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The use of ultrasound in chemical reactions in solution provides specific activation based on a physical phenomenon: acoustic cavitation. Cavitation is a process in which mechanical activation destroys the attractive forces of molecules in the liquid phase. Applying ultrasound, compression of the liquid is followed by rarefaction (expansion), in which a sudden pressure drop forms small, oscillating bubbles of gaseous substances. These bubbles expand with each cycle of the applied ultrasonic energy until they reach an unstable size; they can then collide and/or violently collapse.

For example, sonolysis of Fe(CO)5 in decane under argon produces amorphous iron upon decarbonylation instead of crystalline iron, which shows that both very high temperatures and also rapid cooling rates  (~ 106 K s-1) are involved, the more volatile pentane yields Fe3(CO)12, indicating a somewhat slower collapse. It has been estimated and calculated that the pressure within a bubble in water can rise to more than one thousand atmospheres, and the temperature can reach several thousand degrees during a collapse, as heat conduction cannot keep up with the resulting adiabatic heating. As these bubbles are small and rapidly collapse, they can be seen as microreactors that offer the opportunity of speeding up certain reactions and also allow mechanistically novel reactions to take place in an absolutely safe manner.

Many reactions can be conducted even in a simple ultrasonic cleaning bath, although the amount of energy that reaches the reaction is only between 1 and 5 W cm-2, and temperature control is normally poor. Large-scale reactions can be better conducted using immersible ultrasonic probes that circumvent the transfer of the energy through water and the reaction vessel. The applied energies in this case can be several hundred times higher. Laboratory equipment uses frequencies between 20 kHz and 40 kHz, but cavitation can be generated well above these frequencies and recent research uses a much broader range.

Ultrasound in synthetic organic chemistry

There are two types of effects mediated by ultrasound: chemical and physical. When the quantity of bubbles is low - using standard laboratory equipment - it is mainly physical rate acceleration that plays a role. For example, a specific effect is the asymmetric collapse near a solid surface, which forms microjets. This effect is the reason why ultrasound is very effective in cleaning, and is also responsible for rate acceleration in multiphasic reactions, since surface cleaning and erosion lead to improved mass transport.

For example, when ultrasound is applied to an Ullmann reaction that normally requires a 10-fold excess of copper and 48 h of reaction time, this can be reduced to a 4-fold excess of copper and a reaction time of 10 h. The particle size of the copper shrinks from 87 to 25 μm, but the increase in the surface area cannot fully explain the increase in reactivity. It was suggested that sonication also assists in the breakdown of intermediates and desorption of the products from the surface.

Typically, ionic reactions are accelerated by physical effects - better mass transport - which is also called "False Sonochemistry". If the extreme conditions within the bubble lead to totally new reaction pathways, for example via radicals generated in the vapor phase that would only have a transient existence in the bulk liquid, we speak about "sonochemical switching". Such a switch has been observed for example in the following Kornblum-Russel reaction where sonication favors an SET pathway:

Applications for sonochemistry can be found in many areas, but sonochemical processes are most widely developed for heterogeneous reactions. Currently, sonochemistry is a multidisciplinary field in which chemists, physicists, chemical engineers and mathematicians must cooperate to develop a better understanding of the processes that take place within the collapsing bubbles to develop totally new applications. However, the potential for making improvements in many types of reaction suggests that every chemical laboratory should be equipped with at least one cleaning bath for simple trials. For a detailed discussion of Ultrasound in synthetic organic chemistry please refer to a review by T. J. Mason (Chem. Soc. Rev. 1997, 26, 443. DOI: 10.1039/CS9972600443).

Some ultrasound-promoted reactions can also be found in the recent literature section

Links of Interest

Ultrasonic Devices

Ultrasound Chemistry Highlights

Books on Sonochemistry

Practical Sonochemistry: Power Ultrasound Uses and Applications

Timothy J. Mason, Dietmar Peters
Softcover, 165 Pages
2nd Edition, 2003
ISBN: 978-1-898563-83-9 - Hoorwood Publishing

Recent Literature

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Ga(I)-catalyzed C-C bond formations between allyl or allenyl boronic esters and acetals, ketals, or aminals have proceeded in high yields with essentially complete regio- and chemoselectivity by using a catalytic amount of Ga(0), a Ag(I) cocatalyst, crownether ligation, and ultrasonic activation.
B. Qin, U. Schneider, J. Am. Chem. Soc., 2016, 138, 13119-13122.

A convenient and efficient sonochemical method for methyl esterification of carboxylic acids is catalyzed by polymer-supported triphenylphosphine in the presence of 2,4,6-trichloro-1,3,5-triazine and Na2CO3. Methyl esters of various carboxylic acids bearing reactive hydroxyl groups as well as acid- or base-labile functionalities could be rapidly prepared within short times in good to excellent yields in high purities without column chromatography.
S. Jaita, W. Phakhodee, M. Pattarawarapan, Synlett, 2015, 26, 2006-2008.

Indium-Catalyzed Henry-Type Reaction of Aldehydes with Bromonitroalkanes
R. G. Soengas, A. M. S. Silva, Synlett, 2012, 23, 873-876.

1-Propanephosphonic acid cyclic anhydride (T3P) promotes the synthesis of hydroxamic acids from carboxylic acids. Application of ultrasonication accelerates this conversion. Further, T3P has also been employed to activate the hydroxamates, leading to isocyanates via Lossen rearrangement. Trapping with suitable nucleophiles affords the corresponding ureas and carbamates.
B. Vasantha, H. P. Hemantha, V. V. Sureshbabu, Synthesis, 2010, 2990-2996.

β-Amino-α,β-unsaturated esters are produced by a sonochemical Blaise reaction of nitriles, zinc powder, zinc oxide and ethyl bromoacetate in THF in a commercial ultrasonic cleaning bath.
A. S.-Y. Lee, R.-Y. Cheng, Tetrahedron Lett., 1997, 38, 443-446.

A palladium-catalyzed and ultrasonic promoted Sonogashira coupling/1,3-dipolar cycloaddition of acid chlorides, terminal acetylenes, and sodium azide in one pot enables an efficient synthesis of 4,5-disubstituted-1,2,3-(NH)-triazoles in excellent yields.
J. Li, D. Wang, Y. Zhang, J. Li, B. Chen, Org. Lett., 2009, 11, 3024-3027.

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