Weissler reaction

Gutierrez M, Henglein A, Moeckel H (1995) Observations on the role of MgCl2 in the Weissler reaction. Ultrason Sonochem 2(2) S111-S113... 

It is often difficult to compare the sonochemical results reported from different laboratories (the reproducibility problem in sonochemistry). The sonochemical power irradiated into the reaction system can be different for different instruments. Several methods are available to estimate the amount of ultrasonic power entered into a sonochemical reaction, the most common being calorimetry. This experiment involves measurement of the initial rate of a temperature rise produced when a system is irradiated by power ultrasound. It has been shown that calorimetric methods combined with the Weissler reaction can be used to standardize the ultrasonic power of individual ultrasonic devices. ... 

Reference was made previously to three zones of reaction. The pressure Py, inside the bubble determines the zone in which the reaction occurs. High Pf, values correspond to reaction in zone 1, and high Pi values to reaction in zone 2. For reaction to occur in zone 3 (the Weissler reaction, for instance), Pb should be high, and also the residence time of the radicals generated should not be so high in zones 1 and 2 that they react among themselves and neutralize any enhancing effect. 

Known measuring techniques comprise local physical methods (hydrophones, thermoacoustic sensors, aluminum foil erosion, optical sound pressure and velocity sensors, radiation pressure scale, laser-Doppler anemometry), local chemical methods (Weissler reaction, chemiluminescence, electrochemical sensors), global chemical methods (model reactions), and sonoluminiscence (single-bubble sonoluminescence SBSL, multibubble sonoluminescence MBSL). 

The data set obtained with the local measurements of cavitational yield for the Weissler reaction (Gogate et al., 2002) in the case of an ultrasonic bath can be effectively used for the development of the design equation for cavitational yield in terms of the maximum size of the cavity (recommended when free-radical attack is the controlling mechanism). The mathematical equation relating the two can be given as follows ... 

Cavitational Yield Results Table 8.2.4 also gives the values of cavitational yields obtained for different equipments for the Weissler reaction, ft can be seen from the table that a triple-frequency flow cell gives 2 to 3 orders of magnitude higher cavitational yield as compared to all the other equipments. A dual-frequency flow cell also gives 20% higher yield as compared to an ultrasonic bath, whereas the... 

Figure 33 - Single bubble sonochemistry the filament visualizes the Weissler reaction, the tiny white spot at the lower extremity of the dark thread is the bubble...

The pressure applied to the sonicated medium is directly involved in the resonance between the bubble vibration and the acoustic field, and modifies the cavitation threshold (Gh. 6, p. 251). By increasing this pressure, cavitation becomes more difficult, but the energy released by the collapse is greater. This effect was observed in a pressure range of 0.7-3 bar for the oxidation of iodide ions in water solution (the Weissler reaction, Ch. 8, p. 313) under an oxygen atmosphere, and for the oxidation of indene in a biphasic system. ... 

Figure 8 - Effect of liquid height I (in mm) on the sonochemical yield of the Weissler reaction in a cup-horn reactor...

By knowing the reaction mechanism, the experimentalist will be led to choose the solvent not only on the basis of the usual chemical criteria, but also on its behavior under cavitation. It should be borne in mind that in the case of certain types of solvent, particularly chlorinated materials, sonication induces some decomposition. Radicals can be created whose influence might be considerable on the overall process. A good example is the Weissler reaction, where ultrasound... 

The cylindrical shape of the reactor concentrates the high-frequency waves into the same cavitation area as that of the lower frequency. The efficiency of this array is demonstrated with the Weissler reaction, where the rate under dual irradiation is far higher than when the reaction is performed at 20 kHz or 1.7 MHz under otherwise identical conditions. In heavily loaded industrial systems, it would be... 

Weissler s reaction is a quite attractive chemical probe, due to the ease of handling and the fact that it works well over a wide range of frequency, from 20 kHz up to several MHz. It should be noted, however, that its rate is very frequency-dependent being quite small at low frequency and sharply increasing as the frequency is increased. 

Since this chapter appears in a volume devoted to sonochemistry, chemical probes would appear to be the most attractive since they could allow direct comparisons with other chemical reactions. Chemical dosimeters are generally used to test the effect of an ultrasonic device on the total volume of the reactor. Local measurements can however be made with very small cells containing the dosimeter which could be moved inside the reaction vessel as with a coated thermocouple. Most of these chemical probes are derived from reactions carried out in an homogeneous medium, e.g. Weissler s solution, the Fricke dosimeter, or the oxidation of terephthalate anions. Among these the latter shows promise in that despite the fact that to date it has been much less used than Weissler s reaction it seems to have higher sensitivity and better reproducibility. 

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