Organosilicon sonochemistry

Sonochemical reactions involving organosilicon compounds are reviewed. Possible applications of ultrasonic irradiation for acceleration and initiation of various types of reactions are demonstrated along with examples from organic synthesis using organosilicon compounds. Also described are the sonochemical reactions of organic compounds containing the Group IVB elements germanium and tin, chemically related to silicon.     

Sonolysis and radiolysis of glyceraldehyde in deaerated aqueous solution

The objective of this work was to contribute to the mechanism of the sonolytic and radiolytic reactions leading in deaerated aqueous solutions of sugars to products by radical-radical combination. For this purpose glyceraldehyde, the first homologue of the series of aldoses, was investigated. Primary glyceraldehyde radicals are produced by abstraction of carbon-bonded hydrogen atoms by sonolytic or radiolytic H and OH radicals. Secondary glyceraldehyde radicals are derived from primary radicals by elimination of water. Both kinds of radicals were found to participate in dimer production.     

Sonolysis of organic liquid: effect of vapour pressure and evaporation rate

Various kinds of organic liquids, such as hydrocarbons, ethers, ketones and alcohols, were subjected to ultrasonic irradiation and the effects of vapour pressure and evaporation rate of the liquids on decomposition rates and the distribution of decomposition products were investigated. The main decomposition products from hydrocarbons were hydrogen, methane, ethylene and acetylene, and hydrogen, methane, ethylene, carbon monoxide and aldehydes from alcohols. The decomposition rates of organic liquids were generally faster than that of water, and the reaction would proceed via gas-phase chain reactions in the high temperature site by comparison of the product with pyrolysis data in the literature and by considering the results of DPPH experiments. In the relationship between decomposition rate and vapour pressure, different features were observed in alcohols and other liquids. The hydrocarbons most efficiently decomposed under conditions in which their vapour pressures ranged from 0.1 to 0.5 Torr. On the other hand, the most efficient vapour pressure for alcohol sonolysis was about 15 Torr. The deviations became smaller when the evaporation rate was employed instead of vapour pressure, and as the reactive index of sonolysis of organic liquids, evaporation rate may be a better probe than vapour pressure, which is often chosen as the index.     

β-carboline as a probe for the sonolysis of alcohols and chloromethanes

Sonolysis at 20 and 500 kHz of alcohols, chloromethanes and dilute mixtures of the latter in the former was studied by ultraviolet spectrophotometry in the presence of β-carboline. Acidic degradation products were detected at low frequency sonication, especially for solutions of carbon tetrachloride in alcohols.     

Sonolysis of chlorobenzene in the presence of transition metal salts

Sonolysis of aqueous solution of chlorobenzene at 200 kHz frequency in the presence of transition metals chlorides was investigated. Through analyzing the nature and distribution of the products detected in the reaction mixture, a new mechanism of sonodegradation is advanced. Depending on the metals used and their behavior during sonolysis, we were able to discriminate between inside and outside cavitation bubble mechanisms. Iron and cobalt chlorides, which could undergo redox reactions in the presence of HO radicals generated ultrasonically, give higher amounts of phenolic compounds compared with palladium chloride that undergoes a reduction to metal. Palladium reduction takes place in bulk solution and therefore all organic reactions that compete for hydrogen must occur also in bulk solution. Accordingly, palladium can be a useful tool in determining the reaction site and the decomposition mechanism of organic compounds under ultrasonic irradiation.     

A reaction kinetics model of water sonolysis in the presence of a spin-trap

The time development of the concentration of a spin-trapped OH radical was studied by electron spin resonance at various sound intensities and various 5,5-dimethyl-1-pyrroline N-oxide (DMPO) concentrations in water sonolysis. The lifetime of the spin-trapped OH radical was also studied, and factors governing sonolysis are discussed. We found that the production of spin-trapped OH radical increases with increasing ultrasound intensity. The lifetime of a spin-trapped OH radical decreases linearly with increase in sonication time. This result suggests that an unknown scavenger is produced by ultrasound. Based on the above results, we suggested a model of the reaction kinetics and estimated the production rate of OH radical from this model.     

The size of active bubbles for the production of hydrogen in sonochemical reaction field

The sonication of aqueous solution generates microscopic cavitation bubbles that may growth and violently collapse to produce highly reactive species (i.e. OH, HO₂ and H₂O₂), hydrogen and emit light, sonoluminescence. The bubble size is a key parameter that influences the chemical activity of the system. This wok aims to study theoretically the size of active bubbles for the production of hydrogen in ultrasonic cavitation field in water using a single bubble sonochemistry model. The effect of several parameters such as frequency of ultrasound, acoustic intensity and liquid temperature on the range of sonochemically active bubbles for the production of hydrogen was clarified. The numerical simulation results showed that the size of active bubbles is an interval which includes an optimum value at which the production rate of H₂ is maximal. It was shown that the range of ambient radius for an active bubble as well as the optimum bubble radius for the production of hydrogen increased with increasing acoustic intensity and decreased with increasing ultrasound frequency and bulk liquid temperature. It was found that the range of ambient bubble radius dependence of the operational conditions followed the same trend as those reported experimentally for sonoluminescing bubbles. Comparison with literature data showed a good agreement between the theoretical determined optimum bubble sizes for the production of hydrogen and the experimental reported sizes for sonoluminescing bubbles.