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Autocatalytic Reactions (autocatalytic + reaction)
Selected AbstractsAlkyl halides reactions with cathodes or with magnesium.JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 12 2006Grignard reagent studied with radical clocks. Abstract In the mechanism of reaction of Grignard reagent formation for alkyl halides (RX), it is generally assumed that the alkyl radical, formed by the electron transfer from the metal to this halide, reacts rapidly with the paramagnetic MgX, species. The previous comparisons of aryl halides reactivity toward magnesium and their reactivity toward a cathode strongly suggested that MgX, species are not, for the aryl halides, compulsory to rationalise the observed facts. The aryl radicals formed by electron transfer from the metal to the aryl halide would undergo a rapid second electron transfer to yield carbanions transformed into RMgX by reaction with MgX2. In contrast, for the alkyl halides, the reduction of the rapidly formed alkyl radicals into carbanions has seldom been discussed as a possible fate for these radicals, the main discussed fates being dimerisation, disproportionation, hydrogen abstraction from the solvent, rearrangement or coupling with MgX, radicals. Two main differences distinguish the reactivity of alkyl halides from their aryl halides counterpart. First, the radical anions of aryl halides may have a given lifetime whereas electron transfer to alkyl halides is concerted with the cleavage of the molecule. Second, the aryl radicals display far stronger oxidising properties than the alkyl radicals. The counterpart of this property is that aryl carbanions display weaker reducing properties than the alkyl ones. In this report, putting in perspective Grignard reaction and the experimental results obtained with identical radical clocks in electrochemistry, we tentatively provide an answer to the question raised in the title. The comparison of electrochemical patterns of reactivity of selected alkyl halides and the evolutions of yields in the preparation of Grignard reagent suggest a new explanation for the lower yields generally observed when alkyl iodides are the starting substrates. It involves an autocatalytic reaction where carbanionic species formed from the alkyl radicals and diffusing away from the metal surface, transfer one electron to the alkyl halide; the result would be the creation of two radicals leading to an increased amount of by-products. If the carbanionic mechanism were to be retained for the formation of alkyl Grignard reagent one would have to admit that the magnesium surface behaves as a cathode displaying high current densities reminiscent of microelectrodes. Copyright © 2006 John Wiley & Sons, Ltd. [source] Kinetic parameters estimation for unsaturated polyester-styrene systemsPOLYMER ENGINEERING & SCIENCE, Issue 2 2000Chao-Ming Lin A model modified from the traditional kinetic model is proposed for the determination of the kinetic parameters describing the autocatalytic reaction of polyesterstyrene systems. The proposed method utilizes three experimental data values from the vs. , curve,the initial value, the peak value, and the maximum value,to solve the kinetic parameters. The curing reaction of a thermosetting polyester was investigated by using the isothermal and dynamic techniques of differential scanning calorimetry (DSC). The results show that the reaction orders m, n and the ultimate conversion are not constant, and instead are functions of temperature. The estimated kinetic curves agree well with the experimental DSC data. [source] Thermostability of Lyocell Dopes Modified with Surface-Active AdditivesMACROMOLECULAR MATERIALS & ENGINEERING, Issue 8 2005Frank Wendler Abstract Summary: Cellulose/N -methylmorpholine- N -oxide monohydrate (NMMO) spinning solutions were modified with surface-active additives to yield Lyocell fibers with functional properties. Based on cellulose fibers, a new class of materials with tailored adsorption characteristics are produced. Activated charcoal and carbon black used as additives significantly affect the thermostability of the spinning solutions. Considering the degree of filling three general tendencies become evident. It is most obvious that the onset temperature of dope decomposition is shifted towards lower values accompanied by viscosity reduction after annealing at elevated temperatures and an enhanced formation of degradations products. Morpholine, N -methylmorpholine and formaldehyde as the main degradation products were detected in aqueous distillates by means of HPLC. To study the rate of by-product formation during preparation of the solution kinetic measurements were carried out. Thermal instabilities are not only initiated by heavy metal ions, especially Fe(II), but also by the particle size and porosity of the charcoal. The nano-scaled carbon black used causes autocatalytic reactions as revealed by calorimetric measurements. Relationships between amount of Acc versus onset temperature (Ton) and concentration of N -methylmorpholine. [source] An alternative model for predicting the cure kinetics of a high temperature cure epoxy adhesive,POLYMER ENGINEERING & SCIENCE, Issue 1 2003Angela D. Rogers The purpose of this work was to develop a cure kinetics model for a commercially available high temperature cure epoxy adhesive commonly used in the aerospace industry. While there are several phenomenological cure kinetic models commonly used in the literature for describing the rate of conversion of thermosetting epoxy adhesives as a function of degree of conversion, none of these models adequately depicts the adhesive used in this work over the entire range of conversion. Hence, by curve fitting empirical data collected using differential scanning calorimetry and refinement of existing models, an alternative model is proposed. The form of the present model suggests that chemical curing is the result of the combination of two autocatalytic reactions. The model is able to account for both the chemically controlled and diffusion controlled regimes of the cure. This paper also describes a novel iterative approach for predicting kinetics parameters as a function of isothermal cure temperature. Excellent agreement between experimental measurements and model predictions has been demonstrated over the entire range of conversion. [source] Pyrolysis Kinetics of Wood and Wood ComponentsCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 10 2005T. Willner Abstract The kinetics of pyrolysis decomposition reactions of wood and its components are described employing mathematical modeling tools for first and nth order reactions, autocatalytic reactions, and parallel first order and autocatalytic reactions. Secondary reactions may accelerate the reaction rate and a new autocatalytic reaction model was developed to describe this specific behavior. [source] A new definition of lifeCHIRALITY, Issue 3 2009James D. Carroll Abstract Chirality is often glossed over in theoretical or experimental discussions concerning the origin of life, but the ubiquity of homochiral building blocks in known biological systems demands explanation. Information theory can provide a quantitative framework for understanding the role of chirality in biology. Here I show how conclusions derived from information theory, in particular the concept of equivocation, can explain not only why chiral building blocks are necessary in living systems but also why a homochiral set of building blocks is necessary. These results lead to a new definition of life, and to the conclusion that the simplest form of life exists in the form of self-amplifying, autocatalytic reactions such as the Soai reaction. Chirality, 2009. © 2008 Wiley-Liss, Inc. [source] Emergence of homochirality in far-from-equilibrium systems: Mechanisms and role in prebiotic chemistryCHIRALITY, Issue 8 2007Raphaël Plasson Abstract Since the model proposed by Frank (Frank FC, Biochem Biophys Acta 1953;11:459,463), several alternative models have been developed to explain how an asymmetric non-racemic steady state can be reached by a chirally symmetric chemical reactive system. This paper explains how a stable non-racemic regime can be obtained as a symmetry breaking occurring in a far-from-equilibrium reactive system initiated with an initial imbalance. Departing from the variations around the original Frank's model that are commonly described in the literature, i.e. open-flow systems of direct autocatalytic reactions, we discuss recent developments emphasizing both an active recycling of components and an autocatalytic network of simple reactions. We will present our APED model as the most natural realization of such thermodynamic openness and non-equilibrium, of recycling and of network autocatalysis, each of these in prebiotic conditions. The different experimental and theoretical models in the literature will be classified according to mechanism. The place and role of such self-structured networks responsible for the presence of homochirality in the primitive Earth will be detailed. Chirality 19:589,600, 2007 © 2007 Wiley-Liss, Inc. 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