Apparatus for the determination of melting points, molecular weights, freezing points and purity, and for the study of melting

An apparatus for the determination of the melting point of a partially frozen liquid is described. Thermal equilibrium is rapidly attained, largely because of the presence of glass balls in the experimental flask, and the method of agitation. The use of a thermistor and a recording potentiometer allows charting of the temperature changes with time. The melting point can be calculated from points on the trace. The apparatus has been used to study melting point depressions for solutions of hydrocarbons and phenols in benzene, nitrobenzene, water and dioxan.     

Spectrophotometric, spectrofluorimetric, and densitometric methods for the determination of indapamide

Three sensitive spectrophotometric, spectrofluorimetric, and densitometric methods are described for the determination of indapamide. The first and second methods are based on the oxidative coupling reaction of indapamide with 3-methyl-2-benzothiazolinone hydrazone HCl (MBTH) in the presence of cerium(IV) ammonium sulfate in an acidic medium. The absorbance of the reaction product is measured at the lambdamax, 601 nm. With the same reaction, indapamide is determined by its quenching effect on the fluorescence of excess cerous ions at the emission lambdamax, 350 nm, and the excitation at lambdamax, 300 nm. The reaction conditions were optimized, and Beer's law was obeyed for indapamide at 1.2-9.6 microg/mL with mean recoveries of 99.92 +/- 0.83 and 99.97 +/- 1.11%, respectively. The third method, a stability-indicating densitometric assay, was developed for the determination of indapamide, using toluene-ethyl acetate-glacial acetic acid (69 + 30 + 1, v/v/v) as the developing system and scanning at the lambdamax, 242 nm, in the presence of the degradation product and related substance; for the indapamide concentration range of 0.6-6 microg/spot, the mean recovery was 99.73 +/- 0.71%. The proposed methods were successfully applied to the determination of indapamide in bulk powder and commercial tablets, and the results of the analysis agreed statistically with those obtained with the official method. Furthermore, the methods were validated according to the guidelines of the U.S. Pharmacopeia and also assessed by applying the standard additions technique.     

Asymmetric Methods for the Synthesis of Flavanones, Chromanones, and Azaflavanones

Flavanones, chromanones, and related structures are privileged natural products that display a wide variety of biological activities. Although flavanoids are abundant in nature, there are a limited number of available general and efficient synthetic methods for accessing molecules of this class in a stereoselective manner. Their structurally simple architectures belie the difficulties involved in installation and maintenance of the stereogenic configuration at the C2 position, which can be sensitive and can undergo epimerization under mildly acidic, basic, and thermal reaction conditions. This review presents the methods currently used to access these related structures. The synthetic methods include manipulation of the flavone/flavanone core, carbon-carbon bond formation, and carbon-heteroatom bond formation.     

Mechanism for the deamination of ammeline,guanine, and their analogues

Structural Chemistry - Mechanisms for the deamination of ammeline (AMN), guanine (Gua), and their analogues with nH2O (n = 1–3) have been investigated using B3LYP and...     

Micelle,microemulsions, and gels for the conservation of cultural heritage

Past restorations performed with acrylic and vinyl polymers showed detrimental effects to wall paintings that lead to the complete disfiguration of the painted surfaces. The removal of these materials performed with the traditional solvent-based methodology represents a real challenge to conservators and usually achieves very poor results. This review reports on the new palette, nowadays available to restorers, based on microemulsions, micellar systems, physical and chemical gels specifically formulated for the cleaning of cultural heritage artefacts. These systems have been developed in the last twenty years within the cultural framework of colloids and surface science.     

Electrical,electrochemical, and thermometric sensors for the detection of explosives

The main analytical characteristics of electrical, electrochemical, and thermometric sensors in the detection of vapors and traces of explosives and accompanying substances are compared. The limits of detection, sensitivity, sensor setting time (response speed) and, recovery time after exposure to analytes, and the selectivity of sensors are discussed. The efficiency of using nanodimensional structures in the sensing elements of sensors is investigated.     

Convenient method for the preparation of carbamates, carbonates, and thiocarbonates

A convenient, rapid, and efficient method for the preparation of carbamates from amines with 1-alkoxycarbonyl-3-nitro-1,2,4-triazole transfer reagents is reported. Reactions of newly synthesized stable crystalline reagents with alkyl amines were completed in a few minutes without any additional base, and highly pure carbamates were obtained without chromatographic purification. These highly active reagents are also useful for the selective protection of nucleobases and preparation of carbonates and thiocarbonates.     

On the partitioning of energy for Atoms,Ions, and Molecules

The fact that, for neutral atoms, the interelectronic repulsion energy I is a constant fraction (1/3) of the total energy E is discussed in the light of several approximate energy formulas, theoretical and empirical. We then derive the asymptotic form of the non-relativistic energy of a neutral atom as E=–kZ ^7/3 a.u. (Z = atomic number) with k probably between 1/2(3/2)^1/3 and 9/14(3/2)^1/3. For 6Z90, the actual energies are in fact between these limits, while the formula implies I/E=1/3. The ratio of interelectronic repulsion energy to total electronic energy for ions and molecules is derived.

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Zusammenfassung Mit der Hilfe einiger theoretischer und empirischer Näherungsformeln wird die Tatsache diskutiert, daß für neutrale Atome die zwischenelektronische Abstoßungsenergie I stets etwa 1/3 der elektronischen Gesamtenergie E beträgt. Die asymptotische Form der nicht relativistischen Energie eines neutralen Atoms wird dann zu E=–kZ^1/3 abgeleitet, wobei Z die Kernladungszahl st und k wahr 1/2(3/2)^1/3 und 9/14(3/2)1/3 scheinlich zwischen 1/2 und 9/14(3/2)^1/3 liegt. Für 6Z90 liegen die berechneten Energien tatsächlich innerhalb dieser Grenzen. Gleichzeitig enthält diese Formel das Verhältnis I/E=1/3. Ferner wird das Verhältnis von I zu E für Ionen und Moleküle abgeleitet.

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Résumé Nous discutons le fait que, pour les atomes neutres, l'énergie de répulsion interélectronique I est toujours à peu près 1/3 de l'énergie électronique totale E. Après avoir examiné le rapport entre ce fait et certaines formules théoriques et empiriques pour l'énergie, nous démontrons que la forme asymptotique de l'énergie est E=–kZ ^7/3 u.a. (Z = charge du noyau), avec k probablement entre 1/2(3/2)^1/3 et 9/14(3/2)^1/3. Les énergies calculées pour 6Z90 sont en accord avec cette règle. La formule E=–kZ ^7/3conduit aussi à I/E= 1/3. Nous démontrons également le rapport entre I et E pour les ions et les molecules.

     

New Route for the Synthesis of Pyrazole,Triazole, Triazine,and Triazepine Derivatives

N,N′‐Diphenylpiperidine‐1‐carbohydrazonamide 1 was prepared and treated with halo compounds, active nitriles, diethylhy malonate, ketones, CS₂, phenylisocyanate, phenylisothiocyanate, LR, ethyl 2‐cyano‐3,13‐dithiomethylacetate, and benzylidenenitriles to give the corresponding triazines 2–4, pyrazoles 5 and 6, triazoles 7, 8, and 10, triazaphosphole 11, and triazepines 12 – 14, respectively.     

Determination of the Uncertainty of the Flame Atomic Absorption Spectrometer for Copper,Cobalt, Cadmium,and Nickel

Abstract

The sources of uncertainty in the determination of copper, cobalt, cadmium, and nickel, originating from the flame atomic absorption spectrometer, were studied. They were identified and divided in three main groups according to their origin—from the flame, from the light source (hollow-cathode lamp), and from the optics. The combined uncertainty of the flame atomic absorption determination was experimentally determined.     

Prediction of the enthalpy of vapourisation for anhydrides,formates, acetates,propionates, butyrates,esters, and ethers

Four analytical correlations based on the use of the corresponding states principle were used to calculate the enthalpy of vapourisation of fluids. Three of these correlations require as inputs the critical temperature and the acentric factor. The fourth requires a molecular Lennard–Jones parameter and the acentric factor. Results for 184 polar and non-polar fluids grouped into 9 families are compared with the values accepted by the Design Institute for Physical Property (DIPPR) project. Recommendations are given for the use of each model and for the choice of the adequate model for each family of fluids.     

General strategy for the syntheses of corynanthe, tacaman, and oxindole alkaloids

We report herein the total synthesis of the corynanthe alkaloid dihydrocorynantheol and the formal syntheses of the indole alkaloids tacamonine, rhynchophylline, and hirsutine. The strategies for assembling the corynanthe and tacaman skeletal frameworks comprised of both the classical ABD --> ABCD and ABC --> ABCD approaches wherein the variously substituted piperidinone D-rings were formed via ring-closing metathesis (RCM) followed by a 1,4-addition to introduce the requisite side chain at C(15). Since 1,4-additions to alpha,beta-unsaturated lactams represent an underdeveloped field, we conducted a series of studies with two unsaturated lactams employing organocuprates and metal enolates as the nucleophiles. These studies revealed that organocuprates derived from Grignard reagents and either stoichiometric amounts of CuCN or catalytic amounts of CuBr.DMS complex are excellent nucleophiles for such additions; TMSCl was a crucial additive for optimizing these reactions. The anion derived from ethyl 1,3-dithiolane-2-carboxylate was also an excellent nucleophile in these 1,4-additions, although the stereochemistry of such 1,4-additions to carboline-derived, unsaturated lactams was sensitive to substitution on the indole nitrogen atom. The ABD --> ABCD approach to these alkaloids featured a novel one-pot sequence of an RCM reaction and a zirconocene-catalyzed carbomagnesation followed by a second RCM to generate the D-ring.     

铜、镍、钯催化C-C偶合反应的研究进展

偶合反应是有机合成中的一种重要反应,本文综述了Cu、Ni、Pd等不同形式催化剂催化C-C偶合反应的研究进展,并展望优化该类反应的条件．     

General procedure for the synthesis of ortho-vinylbenzyl-substituted amines, ethers, and sulfides

A convenient procedure has been proposed for the synthesis ortho-vinylbenzyl-substituted ethers, amines, and sulfides via reaction of o-(2-bromoethyl)benzyl bromide with various nucleophiles.     

Membrane Microreactors for the On-Demand Generation,Separation, and Reaction of Gases

The use of gases as reagents in organic synthesis can be very challenging, particularly at a laboratory scale. This Concept takes into account recent studies to make the case that gases can indeed be efficiently and safely formed from relatively inexpensive commercially available reagents for use in a wide range of organic transformations. In particular, we argue that the exploitation of continuous flow membrane reactors enables the effective separation of the chemistry necessary for gas formation from the chemistry for gas consumption, with these two stages often containing incompatible chemistry. The approach outlined eliminates the need to store and transport excessive amounts of potentially toxic, reactive or explosive gases. The on-demand generation, separation and reaction of a number of gases, including carbon monoxide, diazomethane, trifluoromethyl diazomethane, hydrogen cyanide, ammonia and formaldehyde, is discussed.     

Stoichiography for investigation of the functional materials composition,structure, and properties

Principles of the stoichiography and differential dissolution method (DD) providing deep insight into the composition of mixtures containing unknown chemical compounds are considered. Compounds can be identified using their primary feature — the stoichiometry of elemental composition; reference samples of the compounds are not needed. The DD method makes it possible to analyze mixtures of crystalline and/or amorphous phases with constant and/or variable composition in the form of dispersed powders, ceramics, crystals, thin films and nanosize objects. Various aspects of the stoichiography and DD method application to investigation of composition, structure and properties of the functional materials are discussed.