Sonochemistry in Biphasic Systems

Abstract

When a choice is possible, reactions in solution are generally preferred to heterogeneous ones, which are considered to create problems with respect, for example, to rates or reproducibility. In many organic reactions, water is an undesirable contaminant. A kind of revolution occurred when Makosza, Starks, and Brändström discovered the principle of phase transfer catalysis (PIC), by which they demonstrated that not only can the presence of water be tolerated but heterogeneity is no longer a drawback. 1 Since these pioneering works, PIe has undoubtedly demonstrated its efficiency and broad applicability.2 Some procedures could be further improved by using solid-supported reagents, sometimes even in the absence of any solvent. 3 Ultrasonic cavitation has particularly important effects on biphasic systems, emulsification of immiscible liquid-liquids, partic1e breakage and dispersion, and surface cleaning in liquid-solid mixtures. These mechanical effects, even if sometimes oversimplified, appear to be understandable and predictable by nonexperts in the field, who exploited them in a variety of heterogeneous reactions. Sonochemistry of biphasic systems then developed rapidly, and synthetic applications were reviewed in recent artic1es. 4 In many cases, the presence of a phase transfer catalyst becomes unnecessary, and sonication can be considered as a "physical substitute to PIe", according to the expression of Ando. 4a Makosza, M. Pure Appl. Chern. 1975, 43, 439-462. 2 For arecent book, see Dehmlow, E.V.; Dehmlow, 5.5. Phase Transfer Catalysis, VCH, Weinheim, 3 rd Ed., 1993.3 For recent reviews, see (a) THE SONOCHEMICAL REACTIVITYFrom the empirical systematization of sonochemistry,9 it is sufficient to remember that, when the possibility exists, radical pathways are privileged at the expense of polar pathways, and sonochemical switching can result in a number of cases.lO In heterogeneous systems, reactions which follow ionic mechanisms are still sensitive to the mechanical effects of sonication. This "false sonochemistry" can, in prindple, also be observed when effident mixing techniques are applied, but the so-called simple mechanical effects are strongly dependent on geometrical factors and prove to be much more complicated than expected.Shocks on solids produce a complex series of events, the final stage of which is heat. Before this frequently useless point is reached, physicochemical transformations occur at the molecular level, especially if mechanical energy is provided faster than heat can be evolved. Vibrational and electronic excitation, bond deformation and breakage, and atom migration are detected by infrared or X-ray spectroscopies. Covalent bonds, for instance in polymers, can be brought to a predissociation state, and their ionization energy is divided by a factor of 2. In a silicon-oxygen-silicon bond system deformed by friction, the reactivity is practi-caUy that of silicon-centered and silyloxy radicals. On surfaces, the energy-rich domains, named "hot spots", are able to initiate chemical reacti...

Ultrasonic irradiation assisted efficient regioselective synthesis of CF3-containing pyrazoles catalyzed by Cu(OTf)2/Et3N

“…The improvement induced by ultrasound in the above mentioned reactions can be attributed to the well established theory for the cavitation, The collapse of bubbles caused by cavitation produces intense local heating and high pressures [55,56], so reaction time decreases clearly and high% yield was obtained.…”

Synthesis and characterization of octakis(3,3,3-trifluoropropyldimethylsiloxy)cyclotetrasiloxane (I), octakis[tridecafluoro-2,2,5,5-tetramethyl-2,5-disilatridecanoxy]cyclotetrasiloxane (II) and octakis(7-pentafluorophenyl-2,2,5,5-tetramethyl-2,5-disilaheptanoxy)cyclotetrasiloxane (III)

“…29 Si NMR spectra were externally referenced to TMS. 19 F NMR spectra were externally referenced to CFCl 3 . IR spectra of neat films of liquids on AgCl plates or KBr pellets of solids were recorded on a Perkin-Elmer spectrum 2000 FT-IR spectrometer.…”

“…An attempt to increase the yield and shorten the reaction time by applying ultra sonic waves to the reaction media had little effect. Ultrasound has been shown to improve yields and accelerate many heterogeneous reactions [19]. The yield of the reaction did not improve and the reasons for this are not fully understood.…”

“…13 C NMR d: À0.49 (s), 9.75 (s), 28.25 (q, J C-F ¼ 30 Hz), 127.76 (q, J C-F ¼ 275 Hz). 19 In a separate experiment, Ca 8 Si 4 O 12 Cl 8 (5 g, 5.3 mmol), 3,3,3-trifluoropropyldimethylchlorosilane (10 g, 55 mmol) and dry acetone (150 mL) were added into a 250 mL rb flask fitted with a reflux condenser and a Tekmar sonic disruptor micro-tip working at a 20 kHz frequency. Ultra sonic waves were passed into the solution in 5 s pulses of 1 s each.…”

Nanoscale Metal‐Organic Frameworks: Synthesis, Biocompatibility, Imaging Applications, and Thermal and Dynamic Therapy of Tumors

“…Many researchers have pointed out that the effect of ultrasound on chemical reactions originated from acoustic cavitation within collapsing bubbles. This acoustic cavitation could generate localized hot spots, thus induce an exceedingly high transient temperature (5000 K), pressure (1800 atm), and cooling rate (1010 K s −1 ) to improve the activity of reactants . In comparison to solvothermal method, this reaction is milder and faster, even in a few dozen minutes .…”