Jean-Louis Luche and the Interpretation of Sonochemical Reaction Mechanisms

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Article Jean-Louis Luche and the Interpretation of Sonochemical Reaction Mechanisms

Mircea Vinatoru 1,* and Timothy J. Mason 2

1 Faculty of Applied Chemistry and Material Science, University “Politehnica” of Bucharest, 1-7 Gh. Polizu, 011061 Bucharest, Romania 2 Faculty of Health and Life Sciences, Coventry University, Priory Street, Coventry CV1 5FB, UK; [email protected] * Correspondence: [email protected] or [email protected]

Abstract: Sonochemistry can be broadly defined as the science of chemical and physical transformations produced under the influence of sound. The use of sound energy is rather a young branch of chemistry and does not have the clear definitive rules of other, more established, divisions such as those in cycloaddition reactions or photochemistry. Nevertheless, there are a few guidelines which can help to predict what is going to happen when a reaction mixture is submitted to ultrasonic irradiation. Jean-Louis Luche, formulated some ideas of the mechanistic pathways involved in sonochemistry more than 30 years ago. He introduced the idea of “true” and “false” sonochemical reactions both of which are the result of acoustic cavitation. The difference was that the former involved a free radical component whereas only mechanical effects played a role the latter. The authors of this paper were scientific collaborators and friends of Jean-Louis Luche during those early years and had the chance to discuss and work with him on the mechanisms of sonochemistry. In this paper we will review the original rules (laws) as predicted by Jean-Louis Luche and how they have been further developed and extended in recent years.

Citation: Vinatoru, M.; Mason, T.J. Keywords: Jean-Louis Luche; sonochemistry; electron transfer; ordering effect Jean-Louis Luche and the Interpretation of Sonochemical Reaction Mechanisms. Molecules 2021, 26, 755. https://doi.org/10.3390/ 1. Early Attempts at Understanding Sonochemistry molecules26030755 In the 1980’s while based in Grenoble at Joseph Fourier University Jean Louis Luche began his investigations into the effects of ultrasound on chemical reactions and later Academic Editor: Gregory Chatel started to formulate the rules governing this branch of science [1]. Together with oth- Received: 6 January 2021 ers working in the field it was generally accepted that sonochemistry was the result of Accepted: 27 January 2021 acoustic cavitation. This had been clearly identified in a paper published in 1956 entitled Published: 1 February 2021 “Chemical Effects of Ultrasonics—‘Hot Spot’ Chemistry” [2]. The abstract of this paper claimed that “Experimental evidence is presented which supports the conclusions that Publisher’s Note: MDPI stays neutral chemical processes brought about by ultrasonics require cavitation”. When the general with regard to jurisdictional claims in published maps and institutional affil- subjects of power ultrasound and chemistry became unified into sonochemistry some iations. of the comments about it suggested that acoustic cavitation might be just a method of achieving efficient mixing. Using ultrasound we could achieve excellent emulsification and dispersion. Acoustic cavitation could also be used for surface cleaning to keep catalysts active or electrochemical reactions more efficient. Such was the case in the early 1990s and many chemists simply viewed the subject as something that was difficult to reproduce Copyright: © 2021 by the authors. and therefore something of a “Black Art” [3]. Many experimentalists found it difficult to Licensee MDPI, Basel, Switzerland. reproduce the work of other groups in their own laboratories. These difficulties could be This article is an open access article distributed under the terms and traced to a failure of the original report to specify the exact sonication conditions, e.g., the conditions of the Creative Commons frequency of ultrasonic irradiation, the precise power entering the reaction system, the Attribution (CC BY) license (https:// geometry of the reaction vessel, the presence of a bubbled gas or even the temperature creativecommons.org/licenses/by/ of the reaction. But these were in a sense “teething troubles” in a subject which has now 4.0/). achieved international acceptance. An experimental protocol was established including

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even such apparently trivial (but in fact very important) parameters such as the positioning a reaction vessel in a precise position in an ultrasonic cleaning bath. Once the possibilities of different types of ultrasonic systems had been recognized then even in those early years the potential for adoption by industry became apparent [4]. Then attention turned to more fundamental questions about sonochemistry such as what type of reaction was likely to be most affected by sonication and which ones were not. In the beginning for some of us working in the field this question seemed unimportant since it was clear that any reaction involving surface chemistry would almost certainly be enhanced by the application of ultrasound. Most of these could be ascribed to the mechanical effects of cavitation bubble collapse. In 1987, Lorimer and Mason [5] and then Lindley and Mason [6] published consecutive papers in Royal Society of Chemistry Chemi- cal Reviews relating to the physical principles and synthetic aspects of sonochemistry that containing a total of 368 references. Although these articles did not identify specific rules governing sonochemistry an attempt was made to group reactions into four types based on the main interests of a synthetic chemist. The effects of ultrasound were summarized in terms of four different types of reaction. • Reactions involving metal surfaces: divided into those in which the metal is a reagent and is consumed in the process and those in which the metal functions as a catalyst. • Reactions involving powders or other particulate matter: the overall reactivity, will depend upon particle size reduction producing greater reactive surface area. and simultaneous surface activation. • Emulsion reactions: ultrasound is known to generate extremely fine emulsions with a dramatic increase in the interfacial contact area between the liquids. • Homogeneous reactions: in order to explain sonoluminescence or the fragmentation of alkanes must involve more than simply the mechanical effects of cavitation. They may also involve temporary effects on solvent structure resulting in changes to the solvation of reactive species. Generic classifications such as these helped a chemist to appreciate more fully the results which were emerging involving electrochemistry in plating and synthesis [7,8], electro-analysis [9] and the increasingly important fields of organometallic chemistry [10], phase transfer catalysis [11] together with polymer formation and degradation [12,13]. A recent review on the in situ coupling of ultrasound to electro- and photo-deposition methods identified the latter as a relatively unexplored area of sonochemistry involving free radicals [14]. However, the majority of interest in sonochemistry in the years leading up to 1990 was in organic synthesis [6,15]. In 1986, Tim Mason organized the first ever meeting devoted to sonochemistry as part of the Autumn Meeting of the Royal Society of Chemistry held at Warwick University UK. This meeting brought together many of the prominent researchers in the field at that time and it was the first occasion when Tim Mason met Jean-Louis Luche. In the years following that meeting Jean-Louis began to take a particular interest in the underlying rules of sonochemistry. In 1989 he published a review entitled “Sonochemistry—the use of ultrasonic waves in synthetic organic chemistry” which contained 235 references [16]. In the conclusions to that article he wrote about the problems which could delay general applications of this new science and proposed that these would be the result of an incomplete understanding of the underlying theory. Cavitation appeared to explain many things but there were areas that could not be the result of just cavitation. He quoted two examples: one was the almost quantitative yield of adamantanes from the super acid catalyzed rear- rangements of some polycyclic hydrocarbons [17] and the other was some work by another pioneer of sonochemistry, Jacques Berlan, involving the reaction of organometallics with a substrate in the solid state [18]. The final paragraph in the Luche review reproduced here in the exact language that he used read: “To overcome these problems more fundamental research is necessary. In parallel, a satisfactory theoretical understanding will be found more easily by the multiplication of Molecules 2021, 26, x FOR PEER REVIEW 3 of 10

Molecules 2021, 26, 755 3 of 10 “To overcome these problems more fundamental research is necessary. In parallel, a satisfactory theoretical understanding will be found more easily by the multiplication of newnew examples. examples. Thus, Thus, the the final final word word of of this this conc conclusionlusion that that we we can address to chemists is bothboth encouraging encouraging and and short: TRY”. TRY”. TheThe first first paper paper which which had had provided provided evidence evidence for for the the specific specific nature nature of of sonochemistry sonochemistry was published byby AndoAndo et et al. al. who who described described the the first first case case of “sonochemicalof “sonochemical switching” switching” [19]. [19].The originalThe original system system which which led toled this to this discovery discovery (Scheme (Scheme1) consisted 1) consisted of a of suspension a suspension of ofbenzyl benzyl bromide bromide and and alumina-supported alumina-supported potassium potassium cyanide cyanide in toluene. in toluene. The stirredThe stirred reaction re- actionprovided provided regioisomeric regioisomeric diphenylmethane diphenylmethane products products via a Friedel-Craftsvia a Friedel-Crafts reaction reaction between be- tweenthe bromo the bromo compound compound and the and solvent, the solvent, catalyzed catalyzed by the by solid the solid phase phase reagent. reagent. In contrast, In contrast,sonication sonication of the of same the constituentssame constituents furnished furnished only the only substitution the substitution product, product, benzyl cyanide. benzyl cyanide.According According to the authors, to the authors, a structural a structural change onchange the catalytic on the catalytic sites of thesites solid of the support solid supportcould be could responsible be responsible for this effect. for this This effect. type Th ofis result type showsof result that shows sonication that sonication is definitely is definitelynot just another not just method another of method providing of providing agitation in agitation a medium. in a Thismedium. paper This provided paper apro- key videdpiece ofa key information piece of information for those seeking for those to formulateseeking tothe formulate rules of the sonochemistry. rules of sonochemistry.

CH2 Me chani cal agitation CH3

CH2Br+ CH3 + KCN + Al 2O3

Ultrasonic irradiation CH2CN

SchemeScheme 1. 1. Ando’sAndo’s sonochemical sonochemical switch switch re reactionaction reported in 1984 [19]. [19].

InIn 1990, 1990, Jean-Louis Jean-Louis published published consecutive consecutive papers papers in in Tetrahedron Tetrahedron Letters in which he beganbegan to to formulate his ideas about mechanismsmechanisms in sonochemistry [[20,21].20,21]. Sonochemical reactionsreactions were were to be divided into three types: • •Type 1Type as a 1 consequence as a consequence of coordinative of coordinative unsaturated unsaturated or free or free radicals radicals generated generated by by cavitation,cavitation, in inhomogenous homogenous media—true media—true sonochemistry sonochemistry • •Type 2 Type caused 2 caused by only by the only mechanical the mechanical effects of effects sonic of waves sonic are waves operative are operative in heteroge- in heterogenousnous media—false media—false sonochemistry sonochemistry • •Type 3 Type “true” 3 “true” sonochemical sonochemical reactions reactions in which in awhich single a electron single electron transfer transfer is involved is inin volveda keyin a step. key step. InIn the the following following year, year, 1991, 1991, Mircea Mircea Vinat Vinatoruoru visited Tim Mason in Coventry for a one-dayone-day course course on on sonochemistry sonochemistry and following this he went on to visit Jean-Louis Luche who was was working working in in Grenoble Grenoble at at that that time. time. This This was was his hisfirst first meeting meeting with with Jean-Louis Jean-Louis and theand three the three days days spent spent there there allowed allowed plenty plenty of oftime time for for discussions discussions about about sonochemistry. sonochemistry. TogetherTogether they travelled by ca carr to attend the second ESS m meetingeeting in Gargnano, on Lake GardaGarda in in Italy Italy in in September. September. This This was was when when Mircea Mircea became became more more seriously seriously committed committed to sonochemistry.to sonochemistry. NowNow the the history of the development of so sonochemistrynochemistry progresses to the European UnionUnion COST COST programmes. programmes. COST COST is is the the acronym acronym for for "Cooperation "Cooperation in in Science Science and and Tech- Tech- nology"nology" an an organization organization founded founded in in 1971 1971 by by the the European European Community Community to to promote promote inter- inter- EuropeanEuropean cooperation in any field field of scientific scientific and technological research. For many years chemistrychemistry had had not not been been a a part part of of COST COST but but the the Management Management Committee Committee of of the the COST COST Pro- Pro- grammegramme met met in in Brussels Brussels on on December 9 December 9, 1992 and made the decision to include chemistry forfor the the first first time. time. This This was was particularly particularly exciti excitingng news news for for Jean-Louis Jean-Louis and and in in a a FAX FAX sent to TimTim Mason on January 12th, 1993 about this meeting he wrote: “For“For the the first first time, time, a a COST COST programme programme is is specifically specifically devoted devoted to to Chemistry, Chemistry, particu- particu- larlylarly Chemistry Chemistry under under unusual unusual and and extreme extreme conditions. conditions. Under Under this this heading, heading, we we find find high high pressurepressure and highhigh temperaturetemperature chemistries, chemistries, microwaves, microwaves, plasma plasma chemistry, chemistry, reactions reactions in the in absence of solvent, and, of course, our common point of interest: Sonochemistry. This is the reason of this letter. During the meeting in Brussels, it appeared clearly that sonochemistry

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is rather advanced in comparison to other new chemistries. We can envisage the creation of networks involving a multilateral cooperation in various aspects of sonochemistry”. The overall programme was designated as D6 and within it two networks devoted to sonochemistry were created. Jean-Louis took charge of project number D6/0008/93 “Organic Sonochemistry, Mechanisms and Synthetic Applications” together with G. De- scotes Univ. Villeurbanne, S. Toma Univ. Bratislava, R. Miethchen Univ. Rostock, H. Fillion Univ. Lyon, A. Campos Neves Univ. Coimbra and J. Jurczak Academy of Sciences Warszaw. The second network involved Tim Mason in project number D6/0007/93 “Fundamental in Sonochemistry” was led by C. Petrier Univ. de Savoie, and also included J. Reisse Univ. Libre de Bruxelles, T. Lepoint Inst. Meurice Chimie Bruxelles, J. Berlan ENSIGN Toulouse, H. Delmas ENSIGN Toulouse and G. Portenlanger TU München. In 1996 an important text was produced from that D6 programme entitled “Chemistry Under Extreme or non-Classical Conditions”. It was edited by Rudi van Eldik and Colin D Hubbard and included chapters written by participants in the various sections of COST DOne of the chapters “Ultrasound as a new Tool for Synthetic Chemists” was co-authored by Jean-Louis Luche and Tim Mason [22]. In it the three rules governing sonochemistry were written in the following terms: Rule 1 applies to homogeneous processes and states that those reactions which are sensitive to the sonochemical effect are those which proceed via radical or radical-ion intermediates. This statement means that sonication is able to effect reactions proceeding through radicals and that ionic reactions are not likely to be modiﬁed by such irradiation. Rule 2 applies to heterogeneous systems where a more complex situation occurs and here reactions proceeding via ionic intermediates can be stimulated by the mechanical effects of cavitational agitation. This has been termed “false sonochemistry” although many industrialists would argue that the term “false” may not be correct, because if the result of ultrasonic irradiation assists a reaction it should still be considered to be assisted by sonication and thus “sonochemical”. In fact, the true test for “false sonochemistry” is that similar results should, in principle, be obtained using an efﬁcient mixing system in place of sonication. Such a comparison is not always possible. Rule 3 applies to heterogeneous reactions with mixed mechanisms, i.e., radical and ionic. These will have their radical component enhanced by sonication although the general mechanical effect from Rule 2 may still apply. Two situations which may occur in heterogeneous systems involving two mechanistic paths are (a) When the two mechanisms lead to the same product(s), which we will term a “convergent” process, only an overall rate increase results, (b) If the radical and ionic mechanisms lead to different products, then sonochemical switching can take place by enhancing the radical pathway only. In such “divergent” processes, the nature of the reaction products is actually changed by sonication. Several other examples were summarized nearly 20 years later in a paper by Cravotto and Cintas [23] in which electron transfer promoted by ultrasound was associated with sonochemical reactions. The authors made a comment in the paper which perhaps sums up neatly the state of play at that time: “If one asks any synthetic sonochemist about the possibility of obtaining distinctive results, other than under conventional conditions, by applying ultrasound to a chemical reaction, he or she will probably give you the same answer: perhaps.” To this day, apart from the foundations laid down by Jean-Louis Luche as outlined above there have been no further rules brought in that govern sonochemistry to compare with those long-established in photochemistry and thermochemistry. However, the search goes on . . .

2. Explanations for Sonochemical Reactions Based on Electron Transfer If there are no further rules that can be identiﬁed at this stage in the development of sonochemistry then perhaps the way forward is to identify some well documented explanations for sonochemical effects. If these “explanations” are consistent then they might themselves lead on to some new guidelines for sonochemistry. Molecules 2021, 26, x FOR PEER REVIEW 5 of 10

If there are no further rules that can be identified at this stage in the development of sonochemistry then perhaps the way forward is to identify some well documented explanations for sonochemical effects. If these “explanations” are consistent then they might themselves lead on to some new guidelines for sonochemistry. Let us return to Ando’s reaction ‘the sonochemical switch’, the authors claimed that Molecules 2021, 26, 755 potassium cyanide decreases the catalytic activity of alumina [19]. Why is that?5 of 10No plausible explanation was offered. Using a simple ultrasonic cleaning bath, 45 kHz, 200 W power at 50 °C the authors showed that ultrasonic irradiation completely switched the course of the reaction. They also made the very important observation that if there was no Let us return to Ando’s reaction ‘the sonochemical switch’, the authors claimed KCNthat potassiumadded to the cyanide reaction decreases then the the Friedel-Cr catalytic activityafts reaction of alumina occurs [19 under]. Why both is that? mechanical agitationNo plausible and explanation ultrasonic wasirradiation. offered. UsingIt wo auld simple therefore ultrasonic seem cleaning entirely bath, reasonable—as 45 kHz, a suggestion—that200 W power at 50 ultrasound◦C the authors has showed in some that way ultrasonic “assisted irradiation the contact completely of potassium switched cyanide withthe course alumina of the to reaction.decrease They the alsocatalytic made ability the very of important alumina”. observation that if there was no KCNHowever, added to there the reaction is an alternative then the Friedel-Crafts explanation reaction which occurs is that under alumina both mechanical increases the nu- cleophilicagitation and character ultrasonic of CN irradiation.−. In 1979, Itwould Regen therefore and co-workers seem entirely published reasonable—as a paper a entitled “Impregnatedsuggestion—that cyanide ultrasound reagents. has in someConvenient way “assisted synthesis the contact of nitriles” of potassium [24]. cyanideThis was later with alumina to decrease the catalytic ability of alumina”. broadened to other nucleophiles to show that neutral alumina could act as a tri-phase However, there is an alternative explanation which is that alumina increases the − − − catalystnucleophilic not characteronly for ofCN CN but−. In also 1979, for Regen I , Cl and and co-workers acetate in published the displacement a paper entitled of bromide from“Impregnated 1-bromooctane cyanide [25]. reagents. These Convenient studies showed synthesis that of the nitriles” nucleophile [24]. This needed was to later be in inti- matebroadened physical to other contact nucleophiles with alumina to show and that that neutral this could alumina be couldachieved act asby a heating tri-phase up to 90 °Ccatalyst but there not only was for no CN ultrasound− but also foremployed. I−, Cl− and In acetatethe ultrasonic in the displacement switching ofexperiment bromide how- everfrom the 1-bromooctane experiments [25 with]. These and studieswithout showed ultrasound that thewere nucleophile performed needed at 50 to°C be and in no nu- intimate physical contact with alumina and that this could be achieved by heating up cleophilic◦ substitution was observed in the silent stirred reaction. This indicates that ul- trasoundto 90 C but must there have was had no ultrasounda specific effect employed. on CN In− theon ultrasonicthe surface switching of the alumina. experiment however the experiments with and without ultrasound were performed at 50 ◦C and no The impregnation of alumina with cyanide salt requires only three steps, mixing the nucleophilic substitution was observed in the silent stirred reaction. This indicates that aqueousultrasound cyanide must have salt hadwith a alumina, specific effect filtration on CN and− on then the surface drying of of the the alumina. cyanide impregnated aluminaThe impregnation(4 h, 110 °C, of0.05 alumina mmHg) with [24]. cyanide However, salt requires the drying only three procedure steps, mixing cannot the result in theaqueous total cyanideremoval salt of withwater alumina, which filtration has a great and thenaffinity drying to alumina. of the cyanide HO impregnatedgroups can still be detectedalumina (4on h, an 110 alumina◦C, 0.05 surface mmHg) even [24]. after However, being the dried drying at 1000 procedure °C [26]. cannot Thus, result it is reasonable in tothe consider total removal that there of water will which be some has water a great on affinity the alumina to alumina. surface HO during groups this can stillsonochemical be ◦ switchdetected reaction on an alumina especially surface since even the after alumina being dried was at used 1000 justC[26 as]. Thus,supplied it is reasonableby the manufac- turerto consider i.e. not that dried. there Under will be these some conditions water on the the alumina ionic surfacestructure during of KCN this sonochemicaland the availability switch reaction especially since the alumina was used just as supplied by the manufacturer of alumina to offer electrons to support KCN ionization may result in the reaction taking i.e. not dried. Under these conditions the ionic structure of KCN and the availability of placealumina via toelectron offer electrons transfer to from support the cyanide KCN ionization ion to benzyl may result bromide. in the Electron reaction takingtransfer reac- tionsplace are via well electron established transfer fromin organic the cyanide chemistry ion to and benzyl can bromide.be involved Electron in nucleophilic transfer dis- placementreactions are reactions well established [27]. Single in organic electron chemistry transfer and (SET) can be reactions involved were in nucleophilic included in the ideasdisplacement of Luche reactions who considered [27]. Single that electron any transferreactions (SET) in reactionswhich a wereradical included or SET in mechanism the couldideas ofbe Luche clearly who identified considered should that anyprovide reactions useful in examples which a radical of the or efficiency SET mechanism of sonochem- icalcould enhancement be clearly identified [22]. should provide useful examples of the efficiency of sonochemical enhancementAmong examples [22]. of such reactions which are also examples of sonochemical switch- Among examples of such reactions which are also examples of sonochemical switching ing is the KornbIum-Russell reaction (Scheme 2) [28]. is the KornbIum-Russell reaction (Scheme2)[28].

O2N Br + NO2 )))) O2N Li NO2

O2N

SchemeScheme 2. KornbIum-RussellKornbIum-Russell reaction. reaction.

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4-NitrobenzyI bromide reacts with 2-lithio-2-nitropropane via a predominantly polar 4-NitrobenzyI4-NitrobenzyI bromide reacts with 2-lithio-2-nitropropane 2-lithio-2-nitropropane via a predominantly predominantly polar mechanism to give, as a final product, 4-nitrobenzaldehyde. In contrast, the SET pathway mechanismmechanism to give, give, as as a a final final product, 4-nitrobenzaldehyde. In contrast, the SET pathway leads to a dinitro compound. Sonication changes the normal course of the reaction and leadsleads toto aa dinitro dinitro compound. compound. Sonication Sonication changes changes the the normal normal course course of the of reaction the reaction and gives and gives preferentially the latter compound, in amounts depending on the irradiation condi- givespreferentially preferentially the latter the latter compound, compound, in amounts in amounts depending depending on the on irradiation the irradiation conditions conditions and the acoustic intensity. Under optimal conditions of irradiation, the ratio of C/O tionsand theand acoustic the acoustic intensity. intensity. Under Under optimal optimal conditions conditions of of irradiation, irradiation, the the ratio ratio of of C/OC/O alkylation is practically reversed with respect to that of the silent reaction, indicating a alkylationalkylation is is practically practically reversed reversed with with respect respect to that of the silent reaction, indicating a direct intervention of sonic waves in the electron transfer process. directdirect intervention of sonic waves in the electron transfer process. A further example of switching is to be found in another paper from the Ando group AA further further example example of switching switching is to to be be found found in in another another paper paper from from the Ando Ando group describing the reaction of lead tetraacetate with styrene [29]. The addition may follow ei- describingdescribing the reactionreaction ofof lead lead tetraacetate tetraacetate with with styrene styrene [29 [29].]. The The addition addition may may follow follow either either an ionic or a radical pathway, affording the products depicted in Scheme 3. In the theran ionic an ionic or a radicalor a radical pathway, pathway, affording affording the products the products depicted depicted in Scheme in Scheme3. In the 3. former In the former case, the lead reagent adds to styrene to give a gem-diacetate via a carbocation, formercase, the case, lead the reagent lead reagent adds to adds styrene to tostyrene give ato gem-diacetate give a gem-diacetate via a carbocation, via a carbocation, whereas whereasthewhereas first stage thethe fi offirstrst the stagestage radical ofof the pathwaythe radicalradical consists pathwaypathway of the consistsconsists decomposition ofof thethe decompositiondecomposition of lead tetraacetate ofof leadlead to tetraacetategivetetraacetate the methyl toto givegive radical, thethe methylwhichmethyl then radical,radical, reacts which which with styrene. thenthen reactsreacts This withwith was styrene. furtherstyrene. investigated ThisThis waswas furtherfurther some investigatedyearsinvestigated later by some some a team years years involving later later by by both a a team team Ando, involving involving Luche both andboth TimAndo, Ando, Mason Luche Luche [30 and and]. Tim Tim Mason Mason [30]. [30].

Ph Pb(OAc) Ph CH3 Ph OAc Ph OAc Ph Pb(OAc) 4 Ph CH3 Ph OAc Ph OAc 4 + ++ + AcOH AcO AcO AcOH AcO AcO OAcOAc

stirring,stirring, 50 50 °C, °C, 1 1 h h 0 0 0 0 5 5

ultrasound,ultrasound, 50 50 °C, °C, 1 1 h h 38 38 12 12 3 3

Scheme 3. The reaction of lead tetraacetate with styrene [29]. SchemeScheme 3. The 3. The reaction reaction of lead of lead tetraacetate tetraacetate with with styrene styrene [29]. [29 ].

TheTheThe idea ideaidea that thatthat sonication sonication promotes promotes electron electron transfer transfer was was was certainly certainly certainly part part part of of of the the the mecha- mecha- mech- nisticanisticnistic ideas ideas ideas of of ofJean-Louis Jean-Louis Jean-Louis Luche Luche Luche and and and supporte supporte supporteddd by by many bymany many others. others. others. We We propose Wepropose propose however however however that that therethatthere there isis aa possibleispossible a possible nuancenuance nuance toto this tothis this generalgeneral general classificationclassification classification basedbased based onon on thethe the ideasideas ideas suggested suggested by byby EbersonEbersonEberson in inin 1982 19821982 that thatthat there therethere might might be be more moremore than than one one type type of of electron electron transfer transfer mechanism mechanism in in organicorganicorganic chemistry chemistry [31]. [[31].31]. MuchMuchMuch of ofof the thethe original originaloriginal groundwork groundworkgroundwork on onon electron electronelectron transfer transfer came came from from studies studies of of inorganic inorganicinorganic chemistrychemistrychemistry and and formalized formalized formalized by by by the the the 1983 1983 1983 Nobel Nobel Nobel Prize Prize Prize winner winner winner in in chemistry inchemistry chemistry Henry Henry Henry Taube. Taube. Taube. He He hadHehad had aa particularparticular a particular interestinterest interest inin thethe in mechanismsmechanisms the mechanisms ofof electron-transferelectron-transfer of electron-transfer reactionsreactions reactions inin metalmetal in metal com-com- complexes [32]. For the organic chemist the descriptions of electron transfers as applied plexesplexes [32].[32]. ForFor thethe organicorganic chemistchemist thethe descridescriptionsptions ofof electronelectron transferstransfers asas appliedapplied toto to inorganic complexes are rather complicated but Eberson simplified the concept by inorganicinorganic complexescomplexes areare ratherrather complicatedcomplicated butbut EbersonEberson simplifiedsimplified thethe conceptconcept byby pro-pro- proposing that the idea of electrons being transferred one by one was somewhat at odds posingposing that that the the idea idea of of electrons electrons being being transferred transferred one one by by one one was was somewhat somewhat at at odds odds with with with the two-electron centered electronic theory of organic molecules [31]. He described thethe two-electrontwo-electron centeredcentered electronicelectronic theorytheory ofof organicorganic moleculesmolecules [31].[31]. HeHe describeddescribed twotwo two different mechanisms that he identified as inner and outer sphere: differentdifferent mechanisms mechanisms that that he he iden identifiedtified as as inner inner and and outer outer sphere: sphere: Outer Sphere. This type of electron transfer in organic chemistry involves electron OuterOuter SphereSphere. . ThisThis typetype ofof electronelectron transfertransfer inin organicorganic chemistrychemistry involvesinvolves electronelectron movement through space between molecules which do not associate through any form of movementmovement through through space space between between molecules molecules which which do do not not associate associate through through any any form form of of partial bond formation (Scheme4). partialpartial bond bond formation formation (Scheme (Scheme 4). 4).

Scheme 4. Schematic model of outer-sphere electron transfer. SchemeScheme 4. SchematicSchematic model model of of outer-sphere outer-sphere electron transfer. Inner Sphere InnerInner Sphere Sphere.. .When WhenWhen the thethe two twotwo molecules moleculesmolecules form formform a aa temporary temporarytemporary bond, bond,bond, effectively effectivelyeffectively a aa bridge, bridge,bridge, through which the electron could travel from one molecule to another (as in aromatic throughthrough which which the the electron electron could could travel travel from from one one molecule molecule to to another another (as (as in in aromatic aromatic ni- ni- nitration (Scheme5). trationtration (Scheme (Scheme 5). 5).

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SchemeScheme 5. 5. SchematicSchematic model model of of inner-sphere inner-sphere electron electron transfer.transfer.

UsingUsing thisthis concept,concept,concept, it itit is isis possible possiblepossible to toto advance advanceadvance a theoryaa theorytheory in whichinin whichwhich sonication sonicationsonication might mightmight induce in-in- duceeitherduce eithereither a reaction aa reactionreaction switch switchswitch or a reaction oror aa reactionreaction acceleration accelerationacceleration depending dependingdepending on the type onon thethe of electrontypetype ofof electronelectron transfer transferinvolved.transfer involved.involved. This idea ThisThis was ideaidea ﬁrst waswas suggested firstfirst suggestedsuggested by Mircea byby VinatoruMirceaMircea VinatoruVinatoru in a paper inin a publisheda paperpaper publishedpublished in 1994 inaboutin 19941994 the aboutabout reaction thethe reactionreaction between betweenbetween nitrobenzene nitrobenzenitrobenze with triphenylchloromethanenene withwith triphenylchloromethanetriphenylchloromethane [33]. The [33].[33]. reaction TheThe reactionledreaction to several ledled toto products severalseveral products whichproducts he which suggestedwhich hehe suggestedsuggested arose from arosearose a switch fromfrom of aa mechanism switchswitch ofof mechanismmechanism from outer- fromtofrom inner-sphere outer-outer- toto inner-sphereinner-sphere electron transfer. electronelectron This transfer.transfer. would ThisThis involve wouldwould less involveinvolve energy lessless than energyenergy electron thanthan electron transferelectron transferbetweentransfer betweenbetween discreet molecules—outerdiscreetdiscreet molecules—outermolecules—outer sphere electron spspherehere transfer—whichelectronelectron transfer—whichtransfer—which is the more isis the commonthe moremore commonpathwaycommon pathway inpathway organic inin chemistry. organicorganic chemistry.chemistry. NowNow letlet usus re-considerre-consider re-consider thethe the casecase case ofof of thethe the sonosono sonochemicalchemicalchemical switchswitch switch fromfrom from Friedel-CraftsFriedel-Crafts Friedel-Crafts reac-reac- re- tionactiontion toto to nucleophilicnucleophilic nucleophilic displacementdisplacement displacement (Scheme(Scheme (Scheme 1)1)1 )inin in termsterms terms ofof of twotwo types types of ofof electron electron transfer.transfer. TheThe “normal” “normal” course course of of the the reaction reaction (in (in thethe absenceabsence ofof ultrasound)ultrasound) proceedsproceeds viavia aluminaalumina interactioninteractioninteraction with withwith benzyl benzylbenzyl bromide bromidebromide causing causingcausing a aa polarization polarizationpolarization of ofof the thethe carbon carbon bromine bromine bond bond then then thethethe benzyl benzyl positivepositivepositive part partpart attacks attacksattacks toluene toluenetoluene (present (prese(presentnt in inin excess excessexcess as asas the thethe solvent) solvent)solvent) resulting resultingresulting a mixture aa mix-mix- tureoftureO -ofof and OO-- P andand-benzyltoluene. PP-benzyltoluene.-benzyltoluene. However, However,However, in the in presencein thethe presencepresence of ultrasound ofof ultrasoundultrasound the polarized thethe polarizedpolarized carbon carbonbrominecarbon brominebromine bond undergoes bondbond undergoesundergoes an inner-sphere anan inner-sphereinner-sphere electron electron transferelectron astransfertransfer shown asas in shownshown Scheme inin6 SchemefollowedScheme 66 by radical combination of the resulting benzyl and cyanide radicals. followedfollowed byby radicalradical combinationcombination ofof thethe resultingresulting benzylbenzyl andand cyanidecyanide radicals.radicals.

SchemeScheme 6. 6. PotentialPotential ET ET mechanism mechanism for for Ando’s Ando’s switch switch reaction. reaction.

TheThe authorsauthors authors wouldwould would likelike like toto to emphasizeemphasize emphasize thatthat that wewe supportwesupport support thethe overalloverall the overall ideaidea ofof idea “true”“true” of “true” sono-sono- chemistrysonochemistrychemistry asas identifiedidentified as identified byby LucheLuche by Luche involvinginvolving involving electronelectron electron transfertransfer transfer reactions.reactions. reactions. WeWe havehave We simplysimply have addedsimplyadded aa added secondsecond a stepsecondstep inin the stepthe typetype in the ofof typeelectronelectron of electron transfertransfer transfer involvedinvolved involved toto provideprovide to provide aa possiblepossible a possible differ-differ- entiationdifferentiationentiation betweenbetween between electronelectron electron transfertransfer transfer typestypes as typesas shownshown as shown inin SchemeScheme in Scheme ThisThis waswas This presentedpresented was presented byby M.M. VinatorubyVinatoru M. Vinatoru inin aa talktalk in entitled aentitled talk entitled “Exploring“Exploring “Exploring AlAlternativeternative Alternative MechanismsMechanisms Mechanisms andand PathywaysPathyways and Pathyways inin Sono-Sono- in chemistry”Sonochemistry”chemistry” atat thethe at 4th the4th 4thAsia-OceaniaAsia-Oceania Asia-Oceania SonochemicalSonochemical Sonochemical SocietySociety Society ConferenceConference Conference heldheld held inin in Nanjing,Nanjing, ChinaChina in in The The scheme scheme suggests suggests a a possible possible use use of of ultrasound ultrasound as as tool tool to to identify identify the the type type ofof electronelectron transfer transfer involved involved in in a a sonochemical sonochemical reaction reaction depending depending on on whether whether itit involvesinvolves aa switch in products or a rate acceleration. switchswitch inin productsproducts oror aa raterate acceleration.acceleration. 3. Sonochemistry in the Absence of Cavitation 3.3. SonochemistrySonochemistry inin thethe AbsenceAbsence ofof CavitationCavitation Thus far, it is generally accepted that sonochemistry is the result of the effects of Thus far, it is generally accepted that sonochemistry is the result of the effects of acousticThus cavitation. far, it is generally This was firstaccepted clearly that stated sono inchemistry a paper published is the result in 1956 of the introducing effects of acoustic cavitation. This was first clearly stated in a paper published in 1956 introducing whatacoustic the cavitation. authors called This “hot-spot was first chemistry”clearly stated and in in a whichpaper theypublished wrote in that 1956 “experimental introducing whatevidencewhat thethe authorsauthors is presented calledcalled which “hot-spot“hot-spot supports chemistry”chemistry” the conclusions andand inin whichwhich that theythey chemical wrotewrote that processesthat “experimental“experimental brought evidenceaboutevidence by is ultrasonicsis presentedpresented require whichwhich supports cavitation”supports thethe [2 co].conclusionsnclusions However, thatthat there chemicalchemical are some processesprocesses examples broughtbrought in the aboutliteratureabout byby ultrasonicsultrasonics which are more requirerequire difficult cavitation”cavitation” to rationalize [2].[2]. However,However, in terms therethere of simple areare somesome cavitation examplesexamples collapse. inin thethe lit-lit- eratureeratureIn which Maywhich 1994, areare moremore Tim Masondifficultdifficult wrote toto rationalizerationalize to Jean-Louis inin termsterms at C.N.R.S.ofof simplesimple Universite cavitationcavitation collapse. Paulcollapse. Sabatier “HereInIn in MayMay Coventry 1994,1994, Tim weTim will MasonMason work wrotewrote with Mirceatoto Jean-LouisJean-Louis Vinatoru atat onC.N.R.S.C.N.R.S. the ordering UniversiteUniversite effect PaulPaul and Sabatier electronSabatier “Heretransfer“Here inin type CoventryCoventry reactions. wewe willwill He work sayswork he withwith is also MirceaMircea working VinatoruVinatoru with onon you. thethe No orderingordering problems effecteffect for andand me electron butelectron I do transfernottransfer want typetype to be reactions.reactions. either competing HeHe sayssays withhehe isis youalsoalso or workingworking duplicating withwith some you.you. No studies”.No problemsproblems Later, forfor in meme September butbut II dodo notthatnot wantwant same toto year bebe either Mirceaeither competingcompeting Vinatoru presented withwith youyou aoror paper duplicatingduplicating at a COST somesome meeting studies”.studies”. that Later,Later, followed inin Septem-Septem- the 4th bermeetingber thatthat samesame of the yearyear European MirceaMircea Society VinatoruVinatoru of Sonochemistry presentpresenteded aa paperpaper (ESS4) atat aa held COSTCOST in Blankenberg,meetingmeeting thatthat followed Belgium.followed theHethe discussed4th4th meetingmeeting a number ofof thethe European ofEuropean unusual SocietySociety observations ofof SonochemistrySonochemistry in sonochemistry (ESS4)(ESS4) including heldheld inin the Blankenberg,Blankenberg, suggestion Belgium.thatBelgium. ultrasound HeHe discusseddiscussed can induce aa numbernumber order ofof inunusualunusual liquids. observationsobservations At the time inin sonochemistry thissonochemistry was a new includingincluding concept the forthe suggestionsuggestion thatthat ultrasoundultrasound cancan induceinduce orderorder inin liquids.liquids. AtAt thethe timetime thisthis waswas aa newnew conceptconcept

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sonochemistryfor sonochemistry and and received received a mixed a mixed reception reception although although Jean-Louis Jean-Louis Luche Luche was was amongamong thosethose supportingsupporting thethe ideas.ideas. In 1998 Jean-Louis co-authored aa paperpaper withwith MirceaMircea VinatoruVinatoru onon thethe sonochemicalsonochemical and and thermal thermal redox redox reactions reactions of of triphenylmethane triphenylmethane and and triphenylmethyl triphenylme- carbinolthyl carbinol in nitrobenzene in nitrobenzene [34]. [34]. Although Although the sonochemical the sonochemical reactions reactions were were in low in low yield yield the resultsthe results supported supported the ideathe idea that that sonication sonication promotes promotes electron electron transfer. transfer. The subjectsubject ofof ultrasonicultrasonic orderingordering ofof liquidsliquids laylay dormantdormant forfor somesome yearsyears untiluntil atat thethe EleventhEleventh MeetingMeeting of of the the European European Society Society of Sonochemistry,of Sonochemistry, held held at La at Grande-Motte La Grande-Motte near Montpellier,near Montpellier, France France in June in 2008 June Tim 2008 Mason Tim Mason gave a gave plenary a plenary address address entitled entitled Recent TrendsRecent inTrends Sonochemistry in Sonochemistry and at end and of at the end address of the a presentedddress presented one slide one to remindslide to theremind audience the au- of thedience ideas of ofthe Vinatoru ideas of (SchemeVinatoru7 ).(Scheme 7).

SchemeScheme 7.7. Slide presented atat ESS11ESS11 illustratingillustrating non-cavitationnon-cavitation effectseffects inin sonochemistry.sonochemistry.

Some years later in 2014, the ESS 14 in Avignon was dedicated to the memory of Jean- LouisLouis LucheLuche who hadhad passedpassed awayaway onon MarchMarch 24th24th ofof thatthat yearyear agedaged 7373 [[1].1]. AsAs outgoingoutgoing presidentpresident TimTim MasonMason gavegave aa presentationpresentation entitledentitled “Some“Some neglectedneglected oror rejectedrejected pathspaths inin sonochemistry—Asonochemistry—A very personal view” [[35].35]. In the conclusionconclusion to thisthis paperpaper hehe wrotewrote “I“I wouldwould likelike toto thinkthink that that Jean-Louis Jean-Louis Luche Luche would would have have given given serious serious consideration consideration to allto all of theseof these neglected neglected or rejectedor rejected paths paths and and other other strange strange or inexplicable or inexplicable aspects aspects of sonochemistry. of sonochem- Certainly,istry. Certainly, I do notI do believenot believe that that we we as sonochemists,as sonochemists, whatever whatever our our particular particular scientiﬁcscientific leaning,leaning, shouldshould discountdiscount anyany newnew ideasideas inin ourour subjectsubject untiluntil theythey havehave beenbeen thoroughlythoroughly triedtried andand tested”.tested”. This waswas thethe drivingdriving forceforce behind behind the the appearance appearance of of a a new new type type of of paper paper in in Ultrasonics Ultrason- Sonochemistryics Sonochemistry a so-called a so-called discussion discussion paper. paper. In In this this Vinatoru Vinatoru and and Mason Mason hypothesizedhypothesized thatthat sonochemicalsonochemical reactionsreactions maymay occuroccur eveneven inin thethe absenceabsence ofof cavitationcavitation [[36].36]. TheThe ideaidea ofof suchsuch aa paper is, as its name suggests,suggests, to introduce a topic forfor discussion and further work. InIn thethe conclusionsconclusions thethe followingfollowing waswas writtenwritten “Under“Under thethe conditionsconditions outlinedoutlined aboveabove thethe authorsauthors believebelieve thatthat itit isis notnot alwaysalways necessarynecessary toto reachreach thethe cavitationcavitation thresholdthreshold toto induceinduce sonochemical reactions. In other words, sonochemical reactions could occur in the absence sonochemical reactions. In other words, sonochemical reactions could occur in the absence of cavitation. This hypothesis brings with it the need for a new range of experimental of cavitation. This hypothesis brings with it the need for a new range of experimental sonochemistry targeted at shedding light on the idea of an “ordering effect”. Naturally we sonochemistry targeted at shedding light on the idea of an “ordering effect”. Naturally would be happy to see some corroboration of our ideas but, as true scientists, we would we would be happy to see some corroboration of our ideas but, as true scientists, we also be prepared to accept results which disprove it”. would also be prepared to accept results which disprove it”. It was therefore a pleasure to ﬁnd that in 2020 Yeo et al from RMIT University in It was therefore a pleasure to find that in 2020 Yeo et al from RMIT University in Melbourne Australia published a review article entitled “High Frequency Sonoprocessing: Melbourne Australia published a review article entitled “High Frequency Sonopro- A New Field of Cavitation-Free Acoustic Materials Synthesis, Processing, and Manipula- cessing: A New Field of Cavitation-Free Acoustic Materials Synthesis, Processing, and tion” [37]. In this review using much higher frequencies than normally associated with sonochemistry evidence is provided for sonoprocessing in the absence of cavitation.

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4. Final Comments Mechanisms in sonochemistry are not yet fully rationalized although the original observations that radical rather than ionic reactions are affected remain valid. Additional ideas have been proposed in terms of different types of electron transfer mechanism and transformations that occur under the influence of ultrasound but without cavitation. Overall, we could draw an analogy with photochemistry in that the first law of photochemistry states that light must be absorbed for photochemistry to occur. There is certainly a parallel first law of sonochemistry—sound must be absorbed for sonochemistry to occur. Maybe—as we enter the realms of hypothesis—there might be two other laws that involve the types of electron transfer induced by ultrasound. A second law defined as ultrasound accelerates outer-sphere electron transfer reactions and even (although more hypothetical at this stage) a third law which suggests that a sonochemical switch in reaction pathways is possible if an inner-sphere electron transfer pathway is possible. Future investigations should be able to prove or disprove these two further laws, but certainly the first law will stay forever.

Author Contributions: Conceptualization, M.V. and T.J.M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest. Sample Availability: Samples of the compounds are not available from the authors.

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