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Reaction Takes (reaction + take)
Selected AbstractsLow-Temperature Synthesis of Bismuth Titanate Niobate (Bi7Ti4NbO21) Nanoparticles from a Metal-organic Polymeric PrecursorJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2000Pedro Durán This paper describes the preparation of homogeneous Bi7Ti4NbO21 single-phase ceramic powders of ,55 nm crystallite size, at temperatures as low as 400°,500°C using a metal citrate complex method based on the Pechini-type reaction route. The thermal decomposition/oxidation of the polymerized resin, as investigated by TG/DTA, XRD, and SEM, led to the formation of a well-defined orthorhombic Bi7Ti4NbO21 compound with lattice parameters a= 0.544, b= 0.540, and c= 2.905 ± 0.0005 nm. Reaction takes place through an intermediate binary phase with a stoichiometry close to Bi20TiO32 which forms between 300° and 375°C. The metal-organic precursor synthesis method, where Bi, Ti, and Nb ions are first chelated to form metal complexes and then polymerized to give a gel, allows control of the Bi/Ti/Nb stoichiometric ratio leading to the rapid formation of nanosized bismuth titanate niobate (Bi7Ti4NbO21) ceramic powders, at temperatures much lower than usually needed by conventional processing of mixed-oxide powders. [source] Thermodynamic Study of ,H Complexes in Nucleophilic Aromatic Substitution Reactions: Relative Stabilities of Electrochemically Generated RadicalsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2008Iluminada Gallardo Abstract The mechanism for the electrochemical oxidation of ,H complexes, such as 1-hydro-1-alkoxy/sulfoxy or -fluoro-2,4-dinitro/2,4,6-trinitrocyclohexadienyl anions, has been widely studied by means of cyclic voltammetry and controlled-potential electrolysis. Previous studies have shown that the electrochemical oxidation of ,H complexes, formed by the addition of carbon or nitrogen nucleophiles followed by a two electron mechanism, corresponding to the formal elimination of the hydride anion (nucleophilic aromatic substitution of hydrogen mechanism, the NASH mechanism). For these ,H complexes (Nu, = OH,, ,OR, ,SR, ,F), the electrochemical reaction takes place by a one-electron mechanism and is followed by the radical elimination of the leaving group with the consequent recovery of the starting material. This mechanism is similar to that proposed for the electrochemical oxidation of ,X complexes (nucleophilic aromatic substitution of a heteroatom, the NASX mechanism). The operating mechanism in each case, the NASH or NASX, can be rationalized in terms of thermodynamics. The standard potentials of the , complex and/or the leaving group as well as the bond dissociation energies (BDEs) are determinant factors. This study has not led to a significant improvement in the electrochemical preparation of aromatic-substituted compounds, but does help to understand and predict the usefulness or uselessness of using the nucleophilic aromatic substitution route to obtain a desired product. Finally, the current approach extends the electrochemical methodology to different chemical fields, for example, to general nondestructive methods for the detection, identification, and quantification of either organic pollutants or explosives in different solvents. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] The Ritter Reaction under Truly Catalytic Brønsted Acid ConditionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 28 2007Roberto Sanz Abstract Simple organic acids like 2,4-dinitrobenzenesulfonic acid (DNBSA) catalyze the Ritter reaction of secondary benzylic alcohols giving rise to the corresponding N -benzylacetamides in usually high yields. Reactions can be conducted without exclusion of oxygen and without the need of dry solvents. With tertiary ,,,-dimethylbenzylic alcohols a different pathway involving a formal dimerization reaction takes place under the acid-catalytic conditions used. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Asymmetric Ring Opening of Benzo-7-oxabicyclo[2.2.1]heptadienes with Cationic Rhodium ComplexesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-12 2010Angelika Preetz Abstract The efficient design of stereochemically challenging ring systems by ring opening of heterobicyclic alkenes has become a very important reaction in the chemistry of CC and CX bond formation. By using the hitherto applied in situ technique for the generation of the ,2 -bridged, dimeric neutral rhodium complexes, however, the catalytically active species and its concentration remained unidentified. Furthermore, the reaction temperature is at least 80,°C. The application of cationic rhodium(I) solvate complexes (that no longer contain blocking diolefins) shows that a much greater activity and enantioselectivity for the synthesis of 1,2-dihydro-1-naphthols can be reached than was described so far, even at ambient temperature. NMR spectroscopy and X-ray analysis show that a product inhibition during the ring opening reaction takes place that is independent of the nucleophile. [source] Local HSAB principle in the conjugate addition of p -substituted thiophenols to cyclohexenoneINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2005Rocío Meza Abstract In the formation of new bonds through the Michel-type addition of thiolates to a ,,,-conjugate system, it is observed that the reaction takes place faster if the thio donor compound contains an electron-rich group or the enone acceptor is highly electron deficient. In contrast, the hard,soft acid,base (HSAB) principle predicts that this reaction is favored when a soft,soft interaction between the reactants takes place. Taking into account that softness is related to a barely electronegative atom of high polarizability, we consider it of interest to analyze the effect of charge transfer of a p -substituent on the softness of sulfur in thiophenols, as well as its impact in the conjugate addition to 2-cyclohexen-1-one. Experiments-in-competition, net charge of X-groups at the p -position of the aromatic ring, the global and local softness at sulfur, and the electrophilicity, obtained by density functional theory (DFT), led to the observation that the reaction is faster for electron attractor thiophenols. The softness at sulfur increases by delocalization of charge through the aromatic ring. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] Synthesis of Functionalized Indoles with an ,-Stereogenic Ketone Moiety Through an Enantioselective Friedel,Crafts Alkylation with (E)-1,4-Diaryl-2-butene-1,4-dionesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 14-15 2009Gonzalo Blay Abstract Chiral complexes of BINOL-based ligands with hafnium tert -butoxide catalyze the enantioselective Friedel,Crafts alkylation of indoles with (E)-1,4-diaryl-2-butene-1,4-diones at room temperature, with good yields and ee up to 94%. Hafnium(IV) was found to be a more effective Lewis acid than other frequently used metal ions such as titanium(IV) or zirconium(IV). Unlike the enantioselective Friedel,Crafts alkylation of indoles with ,,,-unsaturated compounds where the stereogenic center is generated in the ,-position to a carbonyl group, the Friedel,Crafts alkylation with 2-butene-1,4-diones described here generates an ,-stereogenic center with respect to one of the carbonyl groups. This can be regarded as an inversion of the normal reactivity pattern or umpolung. The enantioselective Friedel,Crafts alkylation of indoles with (E)-4-oxo-4-phenylbutenoates using a zirconium(IV)-BINOL catalyst is also reported. This reaction takes place regioselectively at the carbon in the ,-position to the ketone carbonyl group, generating an ,-ester stereogenic center. [source] Performance of fractionating reactors in the absence of rate limitationsJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 9 2004Jeroen L den Hollander Abstract A fractionating reactor for equilibrium-limited reactions is studied theoretically. Reactant A is fed in the center of the countercurrent fractionating system. Product P is effectively transported with the auxiliary phase, while product Q is effectively transported with the main phase, in which the reaction takes place. Model calculations were based on partition and reaction equilibrium at all stages. These show that if the initial reactant concentration and the flow rates are properly selected, the extent of conversion will significantly exceed the corresponding batch conversion. To approach complete conversion in the fractionating reactor, and to recover both products in a pure form, net transport of reactant in either of the countercurrent directions should be prevented. However, irrespective of the number of equilibrium stages, this situation cannot be fully reached when the reactant feed stream is too large (compared with the main and auxiliary streams). Nonetheless, one of the two products may be recovered in a pure form even for such large feed streams. Copyright © 2004 Society of Chemical Industry [source] A New Farnesyl Diphosphate Synthase Gene from Taxus media Rehder: Cloning, Characterization and Functional ComplementationJOURNAL OF INTEGRATIVE PLANT BIOLOGY, Issue 6 2006Zhi-Hua Liao Abstract Farnesyl diphosphate synthase (FPS; EC 2.5.1.10) catalyzes the production of 15-carbon farnesyl diphosphate which is a branch-point intermediate for many terpenoids. This reaction is considered to be a rate-limiting step in terpenoid biosynthesis. Here we report for the first time the cloning of a new full-length cDNA encoding farnesyl diphosphate synthase from a gymnosperm plant species, Taxus media Rehder, designated as TmFPS1. The full-length cDNA of TmFPS1 (GenBank accession number: AY461811) was 1 464 bp with a 1 056-bp open reading frame encoding a 351-amino acid polypeptide with a calculated molecular weight of 40.3 kDa and a theoretical pI of 5.07. Bioinformatic analysis revealed that TmFPS1 contained all five conserved domains of prenyltransferases, and showed homology to other FPSs of plant origin. Phylogenetic analysis showed that farnesyl diphosphate synthases can be divided into two groups: one of prokaryotic origin and the other of eukaryotic origin. TmFPS1 was grouped with FPSs of plant origin. Homology-based structural modeling showed that TmFPS1 had the typical spatial structure of FPS, whose most prominent structural feature is the arrangement of 13 core helices around a large central cavity in which the catalytic reaction takes place. Our bioinformatic analysis strongly suggests that TmFPS1 is a functional gene. Southern blot analysis revealed that TmFPS1 belongs to a small FPS gene family in T. media. Northern blot analysis indicated that TmFPS1 is expressed in all tested tissues, including the needles, stems and roots of T. media. Subsequently, functional complementation with TmFPS1 in a FPS-deficient mutant yeast demonstrated that TmFPS1 did encode farnesyl diphosphate synthase, which rescued the yeast mutant. This study will be helpful in future investigations aiming at understanding the detailed role of FPS in terpenoid biosynthesis flux control at the molecular genetic level. (Managing editor: Wei Wang) [source] Investigation of reacting flow fields in miscible viscous fingering by a novel experimental methodAICHE JOURNAL, Issue 3 2009Yuichiro Nagatsu Abstract The reacting flow fields in reactive miscible viscous fingering in a Hele-Shaw cell studied by Nagatsu and Ueda had not been completely elucidated, mainly because one cannot exactly recognize where and when the reaction takes place in the reactive fingering pattern. We developed a novel experimental method that allowed us to identify the reaction region in the fingering pattern employed in the previous studies. The novel method involves switching of the less-viscous liquid injected in both the nonreactive and reactive experiments. By using the novel method, we succeeded in showing how the reaction region in the fingering pattern was affected by the initial reactant concentrations, the Péclet number, and time. We propose physical models of the reacting flow field in the cell's gap direction that can explain the obtained experimental results. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Interpolated controllers for the robust transition control of a class of reactorsAICHE JOURNAL, Issue 1 2006Guillermo Fernández-Anaya Abstract Two new theoretical results about interpolated controllers for the transition control and simultaneous stabilization of single input-single output plants with variable operating conditions are shown; recent results are generalized for interpolated controllers. On the basis of these results, an application to the closed-loop transition control of a continuous stirred-tank reactor, where a single exothermic reaction takes place, is presented. A comparison with simple PI controllers is made. © 2005 American Institute of Chemical Engineers AIChE J, 2006 [source] Determination of Size Distributions of Concentrated Polymer Particles Embedded in a Solid Polymer MatrixPARTICLE & PARTICLE SYSTEMS CHARACTERIZATION, Issue 1 2008Ezequiel R. Soulé Abstract In this work we present the results obtained from the size characterization of polymer particles embedded in a solid polymer matrix using Static Light Scattering (SLS) and Scanning Electron Microscopy (SEM). The analyzed samples are the result of the solution polymerization of isobornyl methacrylate (IBoMA) in polyisobutylene (PIB) at complete conversion. Induced by polymerization, the system undergoes phase separation. As a result, spherical micron sized particles rich in PIB are formed. At the end of the polymerization, the particles become trapped in a solid polymer matrix rich in Poly-IBoMA. Size, concentration, and refractive index, make the resulting particle system scatter light under the Rayleigh-Debye-Gans (RDG) regime with interparticle interference. For Light Scattering (LS) characterization the samples are measured with a Flat Cell Static Light Scattering (FCSLS) apparatus, in which the reaction takes place. The resulting SLS spectra are analyzed using the Percus-Yevick approximation to model the interference effects. The local monodisperse approximation is used to consider polydispersity in the particle sizes. The estimated particle size distributions agree well with the measurements from SEM. In this work a concentrated particle system that naturally scatters light according to the RDG regime has been fully characterized in terms of its particle size distribution. This work, against the opinion of other authors, shows the feasibility of measuring still particles using a one dimensional array of light detectors. [source] The Influence of Differences Between Microchannels on Micro Reactor PerformanceCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 3 2005E. R. Delsman Abstract Microstructured reactors most often contain a large number of micrometer-sized, parallel channels, instead of a large undivided reaction volume. Individual microchannels behave as plug-flow reactors without significant axial dispersion and with excellent heat and mass transfer properties. However, since the reaction takes place in a large number of parallel channels, it is important that all channels provide equal residence time and amount of catalyst volume. These issues depend not only on the flow distributor design, but also, for example, on the manufacturing tolerances. Correlations are derived to express the conversion of a multichannel microreactor explicitly as a function of the variance of a number of reactor parameters, viz. the channel flow rate, the channel diameter, the amount of catalyst in a channel, and the channel temperature. It is shown that the influence of flow maldistribution on the overall reactor conversion is relatively small, while the influences of variations in the channel diameter and the amount of catalyst coating are more pronounced. The model outcomes are also compared to experimental results of two microreactors with different catalyst distributions, which show that the presented method is able to provide a quick, though rough estimation of the influence of differences between channels on microreactor performance. [source] Engineering and biotechnological aspects for the manufacturing of high quality fried potato productsBIOTECHNOLOGY JOURNAL, Issue 4 2006Ernst H. Reimerdes Abstract Fried potato products have become very popular foods over the last decades. High quality standards have been established for these products by the food industry including uniform brown color and crispness. During frying, Maillard reactions takes place which contribute to color and taste development in these products. However, safety aspects are also influenced by these reactions, e.g., acrylamide formation. Maintaining high safety standards as well as the expected quality requires systematic research based on an integrated approach including all relevant variables, e.g., raw material properties, processing conditions and equipment concepts. Selected results of these investigations are presented and discussed, regarding influence of composition, e.g., precursor levels for Maillard reactions, treatment of raw materials and addition of reactants to frying fat. It has been demonstrated that a combined treatment of the potato sticks by coating of product surfaces and partial pre-drying can be successfully applied to produce well-browned French fries with lower acrylamide contents. Reductions up to 75% could be reached compared to samples without treatment. Furthermore, addition of a water/oil emulsion containing glutamine in the aqueous phase has been shown to influence Maillard reactions at the product surface, resulting in lower acrylamide contents at the same state of browning. [source] | ||||||||||||||||||||||||||||