Synthesis of homopropargyl alcohols via sonochemical Barbier-type reaction

A series of homopropargyl alcohols were synthesized from the reaction mixture of zinc powder, 1,2-diiodoethane, 3-bromo-1-propyne and aldehyde or ketone in anhydrous THF under ultrasound. The homopropargyl alcohols were obtained as the only product in all cases when aldehydes were reacted with 3-bromo-1-propyne under this sonochemical Barbier-type reaction condition. The homopropargyl alcohol was produced as the major product and the low contamination of allenyl alcohol was also obtained when ketone was used as substrate under the reaction condition.     

Generation and consumption rates of OH radicals in sonochemical reactions

In this study, a KI aqueous solution or Methyl Orange (MO) aqueous solution was irradiated by an ultrasonic wave under the same experimental condition. The rates of oxidation of KI and MO by OH radicals differed by an order of magnitude. When the consumption of OH radicals by chemical reactions with species other than KI or MO is taken into account, numerical analysis of chemical kinetics model yields the same generation rate of OH radicals by the action of an ultrasonic wave for the experiments of KI and MO solutions.     

Effect of equilibrium fluctuations on microcrystalline particles on adsorption isotherms and reaction rates

The effect of the finiteness of face areas and equilibrium fluctuations on the adsorption isotherms of binary molecular mixtures and the rate of surface processes occurring on nanosized particles is studied. The adsorption process is considered in the grand canonical ensemble; the rates of elementary stages are calculated in the kinetic mode. The surface region on the face is found to contain a number of adsorption centers ranging from 10 to 10⁵. The effect of density fluctuations of adsorbed molecules on the partial adsorption isotherms and rate fluctuations is discussed. A calculation procedure for density fluctuations in heterogeneous microparticles with different faces and different binding energies of molecules with the surface is considered. It is shown that the greatest effect of density fluctuations is manifested at a low occupancy of each face of the particle.     

Effects of initial concentration of LASs on the rates of sonochemical degradation and cavitation efficiency

The effect of initial concentration of linear alkylbenzene sulfonate (LAS: p-octylbenzene sulfonate (LAS C₈), p-nonylbenzene sulfonate (LAS C₉), p-dodecylbenzene sulfonate (LAS C₁₂)) on the rate of sonochemical degradation was investigated over a wide concentration range under Ar atmosphere by 200 kHz ultrasonic irradiation. The degradation rate of each LAS increased with increasing initial concentration of LAS and then started to decrease with the different behavior depending on the types of LASs. This result indicated that the cavitation efficiency was gradually changed by their concentrations and the optimum LAS concentrations for their effective degradation existed. The maximum degradation rates were observed at 250 μM of LAS C₁₂, 1250 μM of LAS C₉, and 2500 μM of LAS C₈, respectively. These optimum concentrations were found to be about four to six times smaller than these critical micelle concentrations (CMCs). It was also found that the maximum degradation rates at the optimum concentrations were observed to be almost linearly correlated with their CMCs. Based on the obtained results, it could be suggested that the micelle formation occurs in the interfacial region of cavitation bubbles during rectified diffusion and this phenomenon reduces the cavitation efficiency. In addition, from the results of the rate of the sonochemical degradation of LASs and the yield of hydrogen peroxide, the existence of thermal gradient in the interfacial region of cavitation bubbles was also confirmed.     

Enhancement of sonochemical reaction rate by addition of micrometer-sized air bubbles

The sonochemical reaction rate has been enhanced by the introduction of tiny air bubbles. The bubbles including micrometer-sized ones are produced by method of atomization and are introduced into aqueous luminol solution under 141-kHz sonication in order to investigate the enhancement of sonochemical reaction rate by introduction of tiny bubbles through the intensity measurement of sonochemiluminescence (SCL). It is shown that the introduction of tiny bubbles under sonication accomplishes the large SCL intensity compared to the cases of sonication only and liquid flow under sonication. It is also shown that it is important to adjust the configuration of tiny-bubble addition to the sound field. Through the investigations on the intensity and the spatial pattern of luminol-SCL, it has been clarified that tiny bubbles added into the sonicated liquid not only cause the liquid flow but also increase the number of collapsing bubbles active for sonochemical reaction. It is also shown that the tiny-bubble addition enhances the reaction rate of KI oxidation under sonication. Therefore, the present method of introduction of tiny bubbles is effective for enhancement of sonochemical reaction rate.     

Effect of solvent on chain propagation and termination reaction rates in radical polymerization

The radical polymerizations of acrylamide and methacrylamide and of acrylic, methacrylic and fluoroacrylic acids have been studied in various solvents. The nature of the solvent affects the overall rate due to changes in k_p and E_p. It has been shown that the effect of solvent also depends on the nature of the monomer. A possible explanation of the observed behaviour is discussed.     

A facile and efficient synthesis of aryltriethoxysilanes via sonochemical Barbier-type reaction

A series of aryltriethoxysilanes was synthesized from the reaction mixture of aryl bromide, Mg powder, 1,2-dibromoethane and tetraethyl orthosilicate in THF under ultrasound. This sonochemical Barbier-type reaction provides a simple and efficient method for preparation of aryltriethoxysilanes. The Hiyama cross-coupling reaction of phenyltriethoxysilane with bromobenzene was investigated under different palladium catalysts and Pd(PhCN)₂Cl₂ was found to be the best choice.     

Plasma density effects on atomic reaction rates

Plasma modeling and diagnostics commonly involve solution of rate equations for the population densities of impurity ions in their excited and charge states. Construction of the rate equations requires a complete set of atomic transition rates for all the charge and excited states involved. However, the rates are themselves affected by the host plasma ions and electrons. The ionic and electronic effects in a two-component plasma are intimately interconnected, especially when the rate equations are simplified for computational purpose in the determination of ionization balance. We formulate a coherent approach to the problem of the plasma density effect, and apply it to carbon impurities in a hydrogen plasma. Both the plasma field distortion of atomic states and the corresponding rates by the plasma ions and stochastic plasma collisional transitions caused by the plasma electrons are included. The latter effect is estimated by constructing an effective collisional transition operator, and the electron-ion recombination processes are explicitly evaluated. It is shown that, these two effects of the ionic field distortions and electronic collisions tend to cancel each other, resulting in many cases in reducing the overall effect of the plasma density on the ionization.     

Excluded volume effects on polymer reaction rates

The rates of reaction of acid chloride groups with nitrophenol groups were measured with the groups attached to polymer chains. The average degree of substitution was kept low, near to one or less groups per chain. It was found that the interpolymer reaction rates decreased several fold in dilute solution for high polymers compared with the rates in more concentrated solutions or with those for low-molecular-weight polymers. The results were compared with the models proposed by Morawetz and the effect was of the predicted magnitude. The results also confirmed Morawetz' prediction that smaller effects could be expected for terminally substituted than for randomly substituted polymers.     

Solvent effect on reaction rates in molten salts

Predominant solvation of activated complex is shown to be characteristic for chemical and electrochemical reactions in molten salts.

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Presented at EUCHEM Conference on Molten Salts, September 2–8, 1990, Rio-Patras, Greece     

Correlation between acoustic cavitation noise and yield enhancement of sonochemical reaction by particle addition

The mechanism of the effect of particle addition on sonochemical reaction is studied through the measurements of frequency spectrum of sound intensity for evaluating the cavitation noise and the absorbance for the liberation of iodine from an aqueous solution of KI as an index of oxidation reaction by ultrasonic irradiation in the presence or absence of alumina particles. As it is expected that both the acoustic noise and a rise in temperature in the liquid irradiated by intense ultrasound will increase with the number of collapsing bubbles, these are supposed to be the best tools for evaluating the relative number of bubbles. In the present investigation, it has been shown that the addition of particles with appropriate amount and size results in an increase in the absorbance when both the acoustic noise and the rise in the liquid temperature due to cavitation bubbles also increase. This suggests that the enhancement in the yield of sonochemical reaction by appropriate particle addition comes from an increase in the number of cavitation bubbles. The existence of particle in liquid provides a nucleation site for cavitation bubble due to its surface roughness, leading to the decrease in the cavitation threshold responsible for the increase in the number of bubbles when the liquid is irradiated by ultrasound. Thus, from the present investigation, it is clarified that the particle addition has a potential to enhance the yield in the sonochemical reaction.     

Influence of reaction rates on Raman spectra of molecular complexes

We have performed a study of the influence of reaction rates of formation of iodine complexes on the shape of the iodine vibrational Raman line. The results show that the lineshape provides a very sensitive means for determining complex lifetimes and reaction rates.     

DTA curves and non-isothermal reaction rates

Differential thermal analysis (DTA) is an effective means for studying chemical reactions, but its application to reaction kinetics is handicapped by the involvement of temperature feedback from the reaction heat and by the solvent dependence of the thermal conductivity.General, empirical relationships are derived from digital computer application which allow to transform half width and shape index of a DTA curve of any first-order process in a uniformly temperated sample to the values of the corresponding rate curve at linearly increased temperature.The expressions are complemented by some new relationships for an n-order reaction and are useful for the kinetic study of complex processes.     

Symmetry numbers and chemical reaction rates

This article shows how to evaluate rotational symmetry numbers for different molecular configurations and how to apply them to transition state theory. In general, the symmetry number is given by the ratio of the reactant and transition state rotational symmetry numbers. However, special care is advised in the evaluation of symmetry numbers in the following situations: (i) if the reaction is symmetric, (ii) if reactants and/or transition states are chiral, (iii) if the reaction has multiple conformers for reactants and/or transition states and, (iv) if there is an internal rotation of part of the molecular system. All these four situations are treated systematically and analyzed in detail in the present article. We also include a large number of examples to clarify some complicated situations, and in the last section we discuss an example involving an achiral diasteroisomer.     

Microwave-enhanced reaction rates for nanoparticle synthesis

Microwave reactor methodologies are unique in their ability to be scaled-up without suffering thermal gradient effects, providing a potentially industrially important improvement in nanocrystal synthetic methodology over convective methods. Synthesis of high-quality, near monodispersity nanoscale InGaP, InP, and CdSe have been prepared via direct microwave heating of the molecular precursors rather than convective heating of the solvent. Microwave dielectric heating not only enhances the rate of formation, it also enhances the material quality and size distributions. The reaction rates are influenced by the microwave field and by additives. The final quality of the microwave-generated materials depends on the reactant choice, the applied power, the reaction time, and temperature. CdSe nanocrystals prepared in the presence of a strong microwave absorber exhibit sharp excitonic features and a QY of 68% for microwave-grown materials. InGaP and InP are rapidly formed at 280 degrees C in minutes, yielding clean reactions and monodisperse size distributions that require no size-selective precipitation and result in the highest out of batch quantum efficiency reported to date of 15% prior to chemical etching. The use of microwave (MW) methodology is readily scalable to larger reaction volumes, allows faster reaction times, removes the need for high-temperature injection, and suggests a specific microwave effect may be present in these reactions.     

Quantized RRK theory of unimolecular reaction rates

The main difference between the simple RRK theory and the better based but more complex RRKM theory is explained. Starting from the premise that the classical versus quantum mechanical estimation of the density of states is the major source of the difference, earlier attempts to incorporate the quantum effects in an effective value for the number of oscillators s are noted. By examining the expression for the RRKM rate coefficient it is found that a single effective s value will generally not suffice, but a much better representation of the quantum effects can be obtained if it is recognized that the problem inherently contains two different effective s values. A theory based on this analysis is constructed. It reproduces RRKM results to much improved accuracy, removing difficulties found earlier with single-s-value theories.     

Measuring reaction rates on single particles in a microfluidic device

The development of a simple method to measure reaction rates using magnetic microparticles in a microfluidic device is explored.     

Solvent effects on reaction rates in a catalytic slurry reactor

By carrying out static adsorption measurements, it is shown that absolute values of adsorption strength parameters required in an earlier Langmuir type model for liquid phase catalytic hydrogenation in the presence of an adsorbing solvent can be obtained. These can be used to predict the relationship between reaction rate and reactant concentration in the solvent.

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Enhanced aqueous photochemical reaction rates after freezing

Sunlit snow/ice is known to play an important role in the processing of atmospheric species, including photochemical production of NO(x), HONO, molecular halogens, alkyl halides, and carbonyl compounds, among others. It has been shown that a liquid-like (quasi-liquid or disordered) layer exists on the surface of pure ice and that this quasi-liquid layer is also found on the surface of ambient snow crystals and ice at temperatures similar to polar conditions. However, it is unclear what role the liquid-like fractions present in and on frozen water play in potential photochemical reactions, particularly with regard to organic substrates. Here, we report a detailed study of enhanced rates of photochemical nucleophilic substitution of p-nitroanisole (PNA) with pyridine, a well-characterized and commonly used actinometer system. Reaction rates were enhanced by a factor of up to approximately 40 when frozen at temperatures between 236 and 272 K. Reaction rates were dependent on temperature and solute concentration, both variables that control the nature of the liquid-like fraction in frozen water. The results obtained indicate that a major portion of the organic solutes is excluded to the liquid-like layer, significantly impacting the rate of the photochemical nucleophilic substitution reaction studied here. Also, the direct comparison of liquid-phase kinetics to reactions occurring in frozen water systems is drawn into question, indicating that a simple extrapolation of liquid-phase mechanisms to snow/ice may not be valid for certain reactions.