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and the perforated plate b. A 5-g. q uan tity of CaSi (see p . 946 forprep arat ion ) is p oured from flask m (by rotating the flask in thejoint) into 300 ml. of HCl-saturated absolute alcohol, which coversthe glass f i l ter plate g. The proc ed ure must be ca rr ie d out in anatmosphere of dry CO3 with good a gita tion . The addition of CaSitak es about an ho ur. Good cooling m us t be prov ided. After standingovernight while the evolution of H s ce as es , the brown prec ipi ta teis separated in a CO3 a tmosphere from the supernatant l iquid.The pre cip i ta te is f i rst washed with ice-c old absolute a lcohol andthen with absolute e ther.

The pre pa rat io n of the higher un satu rated Si hyd rides may beaccom plished by cleaving the satura ted hyd rides with ele ctr ic ald i scha rge .

PROPERTIES:

(SiH)x: lemon-yel low subs tance . Hard, am orpho us; oxidizesslowly in air.

(SiHg)x: brown ish sub stance . The dry m ate ria l igni tes spon -taneously in air, leaving a SiO3 res idue whichm ay be gra y becauseof the presence of iron si l icide. Evolves Hswith alkal i hydroxides.

REFEREN ES

G. Schott and W. H errm an n, Z. ano rg. a l lg. Chem . 288, 1(1956); G. Schott and E. Hirsch m ann , Z. an org . al lg . Che m. 288, 9(1956); A. Stock and K. Som ieski, B er . dts ch . ch em . G es . 54, 524(1921); 56 , 247 (1923); R. Schw arz and F . He inrich , Z. an org .all g. Che m. 22 1, 277 (1935).

Silicon Tetrachloride

SiCl4

Si + 2C 12 = SiCl428.1 141.8 169.9

Silicon (prepared as shown above) or coarsely ground ferro-

silicon (which should contain as much Si as possible) is placed in aboat ins erte d into a Py re x tube about 60 cm . long and 2-3 cm . india m ete r, through which a st re am of C l3 is p as se d (Fig. 223). TheC l 3 i s pred r ied over concent ra ted H3SO4. A con de nse r is at tachedto the oth er end of the tube by m ea ns of an ad ap ter ; the tube itselfis heated in an el ec tri c furn ace . The tube should incline tow ard thecondenser to prevent the SiCl4 from backing up.

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furnace

6 8 3

Fig . 223 . Prepara t ion of s i l icon te t rach lor ide .

The co nd en ser end of the rea cti on tube should proje ct out ofthe furnace for som e d istan ce so that the inv ariab le byproductF e C l3 m ay dep osit without plugging the co nd ens er. The con-de ns er di sc ha rg es into a d istill ing flask which is set in an icebath . All join ts m us t be tight and the en tire ap pa ratu s carefullydr ied before the st a rt of the run . A CaCl 3 tube is attached at theend of the sid e a r m of the distillin g flask . If th is pre ca utio n isoverlook ed, the side a r m will im m edia tely be plugged with silicicacid produced by reaction with atmospheric moisture. The reactiontube is heated to about 400C and the Cl3 f low is then started.

If, in addition to SiCl4, the higher Si ch lor id es a re de sire d(SigClg and Si3C la) , th e te m p er at u re of the tube should be as lowas possible, but no lower than just below 400C. When the reactionis well estab lishe d, heating may be sh arply redu ced since thereactio n i tself evolves con siderab le heat . The crud e chlo r ide,which is accu m ulated in the distill ing flask, may be purified byfractional dis t i l lat ion. If an absolutely C l-free pre par at ion isde sire d, a second dist i l lat ion over Cu turnings is perfo rm ed.Both dis t i l lat ion s m ust be run in absolutely d ry equipment.The ampoules into which the SiCl4 is dis tilled should be fusedto the d istillatio n ap pa rat us , since it is im po ssible to obtain anon-turbid product if this is not done. Yield is quantitative.

PROPERTIES:Clear, color less l iquid; fumes heavily upon exposure to air.

Rapidly hy dro lyz es in w ate r to form a SiOa gel. M iscible withbenzene, e ther, ch loroform and sa tura ted hyd rocarbo ns . Fo rm se s te rs of silic ic ac id with alco ho ls. B.p. 57.5C, m .p.d 1.52.

68C;

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Higher Silicon Chlorides

If the preparation of SiCl4 i s car r ied out a t tempera tures be low400C, it is possible to isolate very small quantities of SigClg andSigCl8 from the re sid ue of the final dis t i l lat io n.

Chlorides up to SigCl 14 ar e be st p re pa re d by chlorinat ion ofcalcium si l ic id e. The pro ced ure is a s fol lows: C l3 , dr ied overconcentrated HgSO4, i s pa sse d through a ve rt ic al gla ss tube (34-mm.dia m ete r, about one m e te r long), half fi lled with be an -s iz e lum psof Ca silic id e (about 30-35% C a). The tube should not be too full,for then it might plug du ring the ru n; about 200-250 g. is use d. Thetube is placed inside a short , movable e le ctr ic heating coi l .It is im po rtant that the rea ctio n take plac e at the lowest possibletem pe ra tu re and that only a sho rt sec tion of the tube be heatedat any t im e. The rea ct io n sta r t s a t 250C. The tem pe ra tu rem ust then be im m edia tely lowered to 150C. The C lsf low shouldnot exceed 100 bu bb les/m inu te . The rea ct ion prod ucts a re ac cu -mu lated in a cooled re ce iv er via an at tached con den ser. After 12to 14 day s, during which the heating coil is slowly moved alongthe entire length of the tube, al l the si l icide is reacted, and about700 m l. of chlo ride m ixtu re is col lected . The higher chlo ridesa re obtained from thi s m ixt ure by fraction ation. If a low te m -

perature and a slow flow rate of Cls a r e us ed , then about 35% ofthe product mix ture boi l s a t a h igher te m pe ra tu re than SiCl4.About 30% of this isSi2 C l6 , 4% , S i3Cl8, and 1% re pr es en ts chlor idesup to Si6 C li 4 . After evapo ration of the SiC l4, the res idu e isfractionated at red uce d p re s s u re . Addition of 2 to 5% of analkal i chloride, a lkal ine earth chloride or ammonium chlorideor dilution of the chlorides (e.g., with SiCl4) should imp rov ethe yield.

PROPERTIES:

B.p. : Sis C l s , 147C; Si3C l8 , 216C; Si4 C l1 0 , 150C (at 15 mm.);S iBC l1 3, 190C (15 mm.); Sis C l 1 4 sublimes in vacuum at 200C.

Acc ording to R. Schwarz, the highe r Si ch lo rid es , such asSiioClss, can be prepared by treatment of SiCl4 in the apparatusil lu str ate d in Fig. 224, which the auth or ca lls a quenching tube.The procedure is as fol lows.

A sil icon carbide rod, held in place by two electrode connector

clam ps, is f it ted into a Liebig con den ser. The electro de clampsa re sealed into Py rex cap s, which fit over the gro un d-g las s malejoints of the condenser.

If the e lec tr ic al te rm in al s of the se c lam ps con sist of w ater-cooled copper tubing, then the seal to the glass caps is made withru bb er tubing. A clam p is ma de by clos ing one end of the coppertube, welding on a piec e of thin wall st ai n le ss ste el tube and

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1 2 SILICON AND GERMANIUM 6 8 5

Fig. 224. Pr ep ara t ion of the higher s i licon chlo r id es.

splitting the end of the la tte r w ith two lengthw ise saw cu ts. TheSiC rod fits into tha t end. The open end of the cop per tub e i s closedwith a cap which carries the inlet and outlet tubes for the coolingwater (the inlet tube extends well inside the closed tube).

A lternatively, i ron electrode cla m ps , sealed to the Py rex capswith as be sto s-w ate rg las s m ixtu re, a re used with no cooling. Inthat case, the Pyrex caps may have to be wound with cotton str ing,wetted with water to remove the heat.

The quenching tube itself can be made of co pp er. In th isc a s e , the gla ss ca ps a re sealed on with rub be r tubing (pieces of

bicycle inner tube).The ap pa rat us is f irs t thoroughly drie d by heating in a st re amof i ne rt g a s , with the coo ling wa ter off. After br ief cooling , ast re am of hydrog en is introduced at a and the air is displaced.Then 25 ml. of SiCl4 is placed in f lask a, the hydrogen satu ratedwith the SiCl4 and the heating res um ed [the hydrogen is f irs tpassed over Pd abestos to remove O3 and over P3 O 5 for dry ing(see section on Hydrogen)].

The first run in a new tube should be made with Ha; Si 1OC13OH3

is formed under the se conditions. If oxyg en-free argon and SiC l4ar e used in a la ter run , S i ^ C l ^ is formed. The tem pera ture ofthe SiC ro d should be between 1000C and 1100C. A tr a p bcooled with liquid nit rog en , is conne cted to the dis tillin g flaskwhich serves as the receiver.

I t happ ens occasio nally that the rea ctio n fails to s ta rt. Inthat cas e, the ga s is eith er not com pletely w ate r-fr ee or it contains

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oxygen. W henever a new SiC rod is us ed , the fi rs t ru n should bemade with Hs a s the c a rr ie r ga s, for otherw ise the rea ct io n willnot st a rt . Fr om 112 g. of SiC l4 the yield i s about 35 g. crud eproduct, from which the lower boil ing fractions are stripped off.The Si1 0 C l3 0 H 3 cannot be dist i l le d without deco mp osing. Theprodu ct pr ep ar ed with arg on is fractioned unde r high vacuum . TheS i 1 0 C l2 S co m es over as a highly visc ou s o il between 215 and220C.

The pre pa ra tio n of the higher Si ch lor ide s by pa ssa ge of SiCl4ov er Si at 1000C is de sc rib ed by Rochow and Didtschenko; thepre pa rat io n by m ean s of a glow disc har ge is desc ribed by Hertwigand Wiberg.

PROPERTIES:Visco us, w ith a c onsisten cy rangin g from oily to honey like.

Flammable. Hydrolyzed by water.REFEREN ES

A. Be sso na nd L. F ou rnier, Com ptesRendusHebd. Seances^ Acad.Sci. , 152, 603 (1911); C. F rie d el , Co m ptes Ren dus Hebd. Sean cesAcad . Sci. 73, 1011 (1871); C. F rie d el and A. Lad enb urg , L iebigsAnn. C hem . 203, 253 (1880); L. G atte rm an and E. E lle ry , Ber.dts ch . ch em . G es. 32, 1114 (1899); L. G atte rm an and K. W einlig,B er . dtsch. che m . G es . 27, 1943 (1894); J. W. M ellor, C om pr e-hensive T re at is e on Ino rg. Chem ., VI, p. 971 ; Int. C rit . Tabl. ,Vo l. I, p . 162; G. M ar tin , J . Chem . Soc. (London) 105 , 2836 , 2860(1914); B er . dts ch . ch em . G es. 45 , 2097 (1912); 46, 2442, 3289(1913); L. T ro os t and P . Hau tefeuille, Ann. Chim. Ph ys . (5) 7,4 59(1871); R. Schwarz and H. M eckbach, Z. an or g. allg . Ch em . 232,241 (1937); E. G. Rochow and R. Didtsch enko , J. A m er . Chem .Soc. 74, 5545 (1952); H. S. Gutowski and E . O. Ste jskal, J . Chem .

P h y s . 22, 939 (1954); D. F . Stedmann , U.S. Pate nt 2,621,111;W. J. Walton, U.S. Patent 2,602,728 (1952); H. Schaefer, Z. anorg.al lg . Chem . 274, 265 (1953); C. F. W ilki ns , J. Che m. Soc. (London),1953, 3409; K. A. He rtwig and E . W iberg, Z. N atu rfo rsc h.,6b, 336 (1951); W. C. Schum b and E . L. Gam ble in H. S. Booth,Inor g. Sy nthese s New York and London, 1939 , Vol. I, p . 42 .

Silicon TetrabromideSiBr4

Si + 2B r2 = SiBr428.1 319.7 347.8

A boat, placed in a Vycor tube, is heated in an electric furnaceto about 600C. The bo at co nta ins finely powdere d Si (which is