Transition sulfates, decomposition

A review by Brandt and van Eldik provides insight into the basic kinetic features and mechanistic details of transition metal-catalyzed autoxidation reactions of sulfur(IV) species on the basis of literature data reported up to the early 1990s (78). Earlier results confirmed that these reactions may occur via non-radical, radical and combinations of non-radical and radical mechanisms. More recent studies have shown evidence mainly for the radical mechanisms, although a non-radical, two-electron decomposition was reported for the HgSC>3 complex recently (79). The possiblity of various redox paths combined with protolytic and complex-formation reactions are the sources of manifest complexity in the kinetic characteristics of these systems. Nevertheless, the predominant sulfur containing product is always the sulfate ion. In spite of extensive studies on this topic for well over a century, important aspects of the mechanisms remain to be clarified and the interpretation of some of the reactions is still controversial. Recent studies were... 

Many other cases arise that cannot be considered here. For example, two or more double salts may form, depending upon the ratio of the salts in solution. The two salts may have no common ion, as potassium chloride and magnesium sulfate. On evaporation, either of these two or the two salts formed by their double decomposition may separate as well as any one of a number of double salts. Sometimes a double salt cannot be obtained unless the temperature is below a given point, called the transition point while in other cases the temperature must be above the transition point. Thus, the salt known as kainite, MgS04-KCh3H20, cannot be obtained above 83° while with astrakanite, MgS04 Na2S0v4H20, the temperature must be above 4.5 and below 60°. 

The DSC, TG curves of solvates and hydrates are related to the phase diagrams between substance and solvent (or water). Eutectic are observed. Fusion or decomposition of the solvate may occur during heating. Therefore, one may observe the melting of the solvate followed by recrystallization into the anhydrous form or the endothermic desolvatation in the solid state. In certain cases both phenomena may over-lapp. Details about experimental factors and examples can be found in Ref. If the anhydrous form is metastable, further phase transitions follow the desolvatation. If several solvates or hydrates exist, the transitions observed depend on the pressure, as demonstrated by Soustelle in the case of copper sulfate pentahydrate. Depending on the pressure, the direct dehydration into the anhydrous or the dehydration via the monohydrate, or the three dehydration steps trihydrate, monohydrate and anhydrous forms may be obtained. Hydrates have been the subject of... 

Decompositions of transition-metal sulfides, notably those of Fe, Ni, Cu and Co, have been of technological importance in ore refining. Some of the published work is concerned with naturally-occurring minerals, while other studies used synthetic preparations. Reactions often proceed by a contracting interface mechanism and the rates are decreased when gaseous product is present, or its escape is opposed by an inert gas. On heating in air, several metal sulfides form sulfates or oxysulfates as intermediates in a sequence of reactions which finally yield metal oxides [43]. 

Brittain et al. [112] have reported studies of the decompositions of several sulfates using a torsion efhision detection technique to identify the primary gaseous decomposition products. Zn804 and Zn0.2ZnS04 yielded SO3 as the sole volatile product between 800 and 900 K, together with finely divided residual solids. Mg804 reacted similarly between 900 and 1000 K, but in the presence of additives expected to promote breakdown of SO3 (platinum group metals and transition metal oxides) the products were consistent with the equilibrium mixture ... 

Photochemical decomposition of diazo(trimethylsilyl)methane (1) in the presence of alkenes has not been thoroughly investigated (see Houben-Weyl Vol. E19b, p 1415). The available experimental data [trimethylsilylcyclopropane (17% yield) and la,2a,3j8-2,3-dimethyl-l-trimethylsilylcyclopropane (23% yield)] indicate that cyclopropanation occurs only in low yield with ethene and ( )-but-2-ene. In both cases the formal carbene dimer is the main product. In reactions with other alkenes, such as 2,3-dimethylbut-2-ene, tetrafluoroethene or hexafluoro-propene, no cyclopropanes could be detected.The transition-metal-catalyzed decomposition of diazo(trimethylsilyl)methane (1) has been applied to the synthesis of many different silicon-substituted cyclopropanes (see Table 3 and Houben-Weyl Vol.E19b, p 1415) 3.20a,b,2i.25 ( iQp. per(I) chloride has been most commonly used for carbene transfer to ethyl-substituted alkenes, cycloalkenes, styrene, and related arylalkenes. For the cyclopropanation of acyl-substituted alkenes, palladium(II) chloride is the catalyst of choice, while palladium(II) acetate was less efficient, and copper(I) chloride, copper(II) sulfate and rhodium(II) acetate dimer were totally unproductive. The cyclopropanation of ( )-but-2-ene represents a unique... 

Table IV. Ease of Decomposition of Various Transition Metal Sulfates"...

Luminescence measurements of the decomposition products of Eu2(S04)3-8H20 (Brittain 1983) supported these conclusions. Figure 2 shows the spectra obtained for the Do and Dg -> Fj transition regions for europium sulfate before (a) and... 

The first reaction predominates if the product contains a large amount of water (-18%). This reaction is analogous to the disproportionation of aqueous hypochlorite. However, disproportionation is much slower in solid calcium hypochlorite than in solution. Under dry conditions, the second reaction predominates. It is catalyzed by transition metals including iron and manganese. It may occur explosively 150°C. Thus, calcium hypochlorite products usually contain some water or an additive such as magnesium sulfate heptahydrate. The third reaction is the reverse of chlorination. The fourth reaction is due to the adsorption of carbon dioxide from air or the release of carbon dioxide from carbonate salt impurities. It is accelerated by water and temperature. The first reaction accounts for -70%, and the second reaction -30%, of the decomposition of solid calcium hypochlorite made in the United States and stored in sealed containers at 25°C. ... 

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