Ultrasound-Assisted Oxidation Reactions

Oxidation reactions in anaiyticai chemistry usuaiiy constitute a step preceding derivatization, if present, intended to make the anaiyte amenabie to derivatization or, iess frequentiy direct detection. One can expect US to favour any oxidation reaction taking piace in an aqueous medium through the weii-known radicai formation process. This assumption has been verified with a smaii, but representative, number of exampies. in other cases, aqueous soiutions saturated with a soivent of higher vapour pressure than water have been found to favour oxidation reactions. 

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Chapter 7 deals with other types of US assistance to enhance reactions of analytical interest including derivatization and reagent generation, or ultrasound-assisted oxidation or hydrolysis reaotions. 

The reactive crystallization has some peculiar characteristics like insoluble product, initiation of reaction by change in pH and conductivity. In this case the solution becomes saturated and eventually supersaturated with respect to reactant nucleation [30], The ultrasound assisted decomposition precursors includes dissolving metal organic precursors in organic solvents/water with the assistance of surfactants leads to monodisperse and reduced metal/metal oxide nanoparticles. 

Reductions. Reports of rather simple reductions by indium keep appearing of tx-halocarbonyl compounds in water with ultrasound assistance, of p-nitrobenzyl ethers and esters, disulfides, amine IV-oxides, and organic azides in alcoholic solvents in the presence of NH4CI. When the azide reduction is carried out in DMF the products can be directly converted to carbamates by having a chloroformate ester in the reaction vessel. ... 

The oxidative cyclization of steroidal bromohydrins 609 selectively affords products 610 in high yields (Scheme 3.238) [641]. This reaction can be promoted by either photolysis or ultrasonic irradiation. The yields of products 610 are significantly better under the ultrasound-assisted conditions. 

Zhao et al. have synthesized novel ionic liquids 307 and 308, which contain (3-chloro-2-hydroxypropyl)-functionalized pyridinium cations, by the ultrasound-assisted reaction of pyridine with acid (HCl and HBF4) and 3-chloro-propylene oxide at room temperature, in excellent yields and purity, in whieh the acid provided the eorresponding anionic component of the ionic Uquids. Furthermore, they examined the appheation of new ionic liquids as solvents in the MBH reaction and found that IL-BF4 308 showed a better effect in rate enhanee-ment and an improved yield than IL-Cl 307 in some cases (Scheme 1.121). ... 

Park et al. [61] synthesized ZnO-CuO core-branch hybrid NPs by copper oxide growth and controlled oxidation on ZnO nanospheres, and exhibited remarkable enhancement of catalytic activity and stability for ultrasound-assisted [3+2] azide-alkyne cycloaddition reactions imder ultrasonic irradiation (Scheme 5.19). The surface species of the catalysts were investigated by XPS with the core level of Cu 2p3 2 before and after the reaction. The peaks are deconvoluted into two peaks, where the peak at lower binding energy is attributed to Cu(l) and that at higher binding energy is attribnted to the Cu(ll) species. Before the reaction, the... 

Application of ultrasounds is among the new alternative techniques which can accelerate heterogeneous reactions by increasing the surface (dispersity) of the reagents, while in homogeneous systems ultrasounds assist the even heat distribution in a reactor. Thus, the apphcation of ultrasounds or of MW irradiation may substantially shorten the reaction time in oxidations of alcohols with PCC, from hours to minutes.It is beheved that the ultrasound produces erosion on the PCC surface and therefore accelerates its interaction with the organic substrates. The involvement of ultrasonic irradiation in a drastic reduction on the amount of PCC used was also indicated. 

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