Sulfates, transition-metal, decompositions

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... 

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"...

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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