Thermal studies on dithionate compounds

TG and DTG studies have been carried out on CoS₂O₆ · 6H₂O, NiS₂O₆ · 6 H₂O, CuS₂O₆ · 3.5 H₂O, ZnS₂O₆ · 6 H₂O and CdS₂O₆ · 4H₂O. After partial dehydration, the dithionates of Co(II), Ni(II), Cu(II) and Zn(II) lose water and sulfur dioxide simultaneously to yield the stable metal sulfates in the final step of decomposition. The CdS₂O₆ · 4H₂O dehydrates completely in the first two steps of decomposition with two water molecules being lost in each step. In the third step, it loses only SO₂ to yield CdSO₄. Kinetic parameters are presented for these reactions.

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Zusammenfassung TG- und DTG-Untersuchungen von CoS₂O₆ · 4 H₂O, NiS₂O₆ · 6 H₂O, CuS₂O₆ · 3.5 H₂O, ZnS₂O₆ · 6 H₂O und CdS₂O₆ · 4 H₂O wurden ausgeführt. Nach partieller Dehydratisierung geben die Dithionate von Co(II), Ni(II), Cu(II) und Zn(II) im letzten Schritt des Abbaus gleichzeitig Wasser und Schwefeldioxid unter Bildung der stabilen Metallsulfate ab. CdS₂O₆ · 4 H₂O wird in den ersten beiden Zersetzungsschritten unter Abgabe von je 2 Wassermolekülen vollständing zersetzt. Im dritten Schritt wird nur SO₂ unter Bildung von CdSO₄ abgegeben. Kinetische Parameter dieser Reaktionen werden angegeben.

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CoS₂O₆ · 6 H₂O, NiS₂O₆ · 6 H₂O, CuS₂O₆ · 3.5 H₂O, ZnS₂O₆ · 6 H₂O CdS₂O₆ · 4 H₂O. , , , , . , . , . .

     

Aluminium in the human brain

Abstract  

An inevitable consequence of humans living in the Aluminium Age is the presence of aluminium in the brain. This non-essential, neurotoxic metal gains entry to the brain throughout all stages of human development, from the foetus through to old age. Human exposure to myriad forms of this ubiquitous and omnipresent metal makes its presence in the brain inevitable, while the structure and physiology of the brain makes it particularly susceptible to the accumulation of aluminium with age. In spite of aluminium’s complete lack of biological essentiality, it actually participates avidly in brain biochemistry and substitutes for essential metals in critical biochemical processes. The degree to which such substitutions are disruptive and are manifested as biological effects will depend upon the biological availability of aluminium in any particular physical or chemical compartment, and will under all circumstances be exerting an energy load on the brain. In short, the brain must expend energy in its ‘unconscious’ response to an exposure to biologically available aluminium. There are many examples where ‘biological effect’ has resulted in aluminium-induced neurotoxicity and most potently in conditions that have resulted in an aluminium-associated encephalopathy. However, since aluminium is non-essential and not required by the brain, its biological availability will only rarely achieve such levels of acuity, and it is more pertinent to consider and investigate the brain’s response to much lower though sustained levels of biologically reactive aluminium. This is the level of exposure that defines the putative role of aluminium in chronic neurodegenerative disease and, though thoroughly investigated in numerous animal models, the chronic toxicity of aluminium has yet to be addressed experimentally in humans. A feasible test of the ‘aluminium hypothesis’, whereby aluminium in the human brain is implicated in chronic neurodegenerative disease, would be to reduce the brain’s aluminium load to the lowest possible level by non-invasive means. The simplest way that this aim can be fulfilled in a significant and relevant population is by facilitating the urinary excretion of aluminium through the regular drinking of a silicic acid-rich mineral water over an extended time period. This will lower the body and brain burden of aluminium, and by doing so will test whether brain aluminium contributes significantly to chronic neurodegenerative diseases such as Alzheimer’s and Parkinson’s.     

Kinetic studies on the linkage isomerization and dehydration of silver hexacyanocobaltate(III) hexadecahydrate

Summary Heating Ag₃[Co(CN)₆]·16H₂O results in dehydration and complete conversion of the Ag⁺ NC -Co³⁺ linkages to Ag⁺ -CN -Co³⁺. These processes have been studied kinetically. The dehydration process follows an R1, one dimensional contraction, rate law and has an activation energy of 28.7 kJ mol^–1. The linkage isomerization process was studied nonisothermally by means of DSC. It follows an Avrami rate law with an index of about 1.5. These results are interpreted in terms of processes known for other solid state reactions.     

A general iterative method for obtaining kinetic parameters from TG data

An iterative method is described for determining the reaction order and activation energy from TG curves. The method makes use of equations to represent the temperature integrals which are derived using numerical relationships in terms of E, T, and empirical constants. Like the method of Reich and Stivala, the computation involves varying the value of n until the appropriate linear relationship gives an intercept of zero. The slope of the line is YE^x, where Y and X are constants in the equation -log I = YE^x(1/T) +log E^w+ U The method is tested using data obtained by means of a fourth order Runge-Kutta solution of the rate law for both Arrhenius and non-Arrhenius cases.     

Kinetic and thermodynamic control of axial chirality in biaryl-derived fused oxazolidine lactams exploiting a centre-axis relay of unit efficiency

The condensation of a 2-substituted-2-aminoethanol with methyl 2'-formylbiphenyl-2-carboxylate produces only two of the four possible axially chiral 6,7-dihydrodibenz[c,e]oxazolo[3,2-a]azepin-9(4bH)-ones (fused oxazolidine lactams), with kinetically controlled diastereoisomer ratios of up to 96 : 4. Within each lactam product the central chirality of the oxazolidine-fused benzylic position C(4b) is relayed to the biaryl axis with unit efficiency, the mis-matching of these stereogenic elements being prohibited due to strain, as predicted by molecular mechanics calculations. Diastereoisomeric lactam pairs can be equilibrated by heating with acid, and under these thermodynamic conditions reversed diastereoisomer ratios of up to 26 : 74 are observed.