Titration von Sulfid mit einer sulfidionensensitiven Elektrode

Zusammenfassung Eine Methode zur potentiometrischen Titration von Sulfid mit Natriumplumbat(II)lösung an einer sulfidsensitiven Membranelektrode wird beschrieben. Sie eignet sich zur Bestimmung von 10^–4% Sulfid in Gegenwart von Cl^–, Br^–, J^–, SCN^–, SO₃ ^2–, S₂O₃ ^2–

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Titration of sulphide with a sulphide-ion sensitive electrode

A method is described for the titration of sulphide with sodium plumbate(II) solution using a sulphide-sensitive electrode. The method is suitable for the determination of 10^–4% of sulphide in the presence of Cl^–, Br^–, J^–, SCN^–, SO₃ ^2–, S₂O₃ ^2–.

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Wir danken Herrn Dr. K. Obst für wertvolle Hinweise und Diskussionen.     

Enzymatic desymmetrization of a meso polyol corresponding to the C(19)-C(27) segment of rifamycin S

The stereoselective acylation of meso polyol 2 by vinyl acetate (solvent and acyl donor) in the presence of porcine pancreas lipase gave the corresponding monoester 5 in good yield (76%) and high enantiomeric purity (ee > 98%). The enzymatic reaction was also highly regioselective for a primary alcohol end group, and the two unprotected secondary alcohols were not involved. Compound 5 corresponds to the C(19)-C(27) fragment of rifamycin S.     

LiBF4-Catalyzed three-component coupling of an aldehyde, acetic anhydride and allyltrimethylsilane/TMSCN

Lithium tetrafluoroborate is found to be an efficient catalyst for allylation and cyanation of aldehydes with allyltrimethylsilane and trimethylsilyl cyanide in the presence of acetic anhydride at room temperature to produce homoallylic acetates and α-cyano acetates in excellent yields. A solution of 10 mol % of LiBF₄ in acetonitrile provides a convenient reaction medium to carry out allylation and cyanation reactions under very mild and neutral conditions.     

Investigation of electronic structure and associated nuclear quadrupole interaction of adsorbed fluorine atoms on silicon surface

The electronic energy levels and wave functions of fluorine and chlorine atoms adsorbed at the <111> surface of silicon are obtained by the Hartree-Fock procedure applied to clusters simulating the surface structure. Minimising the total energy with respect to the positions of the halogen atoms, the Si-Cl distance in the adsorbed system was found to be in good agreement with that from a recent SEXAFS measurement, there being no corresponding data presently available for fluorine on silicon surface. The calculated¹⁹ f ^* nuclear quadrupole coupling constants using the electronic wave functions for SiH₃F and the larger cluster Si₄H₉F, simulating the surface, differed substantially in contrast to the situation for the Si-F distance, indicating that the quadrupole coupling is a more sensitive indicator of the adequacy of the cluster chosen to represent the surface system. Experimental results for the coupling constants are awaited for comparison with theory.     

Use of a carboxylate exchanger in the determination of copper in distilled water by neutron activation analysis

Copper in distilled water is collected on Zeokarb 226(NH ₄ ⁺ ) and irradiated with thermal neutrons. By comparison of the induced activity of 0.51 MeV photopeak of⁶⁴Cu with that of a standard, irradiated under comparable conditions, copper at ppb levels is easily determined.     

Determination de C,N, O,F et de quelques autres elements par activation au moyen des photons gamma

The (γ, n) activation curves for 17 elements were determined as a function of energy between 25 and 42 MeV. From these are deduced the detection sensitivities intrinsically possible for an average beam intensity of 100 μA. Our experimental results are compared with values calculated byG. J. Lutz. The activation curves relative to certain competitive nuclear reactions on N, O and Na, liable to interfere in the measurements of C, N and F, were also determined. It is shown that the relative importance of these competitive nuclear reactions decreases rapidly for energies below 30 MeV and becomes practically negligible at 25 MeV.      

Die Konstitution von Roridin D. Verrucarine und Roridine, 12. Mitteilung

The antibiotic roridin D (C₂₉H₃₈O₉), a companion of roridin A, yields on base catalysed hydrolysis verrucarol ( 3 ) and the hitherto unknown 2, 3-epoxy-2-anhydrororidinic acid ( 4 ) (C₁₄H₁₈O₇). Roridin D is a macrocyclic diester to which structure 1 is assigned. In roridin D an epoxy group replaces the hydroxyl group of roridin A.     

Stereochemistry of Macrocyclization of cis-4-Cyclohexene-1,2-dicarboxylic Acid with Dihalogen Derivatives

Stereochemistry of [2+2]- and [1+1]-macrocylization of cis-4-cyclohexene-1,2-dicarboxylic acid with 1,2-dibromoethane, 1,3-dichloro-2-propanol, and 1,5-dichloro-3-oxapentane was studied. The reaction of cis-4-cyclohexene-1,2-dicarboxylic acid with 1,2-dibromoethane according to the [2+2]-cyclization scheme gave a mixture of stereoisomeric macroheterocycles with cis,syn,cis- and cis,anti,cis-junction of the polyether and cyclohexene rings. In the reaction of cis-4-cyclohexene-1,2-dicarboxylic acid with 1,5-dichloro-3-oxapentane, a mixture of crown compounds with cis,anti,cis- and cis-junction of the polyether and cyclohexene rings was obtained. The structure of the products was established on the basis of their chemical transformations and spectral data.     

Methylbis (dithiocarbamato)stibane Synthese und Kristallstruktur

Methylbis(dithicarbamato) stibanes. Synthesis and Crystal Structure . The title compounds are obtained by reaction of methyldibromostibane with the sodium dithiocarbamates in methanol at low temperature (?78°C). The crystal structure of CH₃Sb(S₂CNEt₂)₂ as elucidated by X-ray diffraction shows no special intermolecular interactions. It can be compared to that of the corresponding bismutane CH₃Bi(S₂CNEt₂)₂ and the arsane C₆H₅As(S₂CNEt₂)₂.     

Extraction of Aromatic Hydrocarbons from Hydrofined Diesel Fraction with 2-Methoxyethanol in the Presence of Pentane

Five-step countercurrent extraction of aromatic hydrocarbons from hydrofined diesel fraction with the 2-methoxyethanol-water-pentane extraction system was studied. The process was performed on a rotary-disk extractor.