Redox, internal

A range of chemical analogs of the catalytic centers of Mo and W dithiolene-containing enzymes (pterins) have been prepared. In particular, the rich chemistry of multisulfur transition metal systems allows ligand redox, internal electron transfer, and intermediate redox states. Such redox flexibility may facihtate coupled proton/electron transfer and/or 0x0-transfer mechanisms, which are employed by Mo and W enzymes. 

Stannous Sulfate. Stannous sulfate (tin(Il) sulfate), mol wt 214.75, SnSO, is a white crystalline powder which decomposes above 360°C. Because of internal redox reactions and a residue of acid moisture, the commercial product tends to discolor and degrade at ca 60°C. It is soluble in concentrated sulfuric acid and in water (330 g/L at 25°C). The solubihty in sulfuric acid solutions decreases as the concentration of free sulfuric acid increases. Stannous sulfate can be prepared from the reaction of excess sulfuric acid (specific gravity 1.53) and granulated tin for several days at 100°C until the reaction has ceased. Stannous sulfate is extracted with water and the aqueous solution evaporates in vacuo. Methanol is used to remove excess acid. It is also prepared by reaction of stannous oxide and sulfuric acid and by the direct electrolysis of high grade tin metal in sulfuric acid solutions of moderate strength in cells with anion-exchange membranes (36). 

This method involves very simple and inexpensive equipment that could be set up m any laboratory [9, 10] The equipment consists of a 250-mL beaker (used as an external half-cell), two platinum foil electrodes, a glass tube with asbestos fiber sealed m the bottom (used as an internal half-cell), a microburet, a stirrer, and a portable potentiometer The asbestos fiber may be substituted with a membrane This method has been used to determine the fluoride ion concentration in many binary and complex fluondes and has been applied to unbuffered solutions from Willard-Winter distillation, to lon-exchange eluant, and to pyrohydrolysis distil lates obtained from oxygen-flask or tube combustions The solution concentrations range from 0 1 to 5 X 10 M This method is based on complexing by fluonde ions of one of the oxidation states of the redox couple, and the potential difference measured is that between the two half-cells Initially, each cell contains the same ratio of cerium(IV) and cerium(tll) ions... 

J Marlow, (a) Care and Maintenance of pH and redox electrodes (b) Care and Maintenance oflon-Selective Electrodes. In International Laboratory, Vol XII, Part 2, 1987... 

Potassium chloride (nitrate) bridge 583, 582 Potassium chromate as indicator, 343, 349 Potassium cyanoferrate(II) D. of, (ti) 384 Potassium cyanoferrate(III) D. of, (ti) 399 Potassium cyanonickelate(II) prepn., 328 Potassium dichromate solution analyses involving, 375 oxidising properties of, 375 internal indicators for, 377 preparation of, 0.02M, 375 redox indicators for, 377 standardisation of, by iron, (cm) 546, (ti) 376... 

This considerable enhancement in redox properties may however remain chemically hidden. Several causes may converge to mask these properties. First of all electron transfer is an intermolecular act of reactivity even when thermodynamically feasible it may have to compete with very rapid intramolecular acts of deactivation (fluorescence, phosphorescence, internal conversion)99. The rate of electron transfer is given by the Rehm-Weller equation96,100... 

This is an typical example of a dicarboxylic acid in that C-C cleavage is the only route for oxidation. No study of the Co(III) oxidation has been made although it is highly probable that reaction would proceed through an oxalate complex. The thermal decomposition of Co(Ox)3 has been shown to be a first-order process and probably involves an internal redox reaction, viz. 

This compound breaks down to [Co(CN)5H20] and [(CN)5CoOC5H40H] . The latter in turn undergoes an internal redox reaction to give [Co(CN)5H20] and hydroquinone. Both first-order steps show general acid catalysis. 

Criterion C3 internal redox measure oxygen balance... 

Finally, internal redox is closely bonded to its potential thermal effects by adding an element of fragility to the molecule. 

The final internal boundary condition applies to the interface between phase 1 and phase 2, and relates the flux of species Red] and Red2, at the ITIES, to the rate of the second-order redox reaction occurring at the interface. 

Illustrated in Scheme 7.8 are the mechanisms that give rise to the products shown in Scheme 7.7. These mechanisms involve either electrophilic attack or an internal redox reaction. The internal redox reaction shown in Scheme 7.8 involves proton trapping from the solvent or from the hydroquinone hydroxyl group as shown. This process has been documented for the mitomycin system50 and also occurs in many quinone methide systems.25,30,31... 

The toxicity of 3-methylindole has been attributed to methyleneindolenine trapping of nitrogen and sulfur nucleophiles.57 60-62 Likewise, the ene-imine shown in Scheme 7.9 readily reacted with hydroquinone nucleophiles, resulting in head-to-tail products. Shown in Fig. 7.6 is the 13C-NMR spectrum of a 13C-labeled ene-imine generated by reductive activation. The presence of the methylene center of the ene-imine is apparent at 98 ppm, along with starting material at 58 ppm and an internal redox reaction product at 18 ppm. Thus, the reactive ene-imine actually builds up in solution and can be used as a synthetic reagent. 

Reductive activation of the quinone shown in Scheme 7.9 and incubation in methanol afforded a complex mixture of products consisting mainly of head-to-tail coupling at C-5 or C-7 (Scheme 7.10). Minor reactions involve transfer of H2 from the hydroquinone to the ene-imine (internal redox reaction) and methanol trapping. The structures of the dimers and trimers in Scheme 7.10 were derived from H-NMR,... 

The process that affords the alkenes with13 C = 45 ppm is essentially an internal redox reaction wherein electrons flow from the fused tetrahydropyrido ring to the quinone ring by means of a series of tautomerizations. 

The result of the entire procedure, being a 100% conversion of Fe(II) to Fe(III), thus represents a so-called coulometric titration with internal generation of course, it seems possible to titrate Fe(II) with Ce(IV) generated externally from Ce(III), but in this way one would unnecessarily remove the solution of the 100% conversion problem hence the above titration with internal generation in the presence of a redox buffer as an intermediary oxidant represents an extremely reliable method, unless occasional circumstances are prohibitive for the remainder internal generation offers the advantage of no dilution of the analyte solution. 

The reductive coupling of allyl halides to 1,5-hexadiene at glassy C electrodes was catalyzed by tris(2, 2,-bipyridyl)cobalt(II) and tris(4,4 -dimethyl-2, 2/-bipyridyl)cobalt(II) in aqueous solutions of 0.1 M sodium dodecylsulfate (SDS) or 0.1 M cetyltrimethylammonium bromide (CTAB).48 An organocobalt(I) intermediate was observed by its separate voltammetric reduction peak in each system studied. This intermediate undergoes an internal redox reaction to form 1,5-hexadiene... 

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

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