back-titrated with standard cerium(IV) sulphate solution. Erratic results are obtained, depending upon the exact experimental conditions, because of induced reactions leading to oxidation by air of iron(II) ion or to decomposition of the persulphate these induced reactions are inhibited by bromide ion in concentrations not exceeding 1M and, under these conditions, the determination may be carried out in the presence of organic matter. 

The incorporation of the trapping reaction changes the stoichiometry of the net reaction. The result is an induced reaction. For example, the oxidation of vanadium(IV)... 

The application of techniques of pulse radiolysis offers the potential to determine rates of primary radiolysis induced reaction processes. This knowledge can be of great value in the determination of redox processes of Pu ions occurring in a wide variety of aqueous solutions. As a matter of fact, such information is essential to a prediction of the Pu oxidation states to be expected in breached repository scenarios. For an... 

Nitro-, 4-nitro-, 7-nitro-, 5,7-dinitro-, and 6,8-dinitroquinoline react with the carbanion of chloromethyl phenyl sulfone to give products of substitution of hydrogen at positions 4 3 8 6,8 and 5,7 respectively <96LA641>. The base-induced reaction of benzoyl chloride salts of quinoline iV-oxides with carbonitriles to give 2-quinolyldiacylamines as the main products has been reported <96TL(37)69>. 

The oxidation of oxalic acid by mercuric chloride to give CO2 and mercurous chloride is a classic example of an induced reaction. This reaction is extremely slow unless small quantities of chromic acid and manganous ions are added, whereon facile reduction takes place Addition of permanganate or persulphate and some reducing agents is also effective and the oxidation proceeds readily under photo- or X-irradiation (Eder s reaction). The large quantum yield points to a chain mechanism , which could also function with an inducing oxidant, viz. 

In the majority of cases both the primary and the induced reactions are oxidation-reduction reactions. In such reactions the actor can have either reducing or oxidizing properties. The chemical characteristics of the inductor and acceptor are always identical and opposite to that of the actor. When the latter is a reducing agent the acceptor and inductor are oxidants and vice versa. 

Besides induced oxidation-reduction reactions we often speak of induced dissolution, induced precipitation, as well as of induced complex formation there is even a reference to an induced reaction caused by neutralization. It is only necessary to examine briefly the latter cases. 

There are also examples of induced complex formation, an essential step of which is always an oxidation-reduction reaction. Rich and Taube found that the rate of exchange between PtCl and Cl was considerably increased by addition of cerium(rV). In the presence of this oxidizing agent a labile complex of Pt(III) is formed, the chloride of which is easily exchangeable. Exchange of platinum between PtCl and PtClg is similarly rapid via the intermediate labile PtCIs complex formed by cerium(IV). 

Summarizing, it can be said that induced reactions are connected mostly with oxidation-reduction processes and that this is true for the induced complex formation, too. On the other hand, induced precipitation has nothing to do with genuine induced reactions therefore it is advisable not to use this collective name in order to keep the concept of chemical induction clear. 

Case a, i is well illustrated by the arsenite-induced oxidation of manganese(II) by chromic acid, studied by Lang and Zwerina. The overall equation of this induced reaction is... 

The examples mentioned illustrate well the peculiarities of induced reactions, i.e. a hardly oxidizible substance can be oxidized when a simultaneous reduction... 

Although in the fifties of the last century it had already been recognized that in several oxidation-reduction reactions the co-existence principle i.e. the assumption that the individual processes take place independently of each other) was not valid and to date many examples of chemical induction have been found, there are only a few cases known where the mechanism of the induced reaction has been satisfactorily elucidated. There are several reasons for this. Some of the induced reactions take place too rapidly to be investigated by conventional kinetical methods in other cases a thorough investigation was frustrated by the lack of appropriate analytical methods. 

The observation of induced reactions involving chromate almost coincided with the discovery of the phenomenon of chemical induction itself. According to the the role of chromate ions in these reactions, two groups can be distinguished (i) Chromium(VI) plays the role of actor, whose reaction with various inductors listed in Table 1 results in the oxidation of several acceptor ions or molecules. 

Knowledge of stoichiometry of the induced reaction could help to distinguish whether chromium(V) or chromium(IV) species are involved in the oxidation of benzaldehyde. Thus, the Cr(V) hypothesis predicts that for each molecule of benzaldehyde oxidized two molecules of manganese dioxide should be formed, whereas the Cr(IV) predicts that one molecule of manganese dioxide should be formed for each two molecules of benzaldehyde oxidized. Unfortunately, the attempt to determine the stoichiometry of the induced reaction failed because the oxidized manganese species was not precipitated during the reaction presumably due to formation of acetate complexes in the concentrated acetic acid solution. 

The stoichiometry of the induced reaction depends, as in the Fe(II)-S20 system, on the iron(II)/iron(III) ratio and on the pH. Therefore, it can be expected that under identical experimental conditions (actor, inductor, and hydrogen ion concentration) the induction factors for the two systems should be identical. The data obtained show that this expectation is fulfilled. For the photo-induced oxidation of arsenic(III) the value of ks /k -j was found to be 2, while in the present system k Jk -j = 4. (Comparing these values with the value of k jk = 21, it can be concluded that the SO4 radical, formed by reaction (43), is not removed by the reaction... 

Reaction between arsenic(tll) and chlorate is fairly slow. Although the reaction can be markedly accelerated by osmium tetroxide as catalyst , the quantitative reduction of chlorates takes nearly an hour. In the case of the induced reaction it was assumed that arsenic(III) is oxidized to arsenic(IV) by 1-equivalent oxidizing agents. Chlorate is reduced to chlorine dioxide by the arsenic(IV) intermediate, viz. 

During the induced reduction of chlorate a considerable oxygen effect was observed. The air oxidation of arsenic(ril) is also an induced reaction, the extent of which decreases with increasing acid concentration and is increased by decreasing the rate of primary reaction. The induced oxidation caused by air also can be observed during the osmium tetroxide-catalyzed chlorate-arsenic(III) reaction. 

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