Acceleration of Synthetically Useful Heterogeneous Reactions Using

Chapter 13

Acceleration of Synthetically Useful Heterogeneous Reactions Using Ultrasonic Waves Philip Boudjouk Downloaded by UNIV OF MONTANA on February 25, 2016 | http://pubs.acs.org Publication Date: February 26, 1987 | doi: 10.1021/bk-1987-0333.ch013

Department of Chemistry, North Dakota State University, Fargo, ND 58105

Studies of the effects of low frequency ultrasonic waves on a broad range of synthetically useful reactions are summarized. Discussion is centered on the results obtained in our laboratory where we have concentrated on the reactions of metals with functionalized organic and organometallic compounds. Special emphasis is on lithium and zinc with organic and organosilicon halides. Catalytic systems (platinum, palladium and nickel) have been investigated as have been non-metallic reagents. Our results in these areas are also presented. The pioneering work on the chemical applications of ultrasound was conducted i n the 1920 s by Richards and Loomis i n their c l a s s i c survey of the e f f e c t s of high frequency sound waves on a variety of solutions, s o l i d s and pure liquids(_l). Ultrasonic waves are u s u a l l y defined as those sound waves with a frequency of 20 kHz or higher. The human ear i s most s e n s i t i v e to frequencies i n the 1-5 kHz range with upper and lower l i m i t s of 0.3 and 20 kHz, r e s p e c t i v e l y . A brief but useful general treatment of the theory and applications of ultrasound has been given by Cracknel 1(2). f

Early Studies of Chemical Effects of Ultrasonic Waves Using quartz c r y s t a l s of 6 to 12 m i l l i m e t e r s i n thickness and 50 to 80 m i l l i m e t e r s i n diameter held between two electrodes under o i l , Loomis and his coworkers produced high intensity u l t r a s o n i c waves at frequencies i n excess of 100,000 c y c l e s per second. The high i n t e n s i t i e s are the r e s u l t of the a b i l i t y to generate voltages i n the range of 50,000 with t h i s configuration since the amplitude of v i b r a t i o n of quartz c r y s t a l increases d i r e c t l y with the voltage applied to i t . With power l e v e l s of approximately 2 kilowatts r e a d i l y a v a i l a b l e , i n v e s t i g a t i o n of the e f f e c t s of u l t r a s o n i c waves on chemical systems became f e a s i b l e . Table I summarizes their observations. In spite of the d i v e r s i t y of the chemical e f f e c t s of sonic waves discovered by Richards and Loomis, basic research over the next forty years was sparse and uneven. For the most part, the

0097-6156/87/0333-0209$06.00/0 © 1987 American Chemical Society

In High-Energy Processes in Organometallic Chemistry; Suslick, Kenneth S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

210

HIGH-ENERGY PROCESSES IN ORGANOMETALLIC CHEMISTRY

emphases have been on inorganic reactions, i n p a r t i c u l a r aqueous solutions, and i n nearly a l l cases the systems were homogeneous. Table I. F i r s t Observations of the Chemical E f f e c t s of U l t r a s o n i c Waves

Effects of Ultrasonic Waves

Downloaded by UNIV OF MONTANA on February 25, 2016 | http://pubs.acs.org Publication Date: February 26, 1987 | doi: 10.1021/bk-1987-0333.ch013

System

Nitrogen t r i o d i d e

Rate of explosion

Superheated l i q u i d s

Rate of evaporation

Conversion of yellow Hgl to red Hgl

Rate of conversion

"Atomization" and s o l i d s

Rate of "atomization" of mercury and glass i n water accelerated

of l i q u i d s

accelerated accelerated accelerated

Gases d i s s o l v e d i n water

Rate of expulsion of gases increased

Hydrolysis of methyl s u l f a t e

Rate of hydrolysis increased

The "iodine clock" reaction

Rate of reduction of iodate by s u l f i t e was increased

B o i l i n g points of l i q u i d s

"Apparent" depression of b o i l i n g point

Most studies of heterogeneous systems were i n the applied areas(3). That the sonicators used by the various research groups were often home-built and quite d i f f e r e n t i n configuration and d e l i v e r e d d i f f e r e n t frequencies, i n t e n s i t i e s and wattages may have served to l i m i t study i n the f i e l d . To complicate the matter further, l i t t l e was known of the r o l e of these v a r i a b l e s i n a f f e c t i n g reaction rates. As a r e s u l t , there are few generalizations upon which a new investigator can r e l y and sonochemistry, i n p a r t i c u l a r preparative sonochemistry, must be considered an area that i s i n i t s e a r l i e s t stages and i n need of much further study. A d e t a i l e d review of the l i t e r a t u r e i s outside the scope of t h i s chapter. However, a b r i e f survey of some of the key developments i n the a p p l i c a t i o n of u l t r a s o n i c waves to heterogeneous reactions seems appropriate as an introduction to our work. There were some studies of heterogeneous reactions using u l t r a s o n i c waves i n the four decades following the survey of Richards and Loomis. The r e s u l t s were encouraging and c l e a r l y held promise f o r synthetic chemists. In retrospect, i t i s surprising they did not a t t r a c t more attention. Table II l i s t s some of the reactions investigated.


13.

Acceleration of Useful Heterogeneous Reactions

BOUDJOUK

211

Table I I . U l t r a s o n i c Acceleration of Some Heterogeneous Reactions

REACTION

Zn +

2HC1


R-X

—---—>

r^YV^i a

ZnCl

— DJ~>

+ Mg

YEAR

H

Na

+

)))

CH S(0)CH 3

C H -C1 6

1950(5)

+

+

3

>

5

NaH

—--

1933(4)

2

R-Mg-X

>

l^Jl^^J^J

+

C H5 6

>

N a

+

Na

3

C

H

+

2

1966(8)

0

R C-C-CR 2

2

+

Hg/HgOAc

—--

>

R C-C-CR 2

Br

1978(9)

2

H

OAc

Recent Synthetic Applications of Ultrasound The report by Luche and coworkers that u l t r a s o n i c waves from a common u l t r a s o n i c laboratory cleaner aid the formation of organolithium and Grignard reagents and a l s o improve the Barbier reaction spurred much of the current i n t e r e s t i n the synthetic applications of ultrasound(lO):

R-Br

Li

_)_)_) 1 h

->

RLi (61-95%)

R = Pr, η-Bu, Ph

R-Br

+

f

R C0 2

+ Li —

>

R^RCOH

R = a l k y l , a r y l , benzyl, a l l y l ,

N

L 5 ( 13 9 > 1957(7)

CH S(0)CH " Na

0

Br

+

2

(76-100%) vinyl

f

R C0 = ketones and aldehydes 2

Luche and coworkers extended their studies on the a p p l i c a t i o n s of ultrasound to synthesis to include a variety of systems. Among these a p p l i c a t i o n s are: the syntheses of lithioorganocuprates(ll), of aldehydes from formamides(12), of organozinc intermediates(13),


1

9

5

1

^

212


and of h o m o a l l y l i c alcohols(14). The l a s t development i s p a r t i c u l a r l y noteworthy because organozinc intermediates are employed i n aqueous media, Ishikawa has found the combination of zinc and ultrasound to be p a r t i c u l a r l y useful i n the synthesis of a variety of perfluorinated derivatives(15). Our entry into sonochemistry was spurred by our need for high y i e l d preparations of symmetrical organics and bimetal l i e s . Our f i r s t efforts(16,17) with l i t h i u m wire were s a t i s f a c t o r y but have since been greatly improved by using of l i t h i u m dispersion(18):

2 RoMCl Downloaded by UNIV OF MONTANA on February 25, 2016 | http://pubs.acs.org Publication Date: February 26, 1987 | doi: 10.1021/bk-1987-0333.ch013

J

+ L i ------> RoM-MRo

Ph-C = C L i

+

Mel

-> Ph-C = C-Me

as w e l l as the reductive coupling of diphenylacetylene to form the s y n t h e t i c a l l y useful dilithiotetraphenylbutadienedianion(19): Ph Ph-CsC-Ph + L i

> Ph-C=EC-Ph L i

+

Ph

c_c' // \\ x

Ph-C Li

+

C-Ph L i


+

13.

BOUDJOUK


213

Halides of the l e s s e l e c t r o p o s i t i v e metals are quickly reduced to highly dispersed and very a c t i v e metal powders i f they are exposed to u l t r a s o n i c waves i n the presence of l i t h i u m and other group I metals(20). Ultrasound not only accelerates the reduction of the h a l i d e s but a l s o increases the rate of subsequent reactions of these l e s s a c t i v e metals. These reactions are covered i n the chapter by K. Suslick.


Zinc Zinc promoted reactions are responsive to u l t r a s o n i c waves. The c l a s s i c Simmons-Smith reaction was among our f i r s t successes(21). Using zinc powder and diiodomethane we obtained good y i e l d s of cyclopropanes from styrene and acenaphthene. However, we could not

consistently control the reaction and i t would often get out of hand. Repic and Vogt solved t h i s problem by using mossy zinc with u l t r a s o n i c waves to obtain good y i e l d s of cyclopropanes from olefins(22). More recently F r i e d r i c h found that ultrasound w i l l a c t i v a t e a zinc-copper couple s u f f i c i e n t l y to permit the use of dibromomethane as a source of methylene(23). This method i s almost as e f f i c i e n t as using f r e s h l y prepared zinc powder(24). We f e l t the Reformatsky reaction was a worthwhile target because i t i s the most generally a p p l i c a b l e method for converting aldehydes and ketones to ff-hydroxyesters(25). The improvements i n y i e l d and reaction time exceeded our expectations. E s s e n t i a l l y quantitative conversion to the β-hydroxyester was effected i n a matter of a few minutes(26). The absence of other products, such as α, β-unsaturated esters, r e s u l t i n g from dehydration, and dimers of the bromo ester and the carbonyl are probably the r e s u l t of running R CO 2

+ BrCH C0 Et 2

2

+ Zn

--^_>

R C(OH)CH C0 Et 2

2

2

(94-100%)

R C0 = a l k y l and a r y l aldehydes and ketones 2

the reaction at, e f f e c t i v e l y , room temperature. An interesting modification of t h i s reaction, i n which the ketone was replaced by an imine, leading to a very mild high y i e l d synthesis of /^-lactams has recently been published(27). Reactions of c h l o r o s i l a n e s with carbonyl compounds i n the presence of zinc benefit from i r r a d i a t i o n with ultrasonic waves. «-Dicarbonyls are converted to bis-siloxyalkenes i n very good yields i n less than t h i r t y minutes(28). In the absence of


214


0 0 R-C—C-R ' +

\\\ MeoSiCl

+

Zn

— - - - — >

MeoSiO pSiMeo C= C R R

(60-90%)

N

u l t r a s o n i c waves not even two or three hours of s t i r r i n g w i l l produce the same y i e l d s . The reactions of simple ketones and aldehydes are even more interesting because, with the appropriate s t o i c h i ometry, the carbonyl can not only be s i l y l a t e d , but be persuaded to couple to another carbonyl group to form bis-siloxyalkanes(29): R' Downloaded by UNIV OF MONTANA on February 25, 2016 | http://pubs.acs.org Publication Date: February 26, 1987 | doi: 10.1021/bk-1987-0333.ch013

RCOR' + MeoSiCl + Zn

— - - - — >

R'

MeoSiO^-C^OSiMeo

/

\

Some of our results are summarized i n Table I I I . Table I I I . Reductive Coupling of Carbonyls with Zinc and Trimethylchlorosilane

R

R

£-MeO-Ph £-F-Ph Ph _p_-Me-Ph Ph Ph

f

Me Me Ph Me Me H

Carbonyl : Me SiCl 3

: Zn;

t(h)

)))

8 6 6 2 2 4

74 63 84 76 65 56

Yield % Stir 54 48 65 63 55 42

1:1:5

Benzaldehyde gave modest y i e l d s of the coupled product and 15% of trans-stilbene. By using large excesses of zinc and t r i m e t h y l c h l o r o s i l a n e , the s t i l b e n e y i e l d was increased to 36%. Thus f a r , only 1-indanone has produced high y i e l d s , 83%, of the unsaturated dimer(29):

Most of the carbonyls we have tested couple and undergo a p i n a c o l type rearrangement v i a a bis-siloxyalkane intermediate: Me 1

£-MeO-Ph-C-)Me SiO 3

0 ))) + MeoSiCl + Zn —---—> ( -MeOPh) C-C-Me 11

£

2

Me


(100%)

13.

BOUDJOUK

Reactive


215

Intermediates

When zinc and a,a -dibromo-o-xylene are irradiated with u l t r a s o n i c waves at room temperature, s y n t h e t i c a l l y useful quantities of the r e a c t i v e intermediate, £-xylylene, are generated which can be treated i n s i t u with activated o l e f i n s to give good y i e l d s of cycloaddition products(30). Chew and F e r r i e r used t h i s methodolgy to generate o_-xylylene for the synthesis of o p t i c a l l y pure functionalized hexahydroanthracenes(31). The reaction with lithium takes a d i f f e r e n t course(19). Rather than generate the o-xylylene intermediate, i o n i c species are produced. The two fates of a, a-dibromo-_o-xylene are presented i n the scheme below: T


f

Successes i n producing r e a c t i v e intermediates l i k e o-xylylene and carbene and i n preparing bimetal l i e s i n high y i e l d s using ultrasound led us to attempt to generate West*s novel compound, tetramesityldisilene the f i r s t example of a stable species with a s i l i c o n - s i l i c o n double bond(32). We prepared t h i s species i n one step and trapped i t with methanol(33). The d i s i l e n e i s r e a c t i v e towards lithium, however, and we have found i t very d i f f i c u l t to obtain consistent r e s u l t s . Most often, h e x a m e s i t y l c y c l o t r i s i l a n e i s i s o l a t e d i n very good yield(34).

Mes SiCl 2

2

))) + L i —---—>

MeOH Mes Si=SiMes 2

/ SiMeso / \ Mes Si SiMes

>

2

Mes Si—SiMes H OMe 2

2

•:SiMes

2

2

2

2

The c y c l o t r i s i l a n e i s one of a few c y c l o t r i s i l a n e s and i s , upon photolysis, a useful source of s i l y l e n e s and d i s i l e n e s . Another hindered s i l a n e , di-_t-butyldichlorodisilane, gives high y i e l d s of the r e a c t i v e d i v a l e n t species, d i - t - b u t y l s i l y l e n e , which was characterized by i t s insertion reactions into Si-Η bonds(35):



216 )))

t-Bu SiCl 2

R3S1H

+ Li —

2

> t-Bu Si: R


> R Si-(_t-Bu Si)-H

2

3

3

= E t , PhMe 3

2

2

When no s i l a n e i s used to trap the s i l y l e n e , the mild conditions of the reaction permitted the i s o l a t i o n of the novel ring compound, t r a n s - h e x a - t - b u t y l c y c l o t e t r a s i l a n e i n >15% yield(36). The successful trapping of d i - t - b u t y l s i l y l e n e from a d i c h l o r o s i l a n e suggested the p o s s i b i l i t y of a general reaction mechanism. We examined a v a r i e t y of d i h a l o s i l a n e s using u l t r a s o n i c waves and a l k a l i metals(37). The mild conditions permitted led to fewer products than are normally observed i n these reactions and the f i r s t evidence that s i l y l e n e s can be important intermediates i n metald i h a l o s i l a n e reactions i n s o l u t i o n . Our r e s u l t s for some dichlorosilanes reacting with lithium metal are summarized below.

RR SiCl f

2

+

Li R R R R R

+ R Si-H 3

= = = = =

—---—>

R = _t-Bu t - B u , R = Mes R = Mes t - B u , R' = Ph R = Me, E t , Ph f

f

f

f

RoSi-SiRR'-H 60% 35-40% 15-20%

R CH-CHR 2

2

(95-100%)

In the absence of u l t r a s o n i c waves, the reactions u s u a l l y require two or three hours or heating to 80°. Using our procedure, the cyclopropane ring i n cyclopropylbenzene was e a s i l y opened to give propylbenzene i n >95% y i e l d . Recently we extended t h i s study by replacing formic acid with a stream of hydrogen gas bubbling through the reaction mixture and found that t h i s reaction goes to completion even at 0°(45). A dozen o l e f i n s , including v i n y l ethers and a, β-unsaturated esters and ketones were q u a n t i t a t i v e l y hydrogenated i n one hour or l e s s at 0°. In the absence of u l t r a s o n i c waves, no hydrogénation occurs at t h i s temperature. Our i n t e r e s t i n s i l i c o n chemistry quite n a t u r a l l y l e d to a study of the h y d r o s i l a t i o n reaction, the addition of the Si-Η group across an o l e f i n or an acetylene. This reaction i s one of the most useful methods of making silicon-carbon bonds and i s an important i n d u s t r i a l process. T y p i c a l l y , homogeneous c a t a l y s t s based on platinum, rhodium or ruthenium are used, and while very e f f i c i e n t , they are not recoverable(46). The o r i g i n a l patent uses platinum as the c a t a l y s t and c a l l s for temperatures of 100-300° and pressures of 45-115 psi(47). We found that such rigorous conditions are not required for the h y d r o s i l a t i o n reaction with most commercial sources of platinum on carbon. U s u a l l y vigorous s t i r r i n g at s l i g h t l y elevated temperatures, 40-80°, at 15 p s i w i l l give moderate y i e l d s of the product. The rates and y i e l d s are u s u a l l y highly dependent on the method of preparation of the c a t a l y s t . However, ultrasound permits the reaction to occur at a useful rate at 30° at atmospheric pressure(48): R3S1H

+

C=C

+

·>

Pt/C

RoSi-C-C-H

(30-94%)

Recently we found that f r e s h l y prepared n i c k e l powder i s an e f f i c i e n t h y d r o s i l a t i o n c a t a l y s t when continuously irradiated(49).

Ni*

+

N

C=C

/

/

\

+

H-SiClo

— - - - — >

^

H-C-C-SiClo

(60-70%)

I I

In the absence of u l t r a s o n i c waves n i c k e l does not promote t h i s reaction at or near room temperature. Nonmetallic Heterogeneous Reactions Ultrasound has proven e f f e c t i v e i n promoting a few heterogeneous nonmetallic reactions. As e a r l y as 1933 Moriguchi noted that the reaction of calcium carbonate and s u l f u r i c acid was faster i n the presence of ultrasound(4). In the same year Szalay reported that u l t r a s o n i c waves depolymerized starch, gum arabic and gelatin(50). Examples of s y n t h e t i c a l l y useful a p p l i c a t i o n s are fewer than metal l i c systems but a c t i v i t y i n t h i s arena i s increasing.


218


The u l t r a s o n i c preparation of thioamides from amides and phosphorus pentasulfide "by Raucher(51) and of dichlorocarbene from chloroform and potassium hydroxide by Regen(52) are some of the more recent examples of nonmetallic applications. We were surprised to find that ultrasound greatly accelerates the reduction of haloaromat i c s by l i t h i u m aluminum hydride, permitting the reaction to be


R-Ar-X

+

LiAlH

—HI—y

4

R-Ar-H

(70-98%)

conducted at room temperature(53). This reaction requires, i n some cases, heating to 100° for s e v e r a l hours to obtain even modest y i e l d s of the reduced compounds(54). Lukevics and coworkers extended t h i s methodology to m e t a l l i c halides finding that reduction to metal hydrides was f e a s i b l e even i n nonpolar solvents(55). Brown has demonstrated that ultrasound accelerated some hydroboration reactions(56). Reactions i n v o l v i n g aluminum oxide(57) and potassium permanganate(58) have a l s o been enhanced by u l t r a s o n i c waves. Ultrasound a l s o promotes the reaction of potassium hydride with some s i l i c o n hydrides to give s i l y l anions i n e x c e l l e n t y i e l d s and

f

R R Si-H 2

+

KH

—---—>

f

R R Si:~ 2

K

+

(>90%)

R = Phenyl or v i n y l with a minimum of byproducts(59). This was not only useful for synthesis but a l s o was an important advantage i n obtaining clean spectroscopic data of p o t e n t i a l l y aromatic s i l y l anions l i k e the s i l a c y c l o p e n t a d i e n y l anion to demonstrate that i t i s not aromatic(59). Mechanistic

Considerations

Cavitation. To produce a chemical e f f e c t i n l i q u i d s using u l t r a sonic waves, s u f f i c i e n t energy must be imparted to the l i q u i d to cause c a v i t a t i o n , i.e., the formation and c o l l a p s e of bubbles i n the solvent medium and the consequent release of energy. When u l t r a sonic waves are passed through a medium, the p a r t i c l e s experience o s c i l l a t i o n s leading to regions of compression and rarefaction. Bubbles form i n the rarefaction region which may be f i l l e d with a gas, the vapor of the l i q u i d , or be almost empty depending on the pressure and the forces holding the l i q u i d together. S t r i c t l y defined, c a v i t a t i o n refers only to the completely evacuated bubble or c a v i t y , a true void, but since d i s s o l v e d gases are present unless s p e c i a l steps are taken to remove them, and the vapor of the l i q u i d can a l s o penetrate the c a v i t y , the term c a v i t a t i o n most often encompasses the three kinds of bubbles. The c o l l a p s e of these bubbles, caused by the compression region of the u l t r a s o n i c wave, produces powerful shock waves. The energy output i n the region of the c o l l a p s i n g bubble i s considerable, with estimates of 2-3000°C and pressures i n the 1-10 k i l o b a r range for time periods i n the nanosecond regime(60). In summary, the c a v i t a tion process generates a t r a n s i t o r y high energy environment.


13.

BOUDJOUK


219


Surface Damage and Reaction Rates. Erosion of surfaces r e s u l t i n g i n higher surface area and removal of i n h i b i t i n g impurities are two e f f e c t s of c a v i t a t i o n on s o l i d s i n l i q u i d media, both of which lead to increased reaction rates. The high temperatures and pressures are s u f f i c i e n t to deform and p i t metal surfaces (even cause l o c a l melting of some metals) and to fracture many nonmetallic s o l i d s , i n p a r t i c u l a r , b r i t t l e materials. Mass Transport. Cavitation improves mixing but, on a macroscopic scale, i t i s probably l e s s e f f e c t i v e than a high speed s t i r r e r . On a microscopic scale, however, mass transport i s improved at s o l i d surfaces i n motion as a r e s u l t of sound energy absorption. This e f f e c t i s c a l l e d acoustic streaming and contributes to increasing reaction rates. Experimental Considerations The u l t r a s o n i c cleaning bath i s the most common source of ultrasound in the laboratory and was the equipment used i n most of our investigations. The acoustic intensity i s far l e s s than the immer sion horn but the low price, l e s s than $200 for a 4" χ 9" bath that holds f l a s k s up to 1 l i t e r i n size, compared to nearly $2000 for a modest horn setup probably accounts for the difference i n popularity. The Ultrasonic Cleaning Bath. There i s l i t t l e question that the rate enhancements observed from i r r a d i a t i o n i n the bath would probably be greatly improved i f the horn were used. We have observed such differences and are making an e f f o r t to quantify them. On the other hand, the cleaning bath provides s u f f i c i e n t intensity to accelerate a wide variety of heterogeneous reactions s u f f i c i e n t l y to become very a t t r a c t i v e to the synthetic chemist. A healthy range of metals from the main groups and the t r a n s i t i o n series, including the t r a d i t i o n a l c a t a l y s t s , platinum and palladium, as w e l l as nonmetallic s o l i d s such as lithium aluminum hydride, potassium hydride and aluminum oxide, are e a s i l y activated at room temperature. There i s the a d d i t i o n a l advantage that the bath permits the use of t r a d i t i o n a l glassware. The l i f e t i m e s of the baths i n our laboratory are t y p i c a l l y >8000 hours. Location of the Reaction Flask. We found that i r r a d i a t i o n from the u l t r a s o n i c cleaner i s most e f f e c t i v e when the f l a s k i s positioned i n the bath to achieve maximum turbulence of the reagents. This "sweet spot" i s the point of maximum c a v i t a t i o n and assures optimum energy transfer to the reaction medium. In practice, t h i s f o c a l point of intensity may move after s e v e r a l hours, possibly because of d i s t o r tion of the s t e e l bottom caused by l o c a l heating of the transducer. Coupling Medium. D i s t i l l e d water has proven to be more e f f e c t i v e than tap water as the conducting l i q u i d as evidenced by greater c a v i t a t i o n i n the reaction f l a s k s (and faster reaction rates). Moreover, d i s t i l l e d water leads to s i g n i f i c a n t l y l e s s corrosion of the bath w a l l s . Other low vapor pressure l i q u i d s such as ethylene g l y c o l can be used.


220

H I G H - E N E R G Y PROCESSES IN O R G A N O M E T A L L I C CHEMISTRY

E f f e c t of Dissolved Gases. Dissolved gases play a major r o l e i n the optimizing of c a v i t a t i o n by providing nucleation s i t e s for i n c i p i e n t c a v i t i e s and by a f f e c t i n g the temperature of the c a v i t a t i o n a l c o l l a p s e . Gases with high polytropic r a t i o s , C /C , increase the temperature of the c a v i t a t i o n a l c o l l a p s e i n homogeneous systems (61). We have found that bubbling argon through the reaction mixture often improves the e f f e c t s of u l t r a s o n i c waves. y


S t i r r i n g . S t i r r i n g of the reagents w i l l not i n t e r f e r e with c a v i t a tion and may prove necessary for acceptable reaction rates. S o l i d s must not be allowed to c o l l e c t on the bottom of the f l a s k . I f the s o l i d loading i s high and c a v i t a t i o n cannot suspend a l l of i t , a paddle s t i r r e r should be used. Temperature. Temperature plays a key r o l e i n an unexpected fashion. Most often, lowering the temperature improves u l t r a s o n i c a l l y accelerated reaction rates. This i s attributed, i n the main, to a lowering of vapor pressure and consequently an emptier cavity(62). Implosion of highly evacuated c a v i t i e s i s more energetic than c o l l a p s e of vapor f i l l e d bubbles. Temperatures can be lowered by using slush baths as the conducting media i n the bath. Temperatures from 25° to -25° are easy to reach and maintain c a v i t a t i o n i n reaction f l a s k s containing ordinary organic solvents. A cooling fan mounted on the side of the bath w i l l maintain a temperature of 30 35° without a d d i t i o n a l cooling. Solvents. Solvents with low vapor pressure w i l l lead to c a v i t a t i o n a l implosions of greater energy and p o t e n t i a l l y faster reactions. Optimization of p o l a r i t y and vapor pressure w i l l l i k e l y reap the greatest benefits. Frequency and Intensity. Most u l t r a s o n i c baths operate i n the 30 80 kHz range. Frequency i s r a r e l y an important factor, provided the frequency i s low enough to permit c a v i t a t i o n . The c e l l disruptors normally adapted for sonochemical uses operate at 20 kHz. The intensity must be enough to produce c a v i t a t i o n . Beyond that, optimum i n t e n s i t i e s for heterogeneous reactions have not been determined. Disadvantages of the U l t r a s o n i c Bath. The major disadvantage of the bath as a source of u l t r a s o n i c waves i s the low acoustic intensity. This translates into l e s s than optimum reaction rates and, for some reluctant systems, no rate enhancements at a l l . Companion to t h i s i s v a r i a b i l i t y (often on a day-to-day basis) of the intensity and i t s f o c a l point, which makes precise rate measurements a formidable challenge. Ultrasonic baths are not e a s i l y adapted to flow synthesis as are immersion horns. ACKNOWLEDGMENTS This chapter i s , i n large part, a summary of the work of dedicated graduate students and postdoctoral f e l l o w s with whom I have been a p r i v i l e g e d coauthor and to whom I am deeply indebted. The generous support of t h i s research by the A i r Force Office of S c i e n t i f i c Research i s a l s o g r a t e f u l l y acknowledged.



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Acceleration of Synthetically Useful Heterogeneous Reactions Using

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