Number Of Reaction Steps Research Articles

Copper Catalyzed Borylation of Alkynes: An Experimental Mechanistic Study.

The copper catalyzed hydroboration of alkynes with B2pin2 was studied by in detail studies of individual relevant steps along the catalytic pathway. A number of reaction steps were retraced by in situ NMR spectroscopy as well as central intermediates and side-products were isolated and comprehensively characterized. A copper boryl complex is central to the catalytic process by inserting the terminal alkyne substrate into the B-Cu bond. The selectivity of this step - depending on the NHC auxiliary ligand - determines the α/β selectivity observed in the product. The latter complex is protonated by the auxiliary alcohol reagent resulting in hydroboration product formation and formation of a Cu alkoxido complex. Reaction of the latter with B2pin2 results in the regeneration of the central copper boryl complex. This alcoholysis step depends on the acidity of the alcohol, in particular on the relative acidity of the alcohol vs. the alkyne substrate. A number of side reactions leading to the hydrogenation product of the alkyne substrate and a bis hydroborated product were identified and studied in some detail. It is concluded that the performance of a particular catalytic system depends crucially on the relative acidities of the reagents and generalizations may be difficult.

Formaldehyde Oxidation of Ce0.8Zr0.2O2 Nanocatalysts for Room Temperature: Kinetics and Effect of pH Value.

Ce0.8Zr0.2O2 catalysts were prepared via the co-precipitation method under different pH conditions. The catalysts were characterized via TEM, XRD, XPS, BET, Raman, and FTIR. The oxidation performance of formaldehyde was tested. Precipitation pH affects the physicochemical properties and performance of the Ce0.8Zr0.2O2 catalyst. By controlling the precipitation pH at 10.5, the Ce0.8Zr0.2O2 catalyst with the largest specific surface area, the smallest grain size with the best formaldehyde removal rate (98.85%), abundant oxygen vacancies, and the best oxidation performance were obtained. Meanwhile, the kinetic parameters of the catalyst were experimentally investigated and the calculated activation energy was 12.6 kJ/mol and the number of reaction steps was 1.4 and 1.2.

Self-Organization of Enzyme-Catalyzed Reactions Studied by the Maximum Entropy Production Principle.

The self-organization of open reaction systems is closely related to specific mechanisms that allow the export of internally generated entropy from systems to their environment. According to the second law of thermodynamics, systems with effective entropy export to the environment are better internally organized. Therefore, they are in thermodynamic states with low entropy. In this context, we study how self-organization in enzymatic reactions depends on their kinetic reaction mechanisms. Enzymatic reactions in an open system are considered to operate in a non-equilibrium steady state, which is achieved by satisfying the principle of maximum entropy production (MEPP). The latter is a general theoretical framework for our theoretical analysis. Detailed theoretical studies and comparisons of the linear irreversible kinetic schemes of an enzyme reaction in two and three states are performed. In both cases, in the optimal and statistically most probable thermodynamic steady state, a diffusion-limited flux is predicted by MEPP. Several thermodynamic quantities and enzymatic kinetic parameters, such as the entropy production rate, the Shannon information entropy, reaction stability, sensitivity, and specificity constants, are predicted. Our results show that the optimal enzyme performance may strongly depend on the number of reaction steps when linear reaction mechanisms are considered. Simple reaction mechanisms with a smaller number of intermediate reaction steps could be better organized internally and could allow fast and stable catalysis. These could be features of the evolutionary mechanisms of highly specialized enzymes.

Mechanically Stimulated Solid-State Interaction of Platinum Tetrachloride with Sodium β-Diketonates.

A new mechanically stimulated solid-state reaction of PtCl4 with sodium β-diketonates has been discovered. Platinum (II) β-diketonates were obtained by grinding excess sodium trifluoroacetylacetonate Na(tfac) or hexafluoroacetylacetonate Na(hfac) in a vibration ball mill, followed by subsequent heating of the resulting mixture. The reactions occur under much milder conditions (at about 170 °C) compared to similar reactions of PtCl2 or K2PtCl6 (at about 240 °C). Excess diketonate salt plays the role of a reducing agent in the conversion of Pt (IV) salt to Pt (II) compounds. The effect of grinding on properties of the ground mixtures was studied by XRD, IR, and thermal analysis methods. The difference in the course of the interaction of PtCl4 with Na(hfac) or Na(tfac) indicates the dependence of the reaction on the ligand properties. The probable reaction mechanisms were discussed. This method of synthesis of platinum (II) β-diketonates makes it possible to substantially reduce the variety of reagents used, the number of reaction steps, the reaction time, the use of solvents, and waste generation compared to conventional solution-based methods.

Synthesis, In Vitro Biological Evaluation and In Silico Molecular Docking Studies of Indole Based Thiadiazole Derivatives as Dual Inhibitor of Acetylcholinesterase and Butyrylchloinesterase.

The current study was conducted to obtain hybrid analogues of indole-based thiadiazole derivatives (1-16) in which a number of reaction steps were involved. To examine their biological activity in the presence of the reference drug Donepezil (0.21 ± 0.12 and 0.30 ± 0.32 M, respectively), the inhibitory potentials of AChE and BuChE were determined for these compounds. Different substituted derivatives showing a varied range of inhibitory profiles, when compared to the reference drug, analogue 8 was shown to have potent activity, with IC50 values for AchE 0.15 ± 0.050 M and BuChE 0.20 ± 0.10, respectively, while other substituted compounds displayed good to moderate potentials. Varied spectroscopic techniques including 1H, 13CNMR and HREI-MS were used to identify the basic skeleton of these compounds. Furthermore, all analogues have a known structure-activity relationship (SAR), and molecular docking investigations have verified the binding interactions of molecule to the active site of enzymes.

The Limited Roles of Autocatalysis and Enantiomeric Cross-Inhibition in Achieving Homochirality in Dilute Systems

To understand the effects of fluctuations on achieving homochirality, we employ a Monte-Carlo method where autocatalysis and enantiomeric cross-inhibition, as well as racemization and deracemization reactions are included. The results of earlier work either without autocatalysis or without cross-inhibition are reproduced. Bifurcation diagrams and the dependencies of the number of reaction steps on parameters are studied. In systems with 30,000 molecules, for example, up to a billion reaction steps may be needed to achieve homochirality without autocatalysis.

QM/MM study of the taxadiene synthase mechanism.

Combined quantum mechanics/molecular mechanics (QM/MM) calculations were used to investigate the reaction mechanism of taxadiene synthase (TXS). TXS catalyzes the cyclization of geranylgeranyl diphosphate (GGPP) to taxadiene (T) and four minor cyclic products. All these products originate from the deprotonation of carbocation intermediates. The reaction profiles for the conversion of GGPP to T as well as to minor products were calculated for different configurations of relevant TXS carbocation complexes. The QM region was treated at the M06-2X/TZVP level, while the CHARMM27 force field was used to describe the MM region. The QM/MM calculations suggest a reaction pathway for the conversion of GGPP to T, which slightly differs from previous proposals regarding the number of reaction steps and the conformation of the carbocations. The QM/MM results also indicate that the formation of minor products via water-assisted deprotonation of the carbocations is highly exothermic, by about -7 to -23 kcal/mol. Curiously, however, the computed barriers and reaction energies indicate that the formation of some of the minor products is more facile than the formation of T. Thus, the present QM/MM calculations provide detailed insights into possible reaction pathways and into the origin of the promiscuity of TXS, but they do not reproduce the product distribution observed experimentally. © 2019 Wiley Periodicals, Inc.

Modeling of biomass pyrolysis kinetics using sequential multi-step reaction model

Simultaneous thermal analysis (TGA-DSC) of biomass pyrolysis was conducted to understand the kinetics and energy requirements of biomass pyrolysis. DSC results revealed the energy requirement to heat biomass samples to pyrolysis temperature with contributions from both sensible enthalpy changes along with reaction enthalpy change required to break their chemical bonds. The information obtained was used to determine theoretical energy efficiency of biomass pyrolysis, which was found to be in the range of 86–95%, depending on the sample analyzed, with pinewood providing efficiency of ∼95%. The TGA results were used for inverse modeling of pyrolysis kinetics of various biomass feedstocks that varied in their source of origin. Sequential multi-step pyrolysis model was explored to understand its capability in representing the global pyrolysis reaction of lignocellulosic wastes since they are carbon neutral and can produce clean and sustainable energy to potentially replace the existing fossil fuel resources with minimal change in existing energy consumption infrastructure. Inverse modeling, by fitting with respect to thermogravimetric analysis (TGA), was carried out using global minimization in an elite multi-objective genetic algorithm and with appropriate choice on the number of reaction steps. Biomass/bio-wastes investigated were cardboard, dry-paper waste, pinewood, rice husk and chicken manure. Cellulose was also investigated as a baseline standard sample. The char formation selectivity of the components involved in biomass was obtained from this model and found similar values for most of the biomasses. A set of ∼48%, 65% and ∼70% charring tendencies (based on vi values) was common among paper, pinewood, cardboard. The α-cellulose decomposition also provided the 70% charring reaction suggesting its role in these biomasses. The obtained multi-step kinetic parameters are considered essential to develop simplified reaction mechanism of biomass gasification and pyrolysis for computational fluid dynamics (CFD) models, which can help in the design of energy efficient and feedstock flexible reactors for sustainable energy production from wastes.

Biocatalytic reduction of ketones in a secondary school laboratory

AbstractBiocatalysis is one of the most important industrial methods which has been increasingly attracting attention of scientists as a new and environmentally acceptable method. It is used in the preparation of chiral alcohols - important building blocks for the synthesis of fine chemicals, pharmaceuticals, agrochemicals and analogues of natural substances. In biocatalysis the use of traditional chiral agents based on heavy metals has been replaced by the use of enzymes. In many cases, the number of reaction steps has been limited, the selectivity of the desired products has been increased while the negative impact on the environment has been reduced. The principles of biocatalysis have been applied by us to design a simple chemical experiment for the students of higher secondary education. The modified assignment of a lab task consisted of bioreduction of 4-nitroacetophenone using the enzymes present in plant tissues of carrot, parsley and white radish. Within our pedagogical research, the adequacy of the chemical experiment for secondary school students was examined and the extent of understanding of the green chemistry experiment was analysed by the method of a semi-structured interview.

Synthesis and grafting of carboxymethyl cellulose from environmental pollutant cellulosic wastes of textile industry

Purpose This study aims to explore the use of knitted rag by synthesizing different grades of carboxymethyl cellulose (CMC) by applying multiple-step carboxymethylation techniques. Design/methodology/approach CMC was synthesized from knitted rag, a cellulosic waste of textile and garment industries, in aqueous ethanolic sodium hydroxide and subsequently mono-chloroacetic acid reaction medium. Low-substituted to high-substituted products were obtained from single-step to seven-step carboxymethylation of cellulose. In this way, it was possible to produce low-cost and different grades of substituted carboxymethylated cellulose. The synthesized CMC was characterized, and their physical properties were investigated. The structure of CMC and grafted CMC were investigated by Fourier transform infrared spectroscopy. Findings Solubility, CMC content, degree of substitution and molecular weight of CMC were increased gradually with the increase in the number of reaction steps, although fourth step attained the optimum. The cellulosic waste of knitted rag can easily be used to produce value-added products such as CMC and other cellulose derivatives, and that will ultimately reduce the pollution problems from this waste. Originality/value Grafting of prepared CMC film with methyl methacrylate monomer increased their strength, although decreased rigidity and moisture content because the incorporation of hydrophobic methyl methacrylate monomer was observed.

A summative model for the pyrolysis reaction heats of beech wood

Kinetic models of beech wood decomposition, based on the three main pseudo-components, hemicellulose (one or two steps), cellulose (one step) and lignin (one or two steps), are applied for the first time to predict differential thermogravimetric (DTG) and differential scanning calorimeter (DSC) curves available from the literature. For the conditions that minimize the activity of secondary reactions, it is found that while the DTG curves are already well predicted by the extensively used three-step model, at least an additional step is required for the pseudo-lignin decomposition for a good prediction of the DSC data. The pseudo-cellulose decomposition is a globally endothermic process with the corresponding heat varying from about 601–528J/g (of volatile evolved), as the number of reaction steps is increased from three to five. Pseudo-hemicellulose decomposition also takes place endothermally with reaction heats of 245J/g (one step) or 321 and 226J/g (two steps). Instead pseudo-lignin decomposition occurs with remarkable exothermicity corresponding to −923J/g (one step) or −728 and −635J/g (two steps).

Useful Material Efficiency Green Metrics Problem Set Exercises for Lecture and Laboratory

A series of pedagogical problem set exercises are posed that illustrate the principles behind material efficiency green metrics and their application in developing a deeper understanding of reaction and synthesis plan analysis and strategies to optimize them. Rigorous, yet simple, mathematical proofs are given for some of the fundamental concepts, particularly how metrics for overall plan material performance are related to their composite counterparts for individual reactions. Throughout this exposition, whenever a synthesis scheme is examined, it is converted into a compact tree diagram that is used to depict plans of any degree of complexity (linear or convergent) as a means to conveniently keep track of all reagents, intermediates, reaction yields, stoichiometric coefficients, number of branches, number of reaction steps, and convergent steps. We demonstrate that such tree diagrams facilitate the computation of material efficiency metrics for any individual reaction in a plan as well as for the entire...

Synthesis of functionalized thiophene/phenyl co-oligomers by direct arylation of thiophenes

π-Conjugated rod-like molecules have proven to be powerful candidates for the generation of well-defined nanostructures with interesting optica l and optoelectrical properties. Their synthesis, however, turns out to be rather complex in some cases. Here, we have optimized the synthesis of oligomers consisting of thiophene and phenylene groups with three or four aromatic units using a direct arylation approach, which allo ws us to use cheap starting materials and to minimize the number of reaction steps considerably.

Revisiting the Thermochemical Database of Si-C-H System Related to SiC CVD Modeling

Chemical vapor deposition of silicon carbide (SiC-CVD) is a complex process involving a Si-C-H system wherein a large number of reaction steps occur. To simulate such a system requires knowledge of thermochemical and transport properties of all the species involved in the process. The accuracy of this information consequently becomes a crucial factor toward the correctness of the outcome prediction. The database on thermochemical properties of well-known species such as small hydrocarbons has been established over decades and it is accurate and easily accessible. On the other hand, the database for less frequently used species such as organosilicons is still under development. Apart from the accuracy issue, a consistency in acquiring procedures, whether theoretical or experimental, is another factor controlling the final error of the simulated outcome. In this work, the thermochemical data for several important growth species for SiC CVD using the SiH4/CxHy/H2 system has been calculated. For the most part an excellent agreement is seen with previously reported data, however for the organosilicons a larger deviation is detected and in particular for the CH3SiH2SiH species which shows a stark deviation from the CHEMKIN database.

Finding the Optimum Chloride-Based Chemistry for Chemical Vapor Deposition of SiC

Chemical vapor deposition of silicon carbide with a chloride-based chemistry can be done using several different silicon and carbon precursors. Here, we present a comparative study of SiCl4, SiHCl3, SiH4+HCl, C3H8, C2H4 and CH4 in an attempt to find the optimal precursor combination. We find that while the chlorinated silanes SiCl4 and especially SiHCl3 give higher growth rate than natural silane and HCl, SiH4+HCl gives better morphology at C/Si around 1 and SiCl4 gives the best morphology at low C/Si. Our study shows no effect on doping incorporation with precursor chemistry. We suggest that these results can be explained by the number of reaction steps in the gas phase chemical reaction mechanisms for producing SiCl2, which is the most important Si species, and by formation of organosilicons in the gas phase. As carbon precursor, C3H8 or C2H4 are more or less equal in performance with a slight advantage for C3H8, CH4 is however not a carbon precursor that should be used unless extraordinary growth conditions are needed.

A New Chemical Approach to Human ABO Histo-Blood Group Type 2 Antigens

A new chemical approach to synthesizing human ABO histo-blood type 2 antigenic determinants was developed. N-Phthaloyl-protected lactosaminyl thioglycoside derived from lactulose via the Heyns rearrangement was employed to obtain a type 2 core disaccharide. Use of this scheme lowered the overall number of reaction steps. Stereoselective construction of the α-galactosaminide/galactoside found in A- and B-antigens, respectively, was achieved by using a unique di-tert-butylsilylene-directed α-glycosylation method. The proposed synthetic scheme provides an alternative to existing procedures for preparing ABO blood group antigens.

Controlling chitosan’s molecular weight via multistage deacetylation

Chitosan samples with different molecular weights (Mw) and degree of deacetylation (DD) were prepared by controlling operating conditions throughout the multistage alkali treatment. The temperature of the reaction, time duration and number of reaction steps were considered effective parameters. A database was developed for chitosan preparation in order to achieve high degrees of deacetylation and control the molecular weight of chitosan without changing other molecular structures. The number of treatments and the duration of each step of deacetylation significantly affected molecular weight so that two samples were obtained with a DD of 99% and two different molecular weights ranging from 4.66×105 to 2.93×105 Based on these results, the highest molecular weight obtained using the multistage treatment without decreasing DD was 5.32×105, with a DD of 96.67%. Also, the morphological studies indicate that the molecular weight of chitosan has a significant effect on the pore size of the prepared scaffolds. However, this effect is critical. In other words, the pore size will increase by increasing molecular weight of chitosan from low upto medium molecular weight and when it reached to high molecular weight the pore size is decreased.

NHC-Assisted One-Pot Domino Oxidation of Aldehydes into Carboxylic Acids using Sodium Hydride under Oxygen Atmosphere

Domino reactions are powerful tools in synthetic chemistry for the formation of target molecules in a single flask without the isolation of intermediates, and hence the number of reaction steps is reduced and waste may be minimized. As part of a wider research program aimed at exploring new domino reactions, we became interested in developing a new strategy for the oxidation of aldehydes to the corresponding carboxylic acids. Surprisingly, despite impressive advances in the area of oxidation, domino reactions have not been applied to the oxidation of aldehydes to carboxylic acids. Very recently, we began to explore the utility of sodium hydride-molecular oxygen (NaH-O2) system and successfully applied to the oxidation of benzoins to benzils, and the oxidative cleavage of benzoins to benzoic acids. Based on these results, we envisioned that the combination of the selfbenzoin condensation of aldehyde and the oxidative cleavage of benzoin using NaH-O2 could provide a conceptually new domino oxidation reaction. Herein we report our preliminary results on NHC-assisted one-pot domino oxidation. To identify the feasibility of our approach, we first carried out domino benzoin condensation/NaH-O2 oxidation with pchlorobenzaldehyde 1a. Thus, self-benzoin condensation reaction of 1a in the presence of 50 mol % N-heterocyclic carbene (NHC) precatalyst 3c with K2CO3 as a base in CH2Cl2 for 24 h yielded the intermediate I. The successive addition of 1 equivalent of NaH in a mixed solvent of THF and CH2Cl2 under oxygen atmosphere, provided the desired p-chlorobenzoic acid 2a in a 77% yield after 10 min (Scheme 1). We next attempted to develop one-pot domino reactions by employing NHC precatalyst/NaH-O2 together. Our studies began with an initial examination of the catalytic activities of several NHCs 3a-e for the domino oxidation of 1a. The results are summarized in Table 1. The NHC precatalysts 3a, 3b derived from imidazolium salt produced the desired product 2a in 58% and 78% yields after 30 min and 2 h, respectively (Table 1, entries 1-2). Among the imidazolium salts 3a-c, 3c is capable of catalyzing the reaction in undegassed dry THF to give 2a in a 72% yield after only 10 min under NaH-O2 system (Table 1, entry 3). Noteworthy is that the current one-pot, domino oxidation methodology is superior to the sequencing reaction in terms of reaction rate. Even if the turnover frequency in the benzoin formation step is much lower than that in the oxidative cleavage step, presumably, 1a was quickly consumed in relatively large quantities by NaH/O2, thereby producing the desired product 2a within 10 min. In contrast, the reaction was sluggish and gave lower yield (32%) without oxygen atmosphere (Table 1, entry 4), which was attributed to the remaining selfbenzoin product I. We also investigated the effects of solvent and NaH loading. Replacing the solvent THF with EtOAc or toluene led to lower chemical yields (Table 1, entries 7-8). With the use of precatalyst 3c (10 mol %) and NaH (0.6 equiv) under oxygen atmosphere, the desired product was formed in poor yield (32%) due to the major portion of unreacted aldehyde 1a, the formation of the 4,4'-chloro-

Theoretical Validation of Chemical Kinetic Mechanisms: Combustion of Methanol

A new technique is proposed that uses theoretical methods to systematically improve the performance of chemical kinetic mechanisms. Using a screening method, the chemical reaction steps that most strongly influence a given kinetic observable are identified. The associated rate coefficients are then improved by high-level quantum chemistry and transition-state-theory calculations, which leads to new values for the coefficients and smaller uncertainty ranges. This updating process is continued as new reactions emerge as the most important steps in the target observable. The screening process employed is a global sensitivity analysis that involves Monte Carlo sampling of the full N-dimensional uncertainty space of rate coefficients, where N is the number of reaction steps. The method is applied to the methanol combustion mechanism of Li et al. (Int. J. Chem. Kinet. 2007, 39, 109.). It was found that the CH(3)OH + HO(2) and CH(3)OH + O(2) reactions were the most important steps in setting the ignition delay time, and the rate coefficients for these reactions were updated. The ignition time is significantly changed for a broad range of high-concentration methanol/oxygen mixtures in the updated mechanism.

A Comparative Study of Eight Finite-Rate Chemistry Kinetics for Co/H2 Combustion

We compare the performance and computational cost of 8 kinetic models (3 global and 5 elementary) that describe the finite-rate chemistry of syngas combustion. We apply them in simulating a turbulent jet flame with syngas diluted by 30% nitrogen. We model the turbulence by a modified k-epsilon model and the turbulence-chemistry interaction by the partially stirred reactor approach. To integrate the chemistry equations, we nominally use explicit fifth-order embedded Runge-Kutta ODE solver. But semi-implicit Bulirsch-Stoer and implicit Euler were also used. The computational time depends on the number of reaction steps and the ODE solver. Five models overpredict the maximum flame temperature (by 200 K-320 K). Two models underpredict it by 240 K and 580 K. The global model that is based on the Westbrook-Dryer (1981) model for hydrocarbon fuels gives the best agreement with measurements, and also has low computational demand. Therefore, it is recommended for modeling turbulent syngas flames.