Selen-Atom

Seleninsauren konnen in einfacher Weise quantitativ mit kalter waBriger Hydrazin-Ld-sung zu Selenensauren reduziert werden. Mit Hydrazinium-Salzen gelingt es ferner, verschiedene Seleninsauren zu Diselenanen zu reduzieren (s. Bd. IX, S. 1174f.). 

Allgemein gelten fur die Reduktionen am Stickstoff-Atom mit Hydrazinen folgende 

GD Nitro-Verbindungen lassen sich mit Hydrazin-Hydrat prinzipiell iiber die Stufe der Hydroxylamine zu Aminen reduzieren. Zumindest bei den Nitro-arenen muB we-gen der Explosionsgefahr vor einer Reduktion iiber die Stufe des Hydroxylamins hin-aus gewamt werden. 

Aus Dinitro-benzol kann Nitroso-nitro-benzol isoliert werden. Bei Zusatz von Palla-dium/Kohle oder Raney-Nickel werden in Einzelfallen Diarylazo-Verbindungen erhalten. 

Die Umsetzung von Estem der Salpetersaure bzw. Salpetrigen Saure zur Alkohol-Her-stellung gelingt mit sehr guten Ausbeuten (z.B. Hexanol zu 90% d.Th.) mit Hydrazin in Gegenwart von Platin oder Palladium1  

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Die Oxidation von 1-Trimethylsilyl-benzimidazol mit Benzolselenylchlorid soil iiber einen pri-maren Angriff des Selen-Atoms am N-Atom erfolgen. Unter Abspaltung von Chlor-trimethyl-silan wird 2-Phenylseleno-benzimidazol (93% Schmp. 131-135°) erhalten401 ... 

Auch Dialkyldiselenane iibertragen bei 120° ein Selen-Atom auf Phosphonigsaure-diamide1015 z.B. ... 

The ease with which the dipolarophile interacts with vinylacetylenes depends mainly on a spatial factor. The study of the reactions of alkylthiobuten-3-ynones-l and their selenic and telluric analogs with DPNT shows that, in this case, nitrilimine also acts as a nucleophilic agent with a nucleophilic center on the carbon atom of the 1,3-dipole and always adds to the terminal carbon of the enyne system to form l,3-diphenyl-5-/ -2-pyrazolenines. The oxidation of the latter with chloranil leads to alkynylpyrazoles (65ZOR51). 

Alanyahoglu M, Demir U, Shannon C (2004) Electrochemical formation of Se atomic layers on Au(lll) surfaces the role of adsorbed selenate and selenite. J Electroanal Chem 561 21-27... 

Although selenic and telluric acids contain the central atom in the +6 oxidation state, they are very different. The properties of selenic acid, H2Se04, are very similar to those of H2S04, and many of their salts are similar. The oxyacid that contains Te in the +6 state is HsTeOs which can also be written as Te(OH)s. This acid can be prepared from Te or Te02 by suitable oxidation reactions, and it can be also obtained as a solid hydrate. As expected from the formula, telluric acid is a weak acid, although some salts can be obtained in which one or more protons are replaced. 

Generally, oxo-selenates can be classified according to the oxidation state of the selenium atom as oxu-selenates(IV), and o.Yo-selenates(VI). The same applies for the respective tellurates. Besides this systematically correct naming, chemists usually use the terms selenites and tellurites instead of o. o-selenates(IV) and oxo-tellurates(IV), and selenates and tellurates instead of OAO-selenates(VI) and oxo-tellurates(VI). Therefore, both nomenclatures will be used in parallel throughout this chapter. Compared to the respective sulfur species, the oxo-selenate(IV) and oxo-tellurates(IV) ions are very stable so that numerous compounds with these anions have been prepared. 

The atomic absorption spectrophotometric methods discussed in section 12.10.2.2 [122, 124] has been applied to the determination of selenium in sediments. Itoh et al. [165] and Cutter [122] (section 12.10.2.2) have used hydrogen generation-atomic absorption spectrophotometric techniques to determine selenium in non-saline sediments. Cutter [122] was able to distinguish between selenite, selenate, total selenium and organic selenium in sediments. 

Selenium - the atomic number is 34 and the chemical symbol is Se. The name derives from the Greek Selene, who was the Greek goddess of the moon because the element is chemically found with tellurium (Tellus - the Roman goddess of the earth). It was discovered by the Swedish chemist Jons Jacob Berzelius in 1817, while trying to isolate tellurium in an impure sample. 

This band is at a lower frequenqf than that of the Si-O stretch in pure Si oxide gel (1080 cm ) probably because the Si-O stretch is influenced by neighbouring Fe atoms. For adsorbed arsenate and selenate on ferrihydrite, Harrison Berkheiser (1982) reported a poorly resolved doublet at 805 and 875 cm , and weak bands at 700 cm and at 820, 880 and 910 cm respectively. These were attributed to splitting of the fundamental v vibration. 

If two isomorphous derivatives in which the replaceable atoms have considerably different diffracting powers are available, an improved form of this method can be used. Strychnine sulphate pentahydrate and the corresponding selenate form isomorphous monoclinic crystals of space group C2 Bokhoven, Schoone, and Bijvoet (1951) solved the centrosymmetric b projection by the straightforward method first used... 

Fig. 2X3. Determination of phase angles in non-centrosymmctric crystals, a. Amplitudes and phases for corresponding reflections in isomorphous sulphate and selenate of strychnine. b. Knowledge of -Fguiphi aeb and ifse fs) gives magnitude but not sign of a c. Amplitudes and phases for a protein and two isomorphous derivatives with heavy atoms at different sites, d. Knowledge of Fp, Fp x p+Y fx> fy and gives unique...

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