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C h a p t e r 4R e a c t i o n v e s s e l s a n d g l a s s w a r eIn many cases choice of the reaction vessel in which the synthesis under microwave condi-t ion is p lanned to be car r ied out d epends on the typ e of microwave cavi ty ( reactor) th atis available in the laboratory. In turn, as was mentioned in the Section 3.4, the choice ofmicrowave reactor ( i .e. , s ingle- or multi-mode system if i t is possible to choose betweenthem) depends on the vo lume of the r eac t ion mix tu re. I t can happen tha t in a mul t i -modereactor i t can be d i f f icul t to obtain h igh radiat ion in tens i ty (power dens i ty) wi th in smal lsamples, s ince usually the ca vity is mu ch larger in s ize th an the samp le. In such a case,the react ion mixture does not absorb enough energy to in i t ia te the react ion because onlya smal l par t of the microwave radiat ion is absorbed by the react ion m ixture w hen the res tof radiat ion is d iss ipated an d conver ted in to he at o n t i le wal ls of the cavity . On the o therhand, in s ingle-mode reactors that are character ized by much higher power dens i ty (Ta-ble 3.4) , the am ou nt of the reage nts is l imited by th e sm all size of such cavit ies, an d th ediam eter of the react ion vessels have to be equal or smal ler tha n half of the wavelength ,which for the frequency of 2.45 GH z is ca. 6 cm.

In this book, the reaction vessels most often prescribed for reaction procedures inmu lt imode reactors cons is ts of a rou nd-b ot tom ed f lask with one or two ver t ical necks th atcan be provided with an upr ight condenser , dropping funnel , neutra l gas in le t , and f iber-opt ical therm om eter . Dep ending on the type of addi t ional equipm ent , a l l of these p iecescan be connected with the f lask through a long-neck adapter ( tube) that goes through thespecially shielded inlet s i tua ted on the top of the m icrowave cav ity (Fig. 3.8) . I t mu st bes t ressed again th at such in le ts mu s t be shie lded by specially des igned m etal tube s in orderto prevent leakage f rom the microwave cavity , and unde r an y condi t ions the in lets cannotbe removed from microwave reactors . I t is unn ecessa ry and ev en possible to apply f laskswith more than two necks in mul t i -mode microwave reactors , s ince the upr ight condensercan be combined w ith the d ropping funnel and gas in le t (F ig . 4 .1) . Moreover , mos t of themultimode reactors are equipped with eff icient magnetic s t irrers as well as either f iber-opt ic or inf rared tem per atu re detect ions. In some cases , the react ion m ixture cons istsonly of solids and high bo il ing l iquids (e.g., ne at reactio n or solventless conditions) thu sit is not necessary to use an upright condenser; however, a long-neck adapter has to bealways p laced on the f lask to prevent both the react ion mixture f rom sparks generated in

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34 CHAPTER 4. REACTION VESSELS AND GLASSWARE

the microwave cavi ty as wel l as to protect the microwave cavi ty f rom vapors generatedwithin the reaction mixture (Fig. 4.1) (see also Section 6) .

trap

/ in e r t g a s n le t~ t - - - ~ ~re f lu xc o n d e n s e r ~ ' ~ '~

~ i De a n -S ta rk1 i

a b

Figure 4 .1 . Exper imental se t -up for react ions in mul t i -mode microwave reactors .

Al th oug h in a num ber of publ icat ions on microwave-assis ted synthes is the use of g lass -ware made of quar tz is s t rongly advocated because of i t s t ransparency to microwaveir radiat ion , in laboratory pract ice quar tz vessels can be subs t i tu ted with less f ragi le andchea per b orosil icate glass (e.g. , Pyre x) vessels w ith gro und glass joints . Since borosil icateglassware is commonly used in laboratory pract ice , i t can be expected that any react ionperforme d u nder a tmosp her ic pressure wi th the usage of typical labora tory g lass com-ponents is easy to conf igure and t ransfer to work ins ide mul t imode microwave reactors(cavit ies) . All the rea ctions take place inside the ca vity so the c hem istry is processedsafely and securely inside i ts s teel walls . I t is mu ch more im po rta nt to avoid the use ofmetal c lamps and mater ia l that s t rongly absorb microwave energy to connect g lasswarewith in the microwave cavity. P ieces of me tal can cause arching and sparking in the cavi tywhile p ieces of ma ter ia l (e.g. , b lack rubbe r) t ha t absorb microwaves can be dama ged andeven catch f i re . Thus , Tef lon jo in ts are more advocated than, for example, rubber jo in ts(see Sect ion 6) . Moreover , typical magnet ic s t i r bars that are usual ly coated with Tef loncan be used so as not to cause any problem in the vessels in t roduced in to the microwavereactors provided that the reactor is equipped with a magnet ic s t i r rer .

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React ions under p ressur ized condi t ions are no t i l lus t ra ted in th is book , a l though thereare a num ber o f p ressur ized reac t ion vessels ma de of ceramic and p las t ic ma ter ia ls p rov idedby the m anufac tures o f microwave equipmen t ( see Section 3 .4) . These vesse ls should no tbe rep laced und er any condi t ion s w i th o ther vesse ls and cannot be swi tched be tweendifferent type of microwave reactors ei ther .

For reac t ions tha t a re p lanned to be car r ied ou t in s ing le-mode reac tors there i s ava i l -ab le a g rea t number o f d i f feren t ly shaped vesse ls usual ly p rov ided by the producers o fmicrowave equip me nts (see Section 3.4) . The se vessels, which can b e also ma de of borosil i -care g lass as was me nt ioned in the prev ious pa ragrap h , have d if feren t d iam eters d ependingon the i r in tended vo lume (Fig . 4 .2 ) .

Figure 4 .2 . React ion vesse ls and adp aters for reac t ions in s ing le-mode microwave reac tors(a) - 60 c m 3 , (b) - 25 c m 3 , ( c ) - 90 cm a, (d) - 30 c m 3.

In order to absorb as much of microwave energy as poss ib le , the vo lume of the reac-t ion vesse ls should be la rge enough to keep the en t i re sample of the mater ia l wi th in themicrowave cav i ty bu t a lso smal l enough to no t keep the reac t ion mix ture as on ly a layera t the b o t t om of the reac t ion vesse l (Fig . 4 .3a) . From th e exper ience of the au thor o f thebook , the bes t resu l t s can be ach ieved i f the par t o f the reac t ion vesse l immersed in themicrowave cavity is f rom 30 to 75% fi l led with the reaction mixture (Fig. 4 .3c) .

Simi lar ly to the reac t ion in mul t i -m ode cav i ties , a long-neck adap ter ( tube) can beplaced on top of the reaction vessel in order to connect i t with addit ional pieces of equip-ments such as a cold f inger , dropping funnel , neutral gas inlet , and f iber-optical ther-mom eter . However, in the opposi te to m ul t i -mo de reac tors , the reac t ion f lask togetherwi th long-neck adapter i s p laced addi t ional ly in a quar tz cy l inder to p revent the reac torf rom dam age in case of any breakage of the reac t ion f lask . Since accord ing to some de-s igns of s ing le-mode microwave reac tors , the reac t ion vesse ls a re ro ta te d in o rder to ob ta in

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36 CHAPTER 4. REACTION VESSELS AND GLASSWARE

Figu re 4 .3 . Volume of reac t ion m ix tures d ur ing expe r imen ts in s ing le-mode reac to rs : (a ) -vo lum e of the re ac t ion vesse l is too b ig , (b ) - vo lume of the reac t ion vesse l i s to smal l ,(c) - volume of the react ion vessel i s correct .

F igure 4 .4 . Exper imenta l se t -up fo r reac t ions in s ing le-mode microwave reac to rs .

more un i fo rm i r rad ia t ion a t the reac t ion vesse l s , i t i s recommended to use a co ld f ingerr a t h e r t h an an u p r i g h t co n d en s e r b ecau s e o f g rea t e r s i mp l ic i ty o f s u ch a s e tu p . N ev e r t h e -less, an upr igh t conden ser can be used i f the r eac to r is equ ip ped wi th a m agn et ic s t i r re r .Som e typ ica l se tup s fo r the reac t ions w i th the use o f s ing le-mode sys tem s ( rea c to rs ) a rei l lus t ra ted in Figure 4 .4 .

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Fo r b o t h t y p e o f mi c ro w av e r eac to r s , if t h e r ea c t o r i s n o t s u p p l i ed w i t h a t em p er a t u res en s o r o r mo re l i k e l y accu ra t e t emp era t u re meas u rmen t i s p r e req u i s i t ed d u r i n g an ex p e r -i men t , t h e f i b e r -o p t ic t em p er a t u r e s en so r is d i r ec t ly ap p l i ed t o t h e r eac t i o n m i x t u re . Inorder to secure the sensor f rom harsh chemical s , the sensor i s inser ted in to a cap i l l a ry tha tin tu rn i s inser ted in to the re ac t ion m ix ture . In such a case, i t i s s t rong ly advo cated tou se cap il la r ie s t h a t a r e ma d e o f q u a r t z g la s s an d a re t r an s p a ren t t o mi c ro w av e i r r ad ia t i o n .Any cap i l l a ry tha t i s made of g lass o r even boros i l i ca te g lass can a lways s l igh t ly absorbmicrowave energy , in par t i cu lar , whi le the reac t ion mix ture does no t absorb microwaveseff ic ien tly, and in tu rn l ead to fa i lu res o f f iber -op ti c the rm om ete r per fo rm ance .

However , such reac t ions are no t exempl i f i ed in th i s book . There i s a l so a g rea t numberof s ing le-mode reac t ion sys tem s w i th a u to m ate d feeder o f the rea c t ion vesse l s p rov idedb y t h e mi c ro w av e eq u i p m en t man u fac t u re s , b u t t h e d es c r i p t io n o f t h es e s y s t ems i s o u t o fthe scope o f th i s book . Th e d escr ip t ion o f such sys tem can be found in recen t ly pub l i shedbooks [32,48].