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High Chemical (high + chemical)

Distribution by Scientific Domains
Chemistry75%

Terms modified by High Chemical

  • high chemical yield
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    Selected Abstracts

    Mechanical behavior of carbon nanofibre-reinforced epoxy composites

    JOURNAL OF APPLIED POLYMER SCIENCE, Issue 4 2010
    Sohel Rana
    Abstract Epoxy resins are widely used in a variety of applications because of their high chemical and corrosion resistance and good mechanical properties. But few types of epoxy resins are brittle and possess low toughness which makes them unsuitable for several structural applications. In this work, carbon nanofibres have been dispersed uniformly into the epoxy resin at a very low concentration (0.07 vol. %). Improvement of 98% in Young modulus, 24% in breaking stress and 144% in work of rupture was achieved in the best sample. The emphasis is on achieving uniform dispersion of carbon nanofibers into epoxy resin using a combination of techniques such as ultrasonication, use of solvent and surfactants. The fracture surfaces of the specimens were studied under scanning electron microscope to see the fracture mechanism of nanocomposites under tensile load and correlate it to the enhancement in their properties. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source]

    Studies on Stereoselective [2+2] Cycloadditions between N,N-Dialkylhydrazones and Ketenes

    CHEMISTRY - A EUROPEAN JOURNAL, Issue 23 2004
    Eloísa Martín-Zamora Dr.
    Abstract Staudinger-like cycloadditions between chiral, non-racemic N,N-dialkylhydrazones 1 and functionalized ketenes constitute an efficient methodology for the stereoselective construction of the ,-lactam ring. The potential for fine tuning of the dialkylamino auxiliary structure, the availability of a high-yielding deprotection method for the release of the free azetidinones, and the high thermal and chemical stability of hydrazones as N-dialkylamino imines are highlighted as the key elements for the success of the strategy. This last aspect is of particular importance concerning generality: even hydrazones from easily enolizable aldehydes or from formaldehyde reacted to afford the corresponding cycloadducts with high chemical and stereochemical yields. The syntheses of the ,-amino-,-hydroxyacids (2R,3S)-phenylisoserine (42) and (2R,3S)-norstatin (45) were accomplished as illustrative examples of the synthetic utility of this procedure. A model system for the cycloaddition of g series auxiliaries was studied by ab initio computational methods. The collected results support a two-step mechanism through zwitterionic intermediates, and explain the observed absolute and relative stereochemistry in terms of the preferred outward cycloaddition to the Re face of the hydrazone. La reacción de cicloadición [2+2] de tipo Staudinger entre cetenas funcionalizadas y las hidrazonas quirales 1 constituye un método eficiente para la construcción estereoselectiva del anillo de azetidinona. La modulación de la estructura del grupo dialquilamino empleado como auxiliar, la disponibilidad de un procedimiento eficiente para la desprotección de las , -lactamas libres y la alta estabilidad química y térmica de las hidrazonas como N-dialquilamino iminas son los puntos clave para el éxito de la estrategia. Este último aspecto es de particular importancia por su implicación en la generalidad del método: hidrazonas derivadas de aldehídos fácilmente enolizables o incluso las derivadas de formaldehído reaccionan para dar lugar a los correspondientes cicloaductos con excelentes rendimientos químicos y estereoquímicos. La síntesis de los , -amino- , -hidroxiácidos (2R,3S)-fenilisoserina (42) y (2R,3S)-norestatina (45) se llevaron a cabo como ejemplos ilustrativos del potencial sintético del método. Estudios computacionales ab initio realizados sobre una reacción modelo basada en los auxiliares de la serie g sugieren un mecanismo en dos pasos a través de intermedios zwitteriónicos. Los resultados obtenidos explican la estereoquímica absoluta y relativa, así como el alto grado de inducción observado para estos auxiliares, por la preferencia de la aproximación "outward" de la cetena sobre la cara Re de la hidrazona. [source]

    A high-efficiency preparation, properties and structure of (R, S)-and (S, S)-pyrrolidine-2-carboxylic acid 3,5-dioxa-4-boracyclohepta [2,1- a; 3,4- a,] dinaphthalen-4-yl esters

    CHINESE JOURNAL OF CHEMISTRY, Issue 10 2003
    Zi-Xing Shan
    Abstract A highly-efficient preparative procedure for (R, S) and (S, S)-pyrrolidine-2-carboxylic acid 3,5-dioxa-4-boracyclohepta[2, 1- a; 3, 4- a] dinaphthalen-4-yl esters [namely (R, S)-BNBAP and (S, S)-BNBAP] is described and the crystal structure of (R, S)-BNBAP was obtained. The data indicate that (R, S)- BNBAP is a spirocyclic inner borate salt with almost normal tetrahedral configuration. This structural form may be the basic reason for their high chemical, optical and thermodynamic stability. [source]

    Preparation of enantiomeric gossypol by crystallization,

    CHIRALITY, Issue 6 2003
    Michael K. Dowd
    Abstract Large enantiomorphic crystals of gossypol-acetone (1:3) were grown from acetone solutions of rac -gossypol-acetic acid (1:1) at 4°C. By controlling the initial gossypol concentration, crystallization time, and solution volume, single crystals were grown that weighed >50 mg, equivalent to >37 mg of enantiomeric gossypol. Even larger crystals were possible, but it was difficult to produce these reliably without contamination of the antipode. Essentially all of the acetone within the crystal form was removed by storing the crystals under vacuum for 3,4 days. By employing these techniques, gram quantities of enantiomeric gossypol were prepared in high chemical and optical purity. Based on measured and reported optical rotations, the optical purity of samples prepared by crystallization was greater than the optical purity of samples prepared by chromatographic separation of gossypol-amine diastereomers. The principal limitation of crystallization as a preparative method is the need to determine the chirality and purity of each product crystal. Nevertheless, the method competes favorably with preparative-scale chromatographic procedures. Chirality 15:486,493, 2003. Published 2003 Wiley-Liss, Inc. [source]