Occurrence Of Side Reactions Research Articles

Novel xylylene diaminocellulose derivatives for enzyme immobilization

In the series of soluble and film-forming aminocelluloses for application as enzyme-compatible support matrices, novel derivatives with xylylene diamine residues selectively at C6 and solubilizing carbanilate groups were synthesized. Synthesis of these aminocelluloses was successful in three reaction steps starting from cellulose (Avicel PH 101) via tosylcellulose with degree of substitution of 0.88 (preferably at C6) and tosylcellulose carbanilate. Using a 10-fold excess of xylylene diamine, only one amino group of the diamine reacts during the nucleophilic substitution of the tosylate groups. In the last step, the influence of the structure of the starting xylylene diamine isomers and of the reaction conditions on the degree of substitution of the tosylate, carbanilate and xylylene diamino groups in the final polymer, on the occurrence of side reactions and on the polymer solubility was studied. All xylylene diamino tosylcellulose carbanilates are soluble in DMA, DMSO and DMF in never dried state and form transparent films on glass surfaces. These films are promising support matrices for enzyme immobilization because of their characteristic topography, as is shown by atomic force microscopy examinations of the film surfaces.

Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry and nuclear magnetic resonance analyses of end‐functionalized saccharidic polymers: an example of a useful analytical technique combination

The living cationic polymerization of a saccharidic monomer (1,2:3,4-di-O-isopropylidene-6-O-(2-vinyloxyethyl)-D-galactopyranose, GVE) gives rise to aldehyde end-capped polymers (PGVE--CHO) of low molar masses (<10 000 Da) and low molar mass distribution (<1.2). These polymers were derivatized by selective introduction of either hexamethylenediamine (PGVE-NH2) or 9-fluorenylmethyl carbazate (PGVE-Fmoc) end groups. The resulting polymers were fully characterized by complementary use of nuclear magnetic resonance (1H NMR) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The expected structure according to the polymerization mechanism, the occurrence of side reactions and the success of the post-functionalization reactions were confirmed.

Degradation of mixtures of phenols using boron doped diamond electrodes for wastewater treatment

A model is proposed for describing the electrochemical oxidation of multicomponent mixtures of organic compounds, when significant mineralization (i.e., combustion to CO 2) of the compounds occurs. This is known to be the case of the degradation of many organic compounds on boron doped diamond (BDD) and other new electrocatalytic materials used as anodes. The proposed model predicts the variation of concentrations of each component present in the mixture, chemical oxygen demand (COD) and current efficiency as a function of time, under different rate control regimes (diffusive, kinetic and mixed) at constant current. Mass transfer coefficients of each component of the mixture, needed for the application of the model, were obtained from single solute experiments. Tests were conducted in a conventional three-electrode cell, using a commercial BDD electrode as anode. Good agreement between predicted and experimental results was generally obtained, though some deviations had been observed when the highest concentrations of the solutes were used, with concentrations decreasing faster than predicted. These deviations were found for both single- and multi-component systems, and can be explained by the occurrence of side reactions or the accumulation of intermediate compounds of the combustion mechanism.

Nitration of Diphenylmethane and the Isomeric Nitrodiphenylmethanes in Dichloromethane

Diphenylmethane could be mononitrated to 2- and 4-nitrodiphenylmethane by nitric acid in dichloromethane. Adjustment of experimental conditions gave a mixture of 2,2‘-, 2,4‘-, and 4,4‘-dinitrodiphenylmethane. The occurrence of side reactions was negligible, and the conversions were quantitative in the latter reaction. In both processes conventional separation of the pure isomers appeared realistic from a practical point of view. The reactions ran smoothly and did not necessitate stringent controls against dangerous runaways. This nitration procedure was applied also to the three isomeric mononitrodiphenylmethanes which underwent quantitative mononitration in the unsubstituted ring.

Adsorptive reactors for enhancing equilibrium gas-phase reactions—two case studies

The conversion enhancement potential of fixed-bed adsorptive reactors has been evaluated for two heterogeneously catalysed gas-phase equilibrium reactions: the Claus reaction used in sulphur recovery and the direct synthesis of hydrogen cyanide from carbon monoxide and ammonia. It was found that kinetics of both reaction and adsorption as well as adsorbent capacity have to be very compatible to achieve high conversions. The specific parameters of the reaction/catalyst system, such as the deposition of solids (e.g. sulphur, coke) or the formation of undesirable by-products have to be taken into account for the successful application of adsorptive reactor concepts. A crucial point is the selection of the reaction conditions (i.e. temperature), since the occurrence of side reactions may be enhanced in adsorptive reactor operation due to the inherently distorted concentration profiles.

Synthesis of a polycyclic silazane network and its evolution to silicon carbonitride glass

Polysilazanes have been described as excellent polymeric precursors to amorphous silicon carbonitride (SiCN) glasses. In this work, the synthesis, thermal cross-linking and pyrolysis of a polycyclic silazane network precursor for a SiCN glass, were studied. This polymeric precursor was prepared by free radical polymerization of 1,3,5-trimethyl-1 ′,3 ′,5 ′-trivinylcyclotrisilazane, using dicumyl peroxide as radical initiator. The polymer-to-ceramic conversion was investigated by 29 Si and 13 C magic angle spinning nuclear magnetic resonance (MAS NMR) and infrared spectroscopies, X-ray diffraction and by simultaneous thermogravimetric and differential thermal analyses. Based on the results the polymeric precursor can be characterized as a polycyclic silazane network, containing aliphatic carbon segments produced by the vinyl polymerization. 29 Si and 13 C NMR data of this polymeric precursor heated at different temperatures indicated the occurrence of side-reactions in addition to the vinyl polymerization. The SiCN glass was obtained as an amorphous solid up to 1400 ° C . Above this temperature, the crystallization process of the ceramic product was promoted by rearrangement of the different Si sites in the amorphous network. Heat treatment at 1600 ° C , under N 2 atmosphere, produced a ceramic containing 85% silicon carbide (SiC) and 15% silicon nitride, determined by 29 Si MAS NMR in relation to the total silicon sites, and under argon atmosphere, SiC.

Adsorptive Reactors for Enhancing Equilibrium Gas-phase Reactions — Two Case Studies

The conversion enhancement potential of fixed-bed adsorptive reactors has been evaluated for two heterogeneously catalysed gas-phase equilibrium reactions: the Claus reaction used in sulphur recovery and the direct synthesis of hydrogen cyanide from carbon monoxide and ammonia. It was found that kinetics of both reaction and adsorption as well as adsorbent capacity have to be very compatible to achieve high conversions. The specific parameters of the reaction/catalyst system, such as the deposition of solids (e.g. sulphur, coke) or the formation of undesirable by-products have to be taken into account for the successful application of adsorptive reactor concepts. A crucial point is the selection of the reaction conditions (i.e. temperature), since the occurrence of side reactions may be enhanced in adsorptive reactor operation due to the inherently distorted concentration profiles.

A primary, secondary and pseudo-tertiary mathematical model of a chlor-alkali membrane cell †

A theoretical study of current density and potential at the anode, membrane and cathode, of a chlor-alkali membrane cell where the electrode blades are placed vertically, is presented. A representative unit cell is modelled in primary, secondary and pseudo-tertiary current distribution models. It is shown that electrolyte and membrane resistance has the greatest effect on current distribution. Furthermore, it is shown that there is a surprisingly small influence of mass transport on current distribution, on the assumption that the diffusion layer is of constant thickness. In converse to this, it is shown that mass transport affects the anode overpotential distribution to the extent that conclusions can be made about the occurrence of side-reactions and where they occur. Finally, it is shown that it is possible to estimate tertiary behaviour with a secondary current distribution model, by using an analytic expression at the anode surface.

Electroreduction of Co2 + and Ni2 + at III-V Semiconductors and Properties of the Semiconductor/Metal Interfaces Formed

The electrochemical deposition of Ni and Co on GaAs and GaP at pH 5 was investigated by means of cyclic voltammetry and rotating disk voltammetry. The current‐potential behavior of the two semiconductors in dilute metal ion solutions appeared to be different: at n‐GaP, a stepwise reduction of the metal ions was observed, whereas at n‐GaAs this was not the case. This observation can be explained on the basis of two alternative reaction mechanisms for the reduction of the monovalent intermediate. Under well‐chosen conditions of deposition potential and metal ion concentration, it appeared to be possible to form adherent n‐GaAs/Co, n‐GaAs/Ni, and n‐GaP/Co Schottky contacts with good rectifying properties. The barrier height of the n‐GaAs/Ni contacts appeared to increase as a function of time, leading to values of which were dependent on the deposition potential . At n‐GaAs/Co and n‐GaP/Co junctions, this aging effect was absent, leading to values of the barrier height which were independent of . It was further observed that the occurrence of side reactions during metal deposition may exert a great influence upon the properties of the metal/semiconductor junctions formed. © 1999 The Electrochemical Society. All rights reserved.

The use of crown ethers in peptide chemistry-V. Solid-phase synthesis of peptides by the fragment condensation approach using crown ethers as non-covalent protecting groups.

We have previously described the conditions by which peptide synthesis by the solid-phase fragment condensation approach can be carried out using crown ethers as non-covalent protection for the N alpha-amino group. Here we demonstrate that the procedure can be extended to large, partially protected peptide fragments possessing free Lys and/or Arg residues. The first step was to ensure that complex formation on the side chain of amino acids was not detrimental to the methodology and exhibited the same solubility and coupling properties as N alpha-complexed peptides. Thus, a model hexapeptide was synthesized using Fmoc chemistry containing Lys and Arg residues, which, when complexed with 18-Crown-6, was readily soluble in DCM and coupled quantitatively to a resin-bound tetrapeptide. Two tripeptides were then prepared, one containing a free Ser residue, the other free Tyr, to examine the possible occurrence of side reactions. After coupling using standard conditions only the former tripeptide exhibited the formation of the O-acylation by-product (5%). Another model hexapeptide containing Lys, Tyr, Ser and Asp protected with a TFA-stable adamantyl group was complexed with 18-Crown-6 and coupled to the resin-bound tetrapeptide with near quantitative yield. Extending the length of the peptide to 21 and 40 residues, which represent sequences Gly52 to Leu72 (21-mer) and Pro33 to Leu72 (40-mer) from Rattus norvegicus chaperonin 10 protein, respectively, resulted in partially protected fragments that were readily soluble in water, thus enabling purification by RP-HPLC. Complexation with 18-Crown-6 gave two highly soluble products that coupled to resin-board tetramer with 68% and 50% coupling efficiencies for the 21-mer and 40-mer, respectively. Treatment with 1% DIEA solutions followed by acidolytic cleavage and purification of the major product confirmed that the correct product has been formed, when analysed by amino acid analysis and ESI-MS. These results served to extend the methodology of non-covalent protection of large partially protected peptide fragments for the stepwise fragment condensation of polypeptides.

Acid-catalysed transformation of oleic acid, and 12-hydroxystearic acid, into γ-octadecanolactone. Evidence for the occurrence of a novel side reaction

γ-Octadecanolactone is produced in the reaction between oleic acid (cis-9-octadecenoic acid) and concentrated sulfuric acid at 80 °C. To account for its formation, two types of mechanism have been considered. Both begin with protonation at C-9/C-10 to give a carbocation, following which the positive charge is transmitted to C-4, where nucleophilic attack by the carboxyl group is facilitated. One of them invokes a "zipperlike" succession of 1,2-shifts of intervening methylenic protons, whereas the other amounts to a stepwise migration of the alkene function along the carbon chain. Results obtained through the use of deuterated sulfuric acid to catalyse formation of the lactone allow for the existence of both types. However, each is characteristic of a different region of the C18 structure. Of the total isotope incorporated (individual molecules contained from zero to nine atoms of deuterium), &gt;85% had been distributed relatively uniformly over the chain segment from C-7 to C-17, according to 2H nuclear magnetic resonance spectra recorded in the presence of the shift reagent Yb(fod)3, and 13C nuclear magnetic resonance data. This evidence is consistent with the occurrence of a side reaction that creates many protonation sites over that specific region of the lactone's precursors. By contrast, as very little of the isotope had reached the proximity of the lactone ring, the latter is more likely to have been formed via the 1,2 shift type of mechanism. These events can also be initiated at other sites, as shown by the preparation of γ-octadecanolactone, and a comparably deuterated form of it, from 12-hydroxystearic acid. Reactions of these three stearic acid derivatives with HI, HBr, and HCl are described, by way of comparison.

Products and intermediates formed during the biacetyl‐sensitised photo‐oxidation of some azo dyes

The flash irradiation for a few microseconds of aqueous aerobic solutions of biacetyl(2,3‐butanedione) and a number of different o‐arylazonaphthols resulted in the formation of the corresponding diazonium ions and α‐naphthoquinones. This was verified from the intermediate spectra obtained after the flash. Verification for the formation of diazonium ions was also obtained from flow experiments, in which the diazonium ion was identified by coupling it to H‐acid, forming a new azo compound. Less than equimolar formation of the diazonium ion from one of the azo dyes investigated indicated the occurrence of side reactions in which diazenyl radicals could play a role.

Developments in the production of methyl tert-butyl ether

The catalysis of the etherification of alcohol and iso-olefin mixtures is discussed. Emphasis is placed on the synthesis of MTBE (methyl tert-butyl ether), but mention is also made of other ethers such as ETBE (ethyl tert-butyl ether), SAME ( sec-amyl methyl ether) and MIPE (methyl isopropyl ether) and, to a very limited extent, of hydrocarbon feedstocks other than iso-olefins. Established sulphonated resin catalysts are described together with various aspects of their catalytic characteristics, including kinetics, thermodynamics and the occurrence of side-reactions. The use of alternative inorganic adds, especially zeolites, is covered and comparisons drawn between these systems and the ion-exchange resins. Finally, attention is given to the design of commercial processes for the production of MTBE.

Kinetics of phase-transfer etherification reactions of p-chloronitrobenzene by alkoxides

The substitution reaction of chloride by alkoxide anions in p-chloronitrobenzene in a Phase-transfer catalytic system has been studied and successfully performed in high yield and without the occurrence of side reactions. The effect of several parameters has been experimentally, investigated. In particular, the poisoning effect of the chloride anion produced throughout the reaction is studied and a practical answer to successfully overcome this problem and increase yields is presented. A kinetic model based on the collected data is proposed.

Antimicrobial therapy in dialysis patients. I. Penicillins and cephalosporins.

Antibiotic therapy, using penicillins or cephalosporins, is frequently required in patients on maintenance hemodialysis. Points to consider are dose adjustment for drugs which are excreted via the kidney, drug dialysability, and cumulation with frequent occurrence of side reactions, neurotoxicity and bleeding being the clinically most important ones. For third-generation cephalosporins with N-methylthiotetrazole side chain, impaired intrahepatic vitamin K metabolism may cause problems of hemostasis which can be avoided by dose adjustment and prophylactic administration of vitamin K1.

Diazabicycloalkanes with nitrogen atoms in the nodal positions. 4. Intramolecular cyclization of N-(?-x-ethyl)-1,2,3,4-tetrahydroquinoxalines and behavior of benzo[b]-1,4-diazabicyclo[2.2.2]octenes in acidic media

The intramolecular cyclization of N-(β-hydroxyethyl)-N'-acetyl-1,2,3,4-tetrahydroquinoxaline in refluxing HBr was investigated by liquid microcolumn chromatography. Under these conditions the amide group undergoes rapid hydrolysis, the hydroxy groups undergo relatively slow exchange by bromine, and the resulting N-(β-bromoethyl)-1,2,3,4-tetrahydroquinoxaline undergoes cyclization to give benzo[b]-1,4-diazabicyclo[2.2.2]octene. These transformations terminate with the establishment of equilibrium between VII and I. 7-Methyl-N-(β-chloroethyl)-1,2,3,4-tetrahydroquinoxaline similarly forms an equilibrium reaction mixture in HBr. The effect of various factors (the acid and bromide ion concentrations, the character of the acid, and the temperature) on the position of the equilibrium of the compounds obtained and on the occurrence of side reactions (hydrolysis and dealky lation) was studied.

The selectivity of ethylene oligomerization into higher α-olefins using the system TiCl 4C 2H 5AlCl 2

The effect of the Al/Ti ratio, and of the catalyst and monomer concentrations and of impurities (water, alcohols, AlCl 3, CO 2) that are active with respect to catalyst components has been investigated in relation to the polymer chain structure, the nature of functional groups, the number average molecular weight and the polydispersity coefficient of ethylene oligomerization products, using the system TiCl 4C 2H 5AlCl 2. It is shown that the synthesis of higher linear α-olefins by ethylene oligomerization on the system TiCl 4C 2H 5AlCl 2 in benzene or toluene media with a selectivity exceeding 95 wt. % for ethylene is possible in the temperature range 20–40°C, at pressures 25–30 atm, TiCl 4 concentration 0·5-1·5 g/l., Al/Ti molar ratio = 2–16. It is proposed that such a narrow range of variation in parameters of the process in the ethylene oligomerization into higher α-olefins on the system in question stems from the occurrence of side reactions at active centres of oligomerization, and is due to a difference in the types of active centres formed.

Synthese de polyphosphonates et polyphosphonamides a partir de monomeres diphosphoniques polycondensation en solution

Synthesis of linear polyphosphonates and polyphosphonamides is achieved by polycondensation in solution of aromatic diols and diamines with alkyldiphosphonyl tetrachlorides, in the presence of a hydrochloric acid trapping agent (triethylamine). The main limiting factors for chain growth are the insolubility of the polymers and the occurrence of side reactions.

The Westinghouse Sulfur Cycle for the thermochemical decomposition of water

The Westinghouse Sulfur Cycle is a two-step thermochemical cycle for decomposing water into hydrogen and oxygen. Sulfurous acid and water are reacted electrolytically to produce hydrogen and sulfuric acid. The resultant sulfuric acid is vaporized to produce steam and sulfur trioxide, with the latter compound being subsequently reduced at higher temperatures into sulfur dioxide and oxygen. Following separation of the water and sulfur dioxide for recycle to the electrolyzer, oxygen is available as a process by-product. The cycle has the potential for achieving high thermal efficiencies while using common and inexpensive chemicals. The product hydrogen and oxygen streams are available under pressure and at high purity. As a result, these may be pipelined and used without detrimental environmental effects and without jeopardizing processes which employ the gases. Research has shown that the chemical reactions comprising the cycle proceed at acceptable rates and without the occurrence of side reactions. Conceptual designs and cost estimates indicate that the product gas separations can be done conventionally and economically. This paper discusses the technology supporting the selection of this thermochemical cycle for further development, and potential applications for the hydrogen and oxygen products of the process.

Purine nucleotide cycle. Evidence for the occurrence of the cycle in brain.

Cell-free extracts of rat brain catalyze the reactions of the purine nucleotide cycle. Ammonia is formed during the deamination but not the amination phase of the cycle. The activity of adenylate deaminase in brain is sufficient to account for the maximum rates of ammonia production that have been reported. The activity of glutamate dehydrogenase is not sufficient to account for these rates of ammonia production. The activities of adenylosuccinate synthetase and adenylosuccinase are nearly sufficient to account for the steady state rates of ammonia production observed in brain. Demonstration of the cycle in extracts of brain is complicated by the occurrence of side reactions, in particular those catalyzed by phosphomonoesterase, nucleoside phosphorylase, and guanase.