Intermediate oxides

In general the structural chemistry of Mo oxy-compounds is more complex than that of the analogous W compounds the reason for this difference is not known. 

Dioxides and trioxides. The (isostructural) dioxides have distorted rutile structures (p. 448), but the trioxides are not isostructural. M0O3 has a unique layer structure (Fig. 12.22(c)). Each octahedral MoOg group shares two adjacent ec es with similar groups and, in a direction perpendicular to the plane of the paper in Fig. 12.22, the octahedra are linked through vertices. Three O atoms of each MoOg octahedron are therefore common to three octahedra, two are shared between two octahedra, and the sixth is unshared. (See also p. 181.) 

A mass-spectrometric study has been made of MoOj vapour (at 850°C), in which the most abundant species are M03O9, M04O12, and M05O15. 

In Fig. 12.22(d), (e), and (0 there are respectively 1, 2, and 3 pairs of adjacent MOg octahedra sharing edges the limiting case would be the structure shown in (g), corresponding to the composition M2OS. This represents the idealized structure of VjOs (p. 469). 

In Chapter 11 we distinguished between oxy-salts and complex oxides, but observed that there is no hard and fast dividing line between the two groups of compounds. As regards their crystal structures we may distinguish two main classes of complex oxide. 

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Make acid yields coumaUc acid when treated with fuming sulfuric acid (19). Similar treatment of malic acid in the presence of phenol and substituted phenols is a facile method of synthesi2ing coumarins that are substituted in the aromatic nucleus (20,21) (see Coumarin). Similar reactions take place with thiophenol and substituted thiophenols, yielding, among other compounds, a red dye (22) (see Dyes and dye intermediates). Oxidation of an aqueous solution of malic acid with hydrogen peroxide (qv) cataly2ed by ferrous ions yields oxalacetic acid (23). If this oxidation is performed in the presence of chromium, ferric, or titanium ions, or mixtures of these, the product is tartaric acid (24). Chlorals react with malic acid in the presence of sulfuric acid or other acidic catalysts to produce 4-ketodioxolones (25,26). 

The oxidation potentials also indicate the tendency for intermediate oxidation states to disproportionate. 

All phosphoms oxides are obtained by direct oxidation of phosphoms, but only phosphoms(V) oxide is produced commercially. This is in part because of the stabiUty of phosphoms pentoxide and the tendency for the intermediate oxidation states to undergo disproportionation to mixtures. Besides the oxides mentioned above, other lower oxides of phosphoms can be formed but which are poorly understood. These are commonly termed lower oxides of phosphoms (LOOPs) and are mixtures of usually water-insoluble, yeUow-to-orange, and poorly characteri2ed polymers (58). LOOPs are often formed as a disproportionation by-product in a number of reactions, eg, in combustion of phosphoms with an inadequate air supply, in hydrolysis of a phosphoms trihahde with less than a stoichiometric amount of water, and in various reactions of phosphoms haUdes or phosphonic acid. LOOPs appear to have a backbone of phosphoms atoms having —OH, =0, and —H pendent groups and is often represented by an approximate formula, (P OH). LOOPs may either hydroly2e slowly, be pyrophoric, or pyroly2e rapidly and yield diphosphine-contaminated phosphine. LOOP can also decompose explosively in the presence of moisture and air near 150° C. 

Sulfoxides are compounds that contain a sulfinyl group covalendy bonded at the sulfur atom to two carbon atoms. They have the general formula RS(0)R, ArS(0)Ar, and ArS(0)R, where Ar and Ar = aryl. Sulfoxides represent an intermediate oxidation level between sulfides and sulfones. The naturally occurring sulfoxides often are accompanied by the corresponding sulfides or sulfones. The only commercially important sulfoxide is the simplest member, dimethyl sulfoxide [67-68-5] (DMSO) or sulfinylbismethane. 

The NAD- and NADP-dependent dehydrogenases catalyze at least six different types of reactions simple hydride transfer, deamination of an amino acid to form an a-keto acid, oxidation of /3-hydroxy acids followed by decarboxylation of the /3-keto acid intermediate, oxidation of aldehydes, reduction of isolated double bonds, and the oxidation of carbon-nitrogen bonds (as with dihydrofolate reductase). 

The hydroxyl radical rapidly abstracts an H atom from a second molecule of HCIO4 to give H2O plus CIO and the 2 radicals CIO and CIO then decompose to the elements via the intermediate oxides. Above 450° the CI2 produced reacts with H2O to give 2HC1 plus whilst in the low-temperature range (150-310°) the decomposition is heterogeneous and second order in HCIO4. 

The difference between the two extremes is essentially that, in the former, the Re retains its valence electrons in its d orbitals whereas in the latter it loses 6 of them to delocalized ligand orbitals. In either case paramagnetism is anticipated since rhenium has an odd number of valence electrons. The magnetic moment of 1.79 BM corresponding to 1 unpaired electron, and esr evidence showing that this electron is situated predominantly on the ligands, indicates that an intermediate oxidation state is involved... 

Formation of the first layer (a monolayer) of passivating oxide film on a denuded metal surface occurs very simply by the loss of protons from the adsorbed intermediate oxidation products, such intermediates being common to both dissolution and passivation processes . Thus for example, the first oxidative step in the anodic oxidation of nickel is the formation of the unstable adsorbed intermediate NiOH by... 

Notice that the C103 ion is a much stronger oxidizing agent ( cd = +1.442 V) than reducing agent ( x = —1.226 V). This is generally true of oxoanions and oxoacids in an intermediate oxidation state, at least in acidic solution. Compare, for example,... 

In general, a species in an intermediate oxidation state is expected to disproportionate if the sum + °,d is a positive number. 

In contrast, sulphoxides appear to possess a more classical behaviour in electrochemistry, due to their intermediate oxidation state which allows, in most of the cases, their reduction to sulphides but also their oxidation to sulphones with no cleavage process. Moreover, the increase of the sulphur atom basicity may also produce catalytic hydrogen evolution in acidic solution. 

The white precipitate is presumably jV,jV-di benzyl hydroxy lamine, an intermediate oxidation product, which disappears after the third catalyst addition. 

Figure 2-2. Schematic representation of the radial waveforms for 3d, 45 and 4p orbitals in first row transition-metal ions of intermediate oxidation state (Werner-type complexes).

Complementary to the work with aqueous acidic media is the study of the homolytic decompositions of Co(III) carboxylates in carboxylic acid media by Lande and Kochi . For example, Co(III) is reduced in pivalic acid media with first-order kinetics with E = 30.6 kcal.mole , AS = 8 eu and k ko = 1.28+0.10 (69 °C). The main oxidation products were found to be isobutylene and tert-butyl pivalate, which suggests that (CH3)3C- is an intermediate. Oxidative decarboxylation is the probable course in the analogous oxidations of n-butyric and isobutyric acids, in view of the production of propane and CO2 under normal... 

The anodic oxidation of organic substances is a complex multistep process. The question as to the depth of oxidation required (and sufficient) lias to be answered in each case. Where intermediate oxidation products pose no ecological risk, one can stop at incomplete oxidation. However, in the anodic oxidation of many aromatic substances, the corresponding quinones are formed in the first step, and these are more harmful than the original substances. Upon more profound oxidation, the benzene rings are broken and aliphatic substances are formed that are almost as harmless as carbon dioxide. 

Detailed mechanistic understanding of the allylic oxidation has not been developed. One possibility is that an intermediate oxidation state of Cr, specifically Cr(IV), acts as the key reagent by abstracting hydrogen.160... 

The most selective methods for cleaving organic molecules at carbon-carbon double bonds involve glycols as intermediates. Oxidations of alkenes to glycols was discussed in Section 12.2.1. Cleavage of alkenes can be carried out in one operation under mild conditions by using a solution containing periodate ion and a catalytic... 

Van Bladeren PJ, Delbressine LP, Hoogeterp J J, et al. 1981. Formation of mercapturic acids from acrylonitrile, crotononitrile, and cinnamonitrile by direct conjugation and via an intermediate oxidation process. Drug Metab Dispos 9 246-249. 

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