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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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An ultrasound-promoted three-component reaction provides C3-cyanomethylated imidazo[1,2-a]pyridines under catalyst-free, oxidant-free, and mild conditions. A series of C3-cyanomethylated imidazo[1,2-a]pyridines were rapidly prepared with satisfactory yields and good functional group compatibility.
. Zhang, Y. Zhang, J. Zhang, Q. Wu, H. Yang, Synlett, 2022, 33, 264-268.

(E)-β-Iodo vinylsulfones are synthesized in very good yields under ultrasound irradiation using alkynes, sulfonyl hydrazides, potassium iodide and hydrogen peroxide. The key features of this protocol are the speed and efficiency of the reactions.
C. Zhou, X. Zeng, Synthesis, 2021, 53, 4614-4620.

In the presence of trichloroisocyanuric acid, triphenylphosphine, and sodium cyanamide, readily available carboxylic acids were converted into N-acylcyanamides in good to excellent yields within some minutes at room temperature under ultrasound irradiation. In addition, N-acyl-substituted imidazolones were readily accessible through guanylation-cyclization of in situ generated N-acylcyanamides.
W. Phakhodee, D. Yamano, M. Pattarawarapan, Synlett, 2020, 31, 703-707.

A simple, efficient, and general one-pot reaction of aldehydes and ketones with amines in the presence of indium(III) chloride as a catalyst provides α-amino phosphonates. Sonication accelerates the reaction.
B. C. Ranu, A. Hajra, U. Jana, Org. Lett., 1999, 1, 1141-1143.

An iodine(III)-catalyzed oxidative cyclization of 2-hydroxystilbenes using 10 mol% (diacetoxyiodo)benzene [PhI(OAc)2] as catalyst in the presence of m-chloroperbenzoic acid provides 2-arylbenzofurans in good to excellent yields.
F. V. Singh, S. R. Mangaonkar, Synthesis, 2018, 50, 4940-4948.

The application of sonochemistry for the synthesis of different coumarins from active methylene compounds and 2-hydroxybenzaldehydes or resorcinol was very effective on a multigram scale with a higher yield, higher amount of crystalline compound, and shorter reaction time compared with the compounds obtained using the classical procedures.
L. S. da Silveira Pinto, M. V. N. de Souza, Synthesis, 2017, 49, 2555-2561.

A transition-metal-free synthesis of a series of primary arylamines from potassium aryltrifluoroborates and phenylboronic acids uses hydroxylamine-O-sulfonic acid as a mild, inexpensive source of nitrogen in cooperation with aqueous sodium hydroxide in acetonitrile. Both a sonication and a microwave-assisted method were developed.
D. Kuik, J. A. McCubbin, G. K. Tranmer, Synthesis, 2017, 49, 2555-2561.

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