Kinetics of adiabatic reactions. Kinetic study of reactions resulting in cross-linked products

Kinetic equations expressing adiabatic reactions have been derived. The rate of increase of the temperature under adiabatic conditions has been described. The values of integrals for different orders of reactions have been calculated, and the characteristics of the functions obtained have been discussed. A calorimeter has been constructed for heat measurements, ensuring the adiabatic conditions by a special temperature controller. The formation of polyurethanes was subjected to kinetic investigation. The rate constants of polyaddition reactions for different systems have been calculated. It was established that measurement of reaction heat developed under adiabatic conditions is a useful method for both theoretical and practical purposes. It can be used for the determination of the reaction order and the rate constant even for reactions leading to crosslinkage. It is also well applicable for the control of the reactivity of industrial resins.     

Stereoselective Promiscuous Reactions Catalyzed by Lipases

The ability of lipases to display activity beyond their physiological reactions, so-called “catalytic promiscuity”, has gained increasing interest in the last two decades as an important tool for expanding the application of these enzymes in organic synthesis. Some lipases have been shown to be effective in catalyzing a variety of C-C bond formation reactions and most of the investigations have been directed to the optimization of the products yield through a careful tuning of the experimental parameters. Despite the fact that new stereogenic carbons are formed in many of the tested reactions, the target products have been often obtained in racemic form and examples of an efficient asymmetric induction by the used lipases are quite limited. The aim of this review, mainly focused on those lipase-catalyzed promiscuous reactions in which optically active products have been obtained, is to offer a current state of art together with a perspective in this field of asymmetric synthesis.     

Reactions of the oxirane group

Epoxidized oils and esters have been used commercially for about two decades; their consumption is now of the order of 100 million lb./year, mainly as stabilizer-plasticizers for vinyl chloride polymers. Yet in spite of their low price and ready availability, chemical derivatives of these substances have not achieved similar commercialization. It has long been known that the oxirane group is highly reactive and undergoes a wide variety of ring-opening reactions with a broad range of electrophiles and nucleophiles. During the past ten years, in particular, new and interesting reactions of the oxirane group have been described that provide new routes to other heterocyclic ring systems and functional groups. After a brief background survey, recently published and unpublished reactions from the author’s laboratory, as well as reactions published by other groups, are described. Presented at the AOCS Meeting, New Orleans, April 1970.     

The reactions of formation of selected gas products during coal pyrolysis

Pyrolysis examinations conducted under non-isothermal conditions as well at low heating rate can show that the processes of hydrogen and methane formation are the result of several constituent reactions. In the presented paper a number of these reactions has been determined separately for each of the above mentioned gaseous products. The kinetic parameters of the reactions as well as the yields of products have also been calculated. It has been found that, during coal pyrolysis, methane is formed as a result of six constituent reactions and hydrogen is produced as a result of five constituent reactions. The values of activation energy and frequency factor for the reactions in question were determined. These values fall within the range, which is typical of chemical reactions.     

COMPLEX REACTIONS OF ETHYL-GLUCOSIDES SYNTHESIS OVER ION EXCHANGE RESIN

The glycosidation reactions of D-glucose with ethanol have been carried out over a reusable and separable heterogeneous catalyst, namely, ion exchange resin. Detailed kinetic data for these reactions are reported. A complex reaction model has been developed for interpreting the data. The reactions were found to be global second-order reactions and first -order with respect to each component. Meanwhile, a new regression method is applied to determine the rate constant from time-dependent profiles.     

Modeling the thermal decomposition and residual mass of a carbon fiber epoxy matrix composite with a phenomenological approach: Effect of the reaction scheme

The thermal decomposition of polymer matrix composites is a complex process involving hundreds of reactions and species, which are often modeled with simplified one-step schemes. These schemes can be improved by adding intermediate reactions of different nature (competitive, parallel, and consecutive). However, the optimal number and nature of intermediate reactions are rarely discussed. In this paper, several reaction schemes of increasing complexity have been developed to model the decomposition of a carbon/epoxy composite. The kinetic parameters describing each reaction have been extracted from thermogravimetric analysis (TGA) by means of isoconversional methods. The composite mass loss rate and residual mass have been modeled and compared to TGA and tube furnace data. This research shows that adding parallel or consecutive intermediate reactions improves the agreement against TGA data compared to a single-step model, but only competitive reactions can account for the variation of the residual mass observed in the tube furnace when the heating rate is varied.     

Heterogeneous functionalizing of polysulfone membranes

In recent years efforts have been increased to work with more intelligent membranes which render more than normal filtration properties. Thus, membranes have been prepared using chemically modified polymers or have been furnished with additives in order to increase their selectivity and/or special sorbing capabilities. In this paper, methods are described how commercially available polysulfone membranes can be functionalized by chemical reactions without disintegrating their mechanical properties. For this purpose, Sulfonation, chloromethylation, aminomethylation, and lithiation reactions were applied. A broad spectrum for further conversions modified membranes is opened by these chemical reactions which allow an individual fitting of the membranes with special substitutes. The chemical reactions and the analytical procedures are also described.     

The formation and destruction of NO in turbulent propane diffusion flames

This paper describes a study of the formation and destruction of NO in turbulent propane diffusion flames with recourse to both experiments and modelling. Detailed in-flame measurements of local mean gas species concentrations of O₂, CO, CO₂, unburnt hydrocarbons and NO_x and local mean gas temperature have been performed for three flames — two of them with the same Froude number and two with the same Reynolds number. These experimental data have been analysed with the aid of a mathematical model. For the NO calculations, three reaction schemes have been used: the Zeldovich reactions, an overall approximate prompt reaction, and a 27 reaction scheme, which includes the thermal NO and the prompt NO reactions and the NO to HCN recycle via fuel NO reactions. The main conclusions are that in the present flames: (1) the prompt NO (or Fenimore) mechanism is the dominant route for the NO formation; and (2) the reactions between NO and hydrocarbon radicals, recycling NO to HCN via the fuel NO reactions, play an important role in the global NO reduction.     

Depolymerization of nylon 6: Some kinetic modeling aspects

Depolymerization reactions of nylon 6 [poly(?-caprolactam)] have been modeled based on an existing mechanism for reversible polymerization reactions. The method of moments proposed by Min has been used together with kinetic and equilibrium constants for polymerization reactions to simulate depolymerization reactions. Simulation results thus obtained for batch and semibatch processes compare well with the corresponding experimental results.     

Unerwünschte Reaktionen organischer Stoffe als Gefahren in Chemieanlagen

Undesired reactions of organic substances as sources of danger in chemical plant . In chemical production processes, danger can arise both from the intended exothermal reaction and from unintended exothermal reactions. Such “undesired” reactions may be exothermal reactions of the substances themselves (e.g. decomposition, polymerization) or reactions of the substances involved with one another (e. g. of a reactant with the solvent). Differential thermal analysis and warm storage have been successfully used in various modifications as experimental methods for the study of such reactions. For exothermal decomposition reactions, correlations can be made between the chemical constitution of the substances and the energy released on decomposition, as well as the temperature of incipient decomposition. In mixtures with other substances a modified decomposition behaviour must be expected, usually of such a kind that the temperature range of decomposition is lowered. There are many possible exothermal reactions of substances with one another; only a few have hitherto been studied in detail.     

Modeling and simulation of photofabrication processes using unsaturated polyester resins

Several kinetic models have been proposed to simulate thermosetting cure reactions. The most complex models, based on a mechanistic approach of cure reactions, are developed based on the concepts of free radical polymerization and the mechanism of reactions with diffusion. However, mechanistic models are usually quite impractical for engineering purposes because of the difficulty in obtaining the model parameters. An alternative to these mechanistic models are the phenomenological models, formulated in terms of the degree of cure and much easier to apply. Phenomenological models have been largely used to study thermal‐initiated cure reactions, although only few works used them to model the kinetics of ultraviolet‐initiated cure reaction. This work proposes a photo‐thermal‐kinetic model to study the behavior of unsaturated polyester resins during ultraviolet‐initiated cure reactions. The model considers samples with different amounts of initiator concentration and cure reactions performed under different ultraviolet light intensities. The model has been numerically solved using the finite element technique. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Theoretical perspectives on proton-coupled electron transfer reactions

This Account presents a theoretical formulation for proton-coupled electron transfer reactions. The active electrons and transferring protons are treated quantum mechanically, and the free energy surfaces are obtained as functions of collective solvent coordinates corresponding to the proton and electron transfer reactions. Rate expressions have been derived in the relevant limits, and methodology for including the dynamical effects of the solvent and protein has been developed. This theoretical framework allows predictions of rates, mechanisms, and kinetic isotope effects for proton-coupled electron transfer reactions.     

Chemical modification of Kevlar fiber surfaces and of model diamides

Organic reactions of aromatic diamides as models for PPTA fibers have been investigated to select reagents and conditions suitable for surface-controlled heterogeneous reactions on Kevlar filaments and fabrics. Amine functional groups have been incorporated into fiber surfaces by bromination followed by ammonolysis and by nitration followed by reduction. Modification of filaments has been attained without impairment of fiber properties. Preliminary experiments have also shown that the presence of amino groups on Kevlar fabric can provide remarkably improved peel strength and apparent interlaminar shear strength in epoxy laminates, suggesting a significant role of covalent bonding in improving adhesion in aramidepoxy composites.     

A calorimetric study of the interchange reactions in bisphenol a polycarbonate/nylon-6 blends

The interchange reactions in a partially miscible blend composed of bisphenol A polycarbonate and nylon-6 have been studied by the use of a calorimetric technique. It has been found that these reactions affect the thermal transitions of the mixtures. These effects have been interpreted to be a consequence of the progressive formation of copolymers which homogenize the mixture.     

Theory of asymmetric organocatalysis of Aldol and related reactions: rationalizations and predictions

Computational studies have led to models to understand some classic and contemporary asymmetric reactions involving organocatalysts. The Hajos-Parrish-Eder-Sauer-Wiechert reaction and intermolecular aldol reactions as well as Mannich reactions and oxyaminations catalyzed by proline and other amino acids, and Diels-Alder reactions catalyzed by MacMillan's chiral amine organocatalysts have been studied with density functional theory. Quantitative predictions for several new catalysts and reactions are provided.     

Rapid generation of molecular complexity in the Lewis or Brønsted acid-mediated reactions of methylenecyclopropanes

Although they are highly strained, methylenecyclopropanes (MCPs) are readily accessible molecules that have served as useful building blocks in organic synthesis. MCPs can undergo a variety of ring-opening reactions because the release of cyclopropyl ring strain (40 kcal/mol) can provide a thermodynamic driving force for reactions and the π-character of the bonds within the cyclopropane can afford the kinetic opportunity to initiate the ring-opening. Since the 1970s, the chemistry of MCPs has been widely explored in the presence of transition metal catalysts, but less attention had been paid to the Lewis or Br?nsted acid mediated chemistry of MCPs. During the past decade, significant developments have also been made in the Lewis or Br?nsted acid mediated reactions of MCPs. This Account describes chemistry developed in our laboratory and by other researchers. Lewis and Br?nsted acids can be used as catalysts or reagents in the reactions of MCPs with a variety of substrates, and substituents on the terminal methylene or on the cyclopropyl ring of MCPs significantly affect the reaction pathways. During the past decade, we and other researchers have found interesting transformations based on this chemistry. These new reactions include the ring expansion of MCPs, cycloaddition reactions of MCPs with aldehydes and imines, cycloaddition reactions of MCPs with nitriles in the presence of strong Br?nsted acid, radical reactions of MCPs with 1,3-dicarbonyl compounds, intramolecular Friedel-Crafts reactions of MCPs with arenes, acylation reactions of MCPs, and the reaction of MCPs with 1,1,3-triarylprop-2-yn-1-ols or their methyl ethers. These Lewis or Br?nsted acid mediated reactions of MCPs can produce a variety of new compounds such as cyclobutanones, indenes, tetrahydrofurans, and tetrahydroquinolines. Finally, we have also carried out computational studies to explain the mechanism of the Br?nsted acid mediated reactions of MCPs with acetonitrile.     

Versatile 2-amino-4-substituted-1,3-thiazoles: synthesis and reactions

Syntheses and reactions of 2-amino-4-substituted-1,3-thiazoles are reviewed in a formal way. The title compounds are most easily accessible by various approaches, and even waste-free solid-state procedures have been developed. The substitution in 4-position has synthetic reasons and therefore most interest accumulates around these derivatives of 2-aminothiazole. The high reactivity of both the amino group and the positions 3 and 5 of the 1,3-thiazole ring are used for numerous syntheses in a comprehensive way. The reactions are subdivided in groups that cover reactions at the amino substituent without touching the thiazole ring, reactions which involve both nitrogens in the formal amidine system to give thiazolo-pyrimidinones and -imidazoles as well as more involved polycondensed N,S-heterocycles with multiple possibilities for substituents, and substitution reactions at the 5-position of the thiazole ring. Most of the imaginable reaction types have been successfully applied and used, as many of the synthesized compounds exhibit interesting biological activity in various fields.     

Kinetic studies on the catalytic reduction of o-, m-, and p-nitrophenols by hydrazine

The kinetics of the reduction of o, m- and p-nitrophenols by hydrazine over Raney nickel catalysts have been studied spectrophotometrically. The reactions have been found to be first order with respect to both hydrazine and nitrocompound. The first order behaviour has been observed at the initial part of each of the reactions. The influences of solvent, salt and the amount of catalyst on the reaction rate have been investigated. The rate constants have been found to depend on the nature of the substrate, the dielectric constant, the ionic strength and the amount of catalyst. The activation energies for the reactions in various solvent compositions have been determined and their variations interpreted in favour of an ionic mechanism. The effect of the p-substituent on the activation energy has been explained in terms of the electron-shifting capacity of the substituent.