Reduction of dicarbonyl compounds on the dme: Part VII. Electrochemical behaviour of glyoxal

Polagraphic and kinetic studies of both the first and the second reduction waves of glyoxal have been carried out in aqueous solutions in the pH range 0 – 11.Tafel slopes and reaction orders with respect to glyoxal, H⁺ ion and proton donors present in solutions have been obtained at potentials corresponding to the foot of the waves. Reactions mechanisms are proposed for this potential zone.The proposed mechanism accounts both for our experimental data and for those previously reported in the literature.     

Polarographic reduction of some potential antidiabetic compounds

The polarographic reduction of 4-arylhydrazone-N′-benzysulphonyl-3-methyl-2-pyrazoline-5-ones gave two, two-electron transfer, well defined, well separated, diffusion controlled, irreversible waves in B.R. buffers of pH range 7.0–11.0. In acidic medium the E_1/2 values of the two waves are so close that only one 4-electron transfer, well defined, diffusion controlled, irreversible waves is obtained. The reduction in these compounds takes place at the-NH-N=C-bond. The effect of substituents and its correlation with the Hammett substituent constant (σ) have also been studied.     

Reactions in microemulsion media. Borohydride reduction of mono- and dicarbonyl compounds

Microemulsions were prepared at 26° from mixtures of hexanes (O), a 50: 1 (w/w) solution (W) of 0.1 M KOH-NaBH₄, and a 1.23:1 (w/w) mixture (S) of hexadecyltrimethylammonium bromide (HTABr) and 1-butanol. A pseudoternary phase map contained a significant microemulsion (μE) region, and μE's A and B (60:35:5 and 20:10:70 S:W:O, respectively) were used for reduction of several monocarbonyl compounds [benzophenone (1a), benzaldehyde (2a), acetophenone (3a), and 1-phenyl-1-octadecanone (4a)], an α,β-unsaturated ketone [trans-4-phenyl-3-buten-2-one (6a)], and a diketone [4-(4'-benzoylphenyl)-2-butanone (7a)]at 26°. For comparison purposes, reductions were also performed in aqueous 2-propanol (2-PrOH A and 2-PrOH B) prepared by the substitution of 2-propanol for the S and O components of μE's A and B. Generally, the reductions were slightly faster in the microemulsion media than in the corresponding aqueous 2-propanol media. The significantly slower reduction of 4a relative to that of 3a in μE B indicated that the interphase is the reactive site. With enone 6a, the influence of microemulsions on the competition between 1,2- and 1,4-reduction was determined. In μE's A and B there was 8% and ll% 1,4-reduction, respectively, whereas in 2-PrOH A and B there was only a trace. With diketone 7a, the reactivity of the aromatic carbonyl group relative to that of the aliphatic carbonyl group increased on going from 2-PrOH A and B to μE's A and B, respectively. For the sodium borohydride reduction of ketones, microemulsion catalysis is more effective than phase transfer catalysis or the use of a tetraalkylammonium borohydride in a hydrocarbon solvent.     

Synthesis of 5-hydroxy-N-(2-vinyloxyalkyl)indoles from dicarbonyl compounds and vinyloxyalkylamines

N-(Vinyloxyalkyl)enaminoketones were obtained by the condensation of vinyloxyalkylamines with acetylacetone or ethyl acetoacetate. Their subsequent reactions with 1,4-benzoquinone produce the corresponding 5-hydroxy-2-methyl-N-(2-vinyloxyalkyl)indoles in 17–43% yields. The structures of the compounds obtained were confirmed by IR and¹H NMR spectroscopy.     

Effect of hydration temperature on the structure reconstruction of MgAlY layered materials

This contribution investigates the obtaining of layered double hydroxides (LDHs) modified with different concentrations of yttrium and the effect of the yttrium concentration and temperature during the rehydration of the mixed oxides derived from these solids. Mg₃Al_(1?x)Y_x-LDH (where x = 0.04–0.6) have been prepared using the standard coprecipitation method under low supersaturation conditions. After the calcination of these solids (460 °C, 18 h), the corresponding mixed oxides were obtained. The reconstruction of the layered structure was attempted by rehydration of the mixed oxides under two different conditions: (1) at 25 °C and (2) at 70 °C. All the aforementioned solids have been characterized by chemical analysis, X-ray diffraction, DRIFTS spectra, N₂ adsorption–desorption isotherms, and determination of base sites. Their catalytic activity was tested in cyanoethylation of ethanol with acrylonitrile. The results obtained after the characterization and the catalytic activity tests were compared with those obtained for a standard hydrotalcite Mg₃Al.     

Effect of Base–Acid Properties of Mixtures of Ethanol with Water on the Enthalpy of Solution of Cyclic Ethers in these Mixtures at <Emphasis Type="Italic">T</Emphasis> = 298.15 K

The enthalpies of solution of the cyclic ethers 1,4-dioxane, 12-crown-4 and 18-crown-6 in mixtures of ethanol and water have been measured within the whole mole fraction range at T = 298.15 K. The enthalpy of solvation has been calculated. In pure ethanol and pure water, the solvation enthalpy of the investigated cyclic ethers depends linearity on the number of –CH₂CH₂– groups in the cyclic ether molecules. Based on the analysis of the preferential solvation model proposed by Waghorne, it can be concluded that the 1,4-dioxane, 15C5 and 18C6 molecules are preferentially solvated by water molecules in the range of low water content in these mixtures. The effect of base–acid properties of ethanol–water mixtures on the enthalpy of solution of cyclic ethers in these mixtures has been analyzed. The enthalpy of solution of cyclic ethers correlates with the acidic properties of ethanol–water mixtures in the range of high and medium water content. The results presented are compared with analogous data obtained for the methanol–water and propan-1-ol–water mixtures.     

The Infestation of Olive Fruits by Bactrocera oleae (Rossi) Modifies the Expression of Key Genes in the Biosynthesis of Volatile and Phenolic Compounds and Alters the Composition of Virgin Olive Oil

Bactrocera oleae, the olive fruit fly, is one of the most important pests affecting the olive fruit, causing serious quantitative and qualitative damage to olive oil production. In this study, the changes induced by B. oleae infestation in the biosynthesis of volatile and phenolic compounds in olive (cvs. Picual, Manzanilla, and Hojiblanca) have been analyzed. Despite cultivar differences, the oils obtained from infested fruits showed a significant increase in the content of certain volatile compounds such as (E)-hex-2-enal, ethanol, ethyl acetate, and β-ocimene and a drastic decrease of the phenolic contents. The impact of those changes on the inferred quality of the oils has been studied. In parallel, the changes induced by the attack of the olive fly on the expression of some key genes in the biosynthesis of volatile and phenolic compounds, such as lipoxygenase, β-glucosidase, and polyphenol oxidase, have been analyzed. The strong induction of a new olive polyphenol oxidase gene (OePPO2) explains the reduction of phenolic content in the oils obtained from infested fruits and suggest the existence of a PPO-mediated oxidative defense system in olives.     

Apparent transfer coefficient for ORR at polycrystalline platinum under convection conditions: a potential modulation study

The oxygen reduction reaction has been studied in the thin-layer flow through cell at polycrystalline platinum in 0.5 M H₂SO₄ at different flow rates. The apparent transfer coefficient (α′) and the corresponding Tafel slope have been calculated using the previously introduced ac voltammetry method [1, 2]. In addition to the correction of the high electrolyte resistance in the thin-layer cell, the contribution from other adsorption processes presented by the adsorption resistance has been also subtracted. The effect of increasing convection on the obtained α′ and Tafel slopes has been examined and it was found that the apparent cathodic transfer coefficient is 0.5 (corresponding to a Tafel slope of ?120 mV dec^?1) in all cases independent of the flow rate or frequency, suggesting that the first electron transfer to oxygen is the rate-determining step. The difference between Tafel slopes obtained here, i.e., by ac voltammetry method, and that obtained from usual Tafel slopes is due to the potential dependence of oxygen adsorbates, which in the method used here is—at least in part—separated from the potential dependence of the rate constant.     

A potentiostatic study of hydrophobic ion transfer across lipid bilayer membranes: Part II. Transport of tetraphenylborate ions at high concentration in the aqueous solution

The transfer of tetraphenylborate (TPB^?) ions across black lipid membranes in aqueous solutions ranging from 10^?5M to 10^?3M, has been studied under potentiostatic conditions. A theoretical treatment of the problem accounts for the two time constants given and for the diffusion current for prolonged periods of time if one assumes that the adsorption-desorption transfer currents are large. This is tantamount to considering that these two processes are highly reversible and that the dipolar nature of the drop in potential between the closest approach and adsorption planes determines the adsorption. The resistances corresponding to the translocation and adsorption-desorption processes have thus also been evaluated. Using these results and those given by surface potential measurements on monolayers of glycerylmono-oleate, the number of adsorbed ions has been calculated. This has led to an explanation of the well known conductance maximum encountered when the concentration of the tetraphenylborate ions varies.     

Diagnostic criteria for characterization of mechanisms corresponding to the second reduction polarographic wave of carbonyl compounds in acidic medium

Expressions of i-E funtions are developed, either by the reaction layer approach or from the assumption that the reactions take place at the surface of the electrode, for different possible mechanisms corresponding to the second reduction polarographic wave of carbonyl compounds in acidic medium.Diagnostic criteria for their characterization are presented and applied to the pyridine-4-aldehyde reduction. Close agreement between the experimental and theoretical results is obtained for a mechanism with a reversible transfer reaction followed by a protonation reaction of the compound formed.     

Adsorption of ethanol on the modified ruthenium catalysts in alkaline media

Regularities of ethanol adsorption in alkaline medium on the surface of the monoruthenium Ru/C catalyst and systems promoted by nickel and vanadium oxides, Ru-VO _x /C and Ru-NiO _x /C, were studied by non-stationary voltammetry techniques. An analysis of the kinetic and concentration dependences of adsorption showed that an increase in the content of the promoting component in the bimetallic systems enhances the energy heterogeneity of the catalyst surface. The data on adsorption measurements were compared with the results of previous studies of the kinetics and depth of ethanol electrooxidation on the examined catalysts. The larger quantity of electricity consumed for the oxidation of adsorbed particles formed on the surface of the metallooxide systems corresponds to higher extents of electrooxidation and rate than those for the monoruthenium catalyst.     

H2 adsorption by noble gas insertion compounds: A computational study

In order to find a solution of energy-related problems, sophisticated hydrogen storing materials are needed as hydrogen is an abundant and environment friendly fuel. We have investigated the hydrogen storage potential of Ng inserted metal acetylide and metal cyanide compounds (metal ​= ​Cu, Ag and Au) at the ωB97X-D/cc-pVTZ-PP level of theory. Due to the difference in electronegativity and formal charge on metal atoms in the insertion compounds, the interaction with the hydrogen molecule is expected to be different. The adsorption energies, the free energy of adsorption, natural charges on atomic centers/moieties are obtained through the natural population analysis, and energy decomposition analysis has also been carried out for nH₂···MNgCCH and nH₂···MNgCN (n ​= ​1–3). The hydrogen adsorption capacity of the strongest and the weakest cases has also been investigated. Both the insertion compounds, MNgCCH and MNgCN, are found to adsorb a maximum of three hydrogen molecules on the metal site. The single H₂ adsorbed minimum energy structures of studied compounds show a “T-shaped” orientation while double H₂ adsorbed minimum energy structures are of “Y- shaped” geometry and those of tricoordinated structures resemble “T_d-like” shape. The negative value of Gibbs free energy change suggests the thermodynamical spontaneity of the hydrogen adsorption process.     

The potential dependence and kinetics of HCOOH adsorption on rhodium electrodes

The formic acid adsorption on an electrochemically prepared rhodium electrode has been studied by the radiochemical method. Electrochemical properties of the rhodium electrode surface in 0.5 M H₂SO₄ have been investigated by cyclic voltammetry. It has been shown that starting from E=0.20 V the rate of HCOOH adsorption is markedly potential dependent being practically independent of the electrode potential up to E=0.20 V. It seems that the HCOOH adsorption process may be explained on the basis of the two-sites kinetics model. The data obtained for HCOOH adsorption on a rhodium electrode have been compared with those for a platinum electrode reported previously.     

Dehydration of Ethanol by Facile Synthesized Glucose-Based Silica

Bioethanol is considered a potential liquid fuel that can be produced from biomass by fermentation and distillation. Although most of the water is removed by distillation, the purity of ethanol is limited to 95–96 % due to the formation of a low-boiling point, water–ethanol azeotrope. To improve the use of ethanol as a fuel, many methods, such as dehydration, have been proposed to avoid distillation and improve the energy efficiency of extraction. Glucose-based silica, as an adsorbent, was prepared using a simple method, and was proposed for the adsorption of water from water–ethanol mixtures. After adsorption using 0.4 g of adsorbent for 3 h, the initial water concentration of 20 % (water, v/v) was decreased to 10 % (water, v/v). For water concentrations less than 5 % (water, v/v), the adsorbent could concentrate ethanol to 99 % (ethanol, v/v). The Langmuir isotherms used to describe the adsorption of water on an adsorbent showed a correlation coefficient of 0.94. The separation factor of the adsorbent also decreased with decreasing concentration of water in solution.     

Thermodynamics of adsorption of aromatic compounds from non-aqueous solutions by MIL-53(Al) metal-organic framework

The thermodynamics of adsorption of mono-, di-, and tricyclic aromatic compounds by MIL-53(Al) metal-organic framework from their solutions in MeCN, MeOH and n-C₆H₁₄–PrⁱOH was studied for the first time. It was found that the adsorption of the test substances from solutions in MeCN and MeOH is characterized by positive values of enthalpy and entropy changes, and the adsorption from n-C₆H₁₄–PrⁱOH medium is characterized by negative enthalpy and entropy changes. Upon adsorption by MIL-53(Al) framework from polar media, aromatic compounds were proposed to transfer from the liquid phase with a higher degree of association into the solvent medium with a lower degree of association, molecules of which are disordered due to the strong interaction with the hydrophobic walls of the framework pores. It was concluded that the driving force of adsorption by MIL-53(Al) from MeCN and MeOH is increase in entropy of the system, while the factor of adsorption from n-C₆H₁₄–PrⁱOH medium is decrease in enthalpy of the adsorption system. The compensation effect in liquid-phase adsorption of aromatic compounds by MIL-53(Al) framework was discovered. The effect of the liquid phase nature on selectivity of adsorption from solutions onto investigated metal-organic framework was demonstrated.     

The nature of species adsorbed on platinum from SO2 solutions

The present studies have shown that at θ_S<0.9, the species adsorbed on platinum from SO₂ solutions are virtually identical with those obtained from H₂S solutions. The layer at θ_S>0.9 can be explained by the formation of a partial bilayer on top of the first layer wherein 70% of the sites are covered with S (2-site adsorption) and 30% with S (1-site adsorption). It has also been shown that the ‘difficultly reducible’ oxygen referred to by earlier workers is identical with the species formed by exposure of the electrode to SO₂ at potentials above 0.2 V. Evidence for the presence of two forms of ‘difficultly reducible’ oxygen, corresponding to the two forms of chemisorbed sulfur, is presented.     

Indirect radiotracer study of the adsorption of organic compounds at platinized platinum electrodes

The possibility of the use of an indirect radiotracer method for the study of the adsorption of organic species has been discussed. It has been shown that by study of the adsorption of Cl-36 labelled chloride ions in the presence of organic species such as maleic, benzoic and m-nitrobenzoic acids there is a possibility of obtaining information concerning the adsorption of the latter compounds. Change in the adsorption behaviour of organic species connected with their electroreduction were detected and followed by indirect tracer measurements. In the light of the experimental results, some mechanistic aspects of the reduction of aromatic nitro compounds are discussed.     

Plant-mediated asymmetric reduction of 1-(3,4-dimethylphenyl)ethanone

Bioreduction mediated by plants is a highly selective and environmentally friendly approach to synthesise optically active alcohols. Herein the bioreduction of 1-(3,4)-dimethylphenyl)ethanone to the corresponding chiral alcohol 1-(3,4-dimethylphenyl)ethanol has been evaluated using nine different vegetables roots as biocatalysts. The chiral alcohol was prepared in yields ranging from 44.1% to 88.2%, and with enantiomeric excess up to 97.2% for the (S)-enantiomer. Sugar beet was the most promising among the tested vegetables roots. Therefore, sugar beet cell cultures (normal and transformed) were obtained and tested for the reduction as well, yielding the corresponding alcohol in the range from 62.1% to 88.2% yield and with enantiomeric excesses of >99%. Based on these results reported, there are no profound differences in the reductive capacity of undifferentiated cells and organs of the same plant species (sugar beet). Due to overall advantages of using cell cultures, we have highlighted this approach as a promising method for preparation of enantiomerically pure 1-(3,4-dimethylphenyl)ethanol.     

What can positronium tell us about adsorption?

The condensation and evaporation of n-heptane at 298 K in mesopores of silica material obtained by the polymer templating method have been studied by PALS measurements. It is demonstrated that the ortho-positronium lifetimes and intensities provide valuable information on pore filling and emptying which are not accessible from a conventional adsorption experiment. The results confirm the specific adsorption mechanism of n-heptane in pores with narrow openings (ink-bottle shape) which is different from that known for other pore geometries. The results from PALS experiment are compared to those derived from the conventional n-heptane and nitrogen adsorption data.     

Optimized Fed-Batch Fermentation of Scheffersomyces stipitis for Efficient Production of Ethanol from Hexoses and Pentoses

Scheffersomyces stipitis was cultivated in an optimized, controlled fed-batch fermentation for production of ethanol from glucose–xylose mixture. Effect of feed medium composition was investigated on sugar utilization and ethanol production. Studying influence of specific cell growth rate on ethanol fermentation performance showed the carbon flow towards ethanol synthesis decreased with increasing cell growth rate. The optimum specific growth rate to achieve efficient ethanol production performance from a glucose-xylose mixture existed at 0.1 h^?1. With these optimized feed medium and cell growth rate, a kinetic model has been utilized to avoid overflow metabolism as well as to ensure a balanced feeding of nutrient substrate in fed-batch system. Fed-batch culture with feeding profile designed based on the model resulted in high titer, yield, and productivity of ethanol compared with batch cultures. The maximal ethanol concentration was 40.7 g/L. The yield and productivity of ethanol production in the optimized fed-batch culture was 1.3 and 2 times higher than those in batch culture. Thus, higher efficiency ethanol production was achieved in this study through fed-batch process optimization. This strategy may contribute to an improvement of ethanol fermentation from lignocellulosic biomass by S. stipitis on the industrial scale.