SECTION 14

BoronH. J. BECHER

Boron

I. According to Moissan, very impure amorphous boron, containingabout 80-90% B, is obtained by the reaction of B3O3 with magnesium.According to Kroll the optimum yields are obtained as follows: Afireclay crucible, approximately 20 cm. high and 16 cm. in diam-eter, is painted with a paste of ignited MgO and sintered MgCl3 anddried in a low-temperature oven. A mixture of 110 g. of B2O3,115 g. of Mg shavings (the use of Mg powder frequently leads toexplosive reactions) and 94 g. of powdered S is placed in the cruc-ible. The reaction is started with an ignition pellet, after which itproceeds vigorously. After the mixture has cooled, it is extractedin water and then in dilute HC1 for a week. The residue is treatedseveral times by heating with HF and HC1, washed with water anddried in vacuum at 100C. The yields are variable, with a maximumof 46%.II. According to Kiessling, pure boron can be made by reducingBBr3 with H3 at 800C. The reaction takes place in the apparatusshown in Fig. 238. The BBr3 is prepared by the method of Meyerand Zappner from Br s and commercial boron (usually 70-80% pure)(cf. the method described on p. 782). Thus, 15 g. of B is pressedinto pellets, and the quartz tube b is filled with them. The tube isheated to 700C and dry Br a is added in drops from dropping funnela. The resulting BBr3 will then collect in trap c, which is cooledwith an ice-salt mixture. After about 30 minutes, 5-10 ml. ofBBr3 will have accumulated. The addition of Br3 is stopped andexcess Br 3 from b and c is flushed out with H3. The resultingBBr3 should be colorless. The H3 flow is then adjusted to 2-4bubbles per second, the temperature of the quartz tube heater israised to 750-800C, and the BBr3 in a allowed to evaporate in theH3 stream at ambient temperature. As a result elemental B pre-cipitates in d. Unreacted BBr3 recondenses in e, which is cooledwith ice-salt mixture. When no further BBr3 is left in o, traps eand a are interchanged and the decomposition continued in d. Whenthe BBr3 is all reacted, more material is prepared by allowingfresh Br 3 to drop into quartz tube b. One charge of 15 g. of B will

770

1 4 . BORON 771

be sufficient for preparation of 75 ml. of BBr3. Boron that hasprecipitated in d appears to catalyze further decomposition. There-fore, this tube should not be emptied too early. The resulting B iswashed and dried with hot HSO. According to Kiessling the com-position is: 98.9% B, 0.04% Al, 0.1% Si and traces of O, H and Mg.

hood

1Fig. 238. Preparation of high purity boron,adropping funnel for Br s , protected againstatmospheric moisture; b quartz tube for Bpellets (10 mm. in diameter and 700 mm.long); cfirst condensation trap for BBr3,capacity about 25 ml.;dquartz tube for r e -duction of BBr3; esecond condensation trapfor BBr3, capacity 25 ml.; o tubular elec-trical heaters, about 600 mm. long; pjointscemented withpicein; gmercury seals; theirdesign is shown enlarged next to the principalfigure; it illustrates the seal for the top of the

condensation flask.PROPERTIES:Atomic weight 10.82. Gray-brown to yellow-brown powder.

M.p. 2300 C; d 2.3. Ignites in air at 700 C. Reacts violently withconcentrated HNO3.

REFERENCES:

I. H. Moissan. Compt. Rend. Hebd. Seances Acad. Sci. 114, 392(1892); W. Kroll. Z. anorg. allg. Chem. 102, 1 (1918).

II. R. Kiessling. Acta Chem. Scand. 2, 707 (1948).For other procedures, cf. A. W. Laubengayer, D. T. Hurd,

A. E. NewkirkandJ. L. Hoard. J. Amer. Chem. Soc. 65, 1924 (1943).

780 H. J . BECHER

interrupted and the excess BC13 still remaining in the flask isallowed to recondense on the inlet cold finger for another hour.The cooling is then stopped and the excess BC13 distilled off. Theliquid phase is siphoned out from the flask and centrifuged forfurther clarification, if necessary. The chlorobenzene is distilleduntil solid (ClBNH)

3 remains as a residue. The latter is purifiedby vacuum sublimation at 50-60C. The yield is approximately40%. If the recovered chlorobenzene is recycled back to the flaskwhich still contains the solid residue and if fresh NH4C1 is added,the yield can be increased appreciably by further reaction.

PROPERTIES:

Colorless crystals, exceedingly sensitive to moisture. M.p.84C; d (25C) 1.58. Soluble in benzene, CC14 and other organicsolvents.

REFERENCES:

Ch. A. Brown and A. W. Laubengayer. J. Amer. Chem. Soc. 77,3699 (1955).

R. Schaeffer, M. Steindler, L. Hohnstedt, H. S. Smith, Jr., L. B.Eddy and H. I. Schlesinger. J. Amer. Chem. Soc. 76, 3303(1954).

For preparation and properties of some other borazole deriva-tives, see E. Wiberg, Naturwiss. 35, 182, 212 (1948); H. J. Becherand S. Frick, Z. anorg. allg. Chem. 295, 83 (1958).

Boron Trichloride

BCI3

Fairly large quantities of BC13 can be produced by heatingdilute borax with charcoal in a stream of Cl3 at temperatures of400 to 700C. In the laboratory it is more convenient to produce itfrom BF3 and A1C13.

I. BF3 + A1C13 = BCI3 + A1F3.67.8 133.4 117.2 84.0

The reaction is carried out in the apparatus shown in Fig. 241.The lower flask has a capacity of one liter; the upper bulb, halfthat. Anhydrous A1C13(67 g. = 0.5 mole) is placed in the lower flask.The inlet tube is connected to a BF 3 generator and the BF3 flowadjusted in such a manner that 132 g. or two moles of BF 3 areadded to the vessel over a period of 30 minutes. At the same time,

14 . BORON 781

Fig. 241. Preparation of boron trichloride.

the lower flask is heated with an open flame. Later the bulb is alsoheated. The BC13 distills off while the A1F3 peels off from thewalls of the flask as a light powder. The BC13 is cooled in a Utube cooled to 80C. Moisture is excluded by means of a dryingtube. The impure product is shaken with some Hg and recondensed.The yield is 47 g.

II. KBF4125.9

A1C13 = BC13133.4 117.2

KF A1F3

An i n t i m a t e m i x t u r e of 133.3 g. of A1C1 3 and 62 g. of K B F 4 i splaced in the apparatus described above, which is then slowlyheated in an oil bath to 150-170C. Using the method describedabove, the BC13 that has been distilled off is trapped and purified.The yield is poorer than that produced by method I.

PROPERTIES:M.p. 107C, b.p.

fuming in moist air.REFERENCES:

12.5C; (0C) 1.434. Colorless liquid,

I and II: E. L. Gamble in: L. F. Audrieth, Inorg. Syntheses, Vol.Ill, New York-London, 1950, p. 27. Synthesis of BC13 fromamorphous B and Cl3: L. Gattermann. Ber. dtsch. chem. Ges.22, 195 (1889).

Boron TribromideBBrs

I. AlBr3 + BF3 = BBr3 + A1F3266.7 67.8 250.6 84.0

One half mole (133.4 g.) of AlBr3 is distilled into the flaskdescribed for the preparation of BC13; BF3 is added while heating

782 H. J . BECHER

the flask. After some time the flask contents solidify. The heatingand admission of BF 3 are continued. As a result, the BBr3product distills over into the 78C trap. The distillate still con-tains some Br2 which can then be removed by shaking with Hg. TheBBr3 must be distilled for further purification. Theyield is 87.7 g.(70%).

An alternative is to heat a mixture of AlBr3 and KBF4 in theflask itself, but this results in a much lower yield of BBr3.

II. B + 17a Br2 = BBr310.8 239.8 250.6

This procedure has been previously described in connection withthe Kiessling method for preparing pure boron. If BBr

3 is desired,the apparatus shown in Fig. 238 is closed off by means of a dryingtube inserted behind condensation trap o. Before the reaction theboron is thoroughly dried by prolonged heating at 600C in aquartz tube flushed with a stream of H3. Then dropwise Br3 ad-dition is started, and the temperature of the reaction tube israised to 700-750 C. The product BBr3 is purified as in method I.

PROPERTIES:

M.p. -46C, b.p. 90.8C; d (0C) 2.65. Colorless, easily hydro-lyzed liquid.

REFERENCES:

I. E. L. Gamble in: L. F. Audrieth, Inorg. Syntheses, Vol. Ill,New York-London, 1950, p. 27.

II. Fr. Meyer and R. Zappner. Ber. dtsch. chem. Ges. 54, 551(1921); H. Menzel. Unpublished.

Boron Triiodide

BI3

3LiBH4 + 8I2 = 3LiI + 3BI3 + 4H2 + 4 HI65.37 2030.56 401.55 1174.65 8.06 511.67

The apparatus shown in Fig. 242 is used and 170 g. of Ia isadded to flask a, while 5.1 g. of LiBH4 is placed in addition bulb bunder an N s blanket. The entire assembly is thoroughly flushedwith N3 by evacuating it several times through u and introducing

4. FLUORINE COMPOUNDS 223

Potassium FluoroborateKBF4

H3BO3 + 4HF + KOH = KBF4 + 4H2O61.82 80.04 56.11 125.92 72.05

Boric acid (6.2 g.) is added to 25 g. of 40% hydrofluoric acidsolution contained in an ice-cooled platinum dish. The solution isallowed to stand at room temperature for six hours. At the endof this period it is again chilled with ice, and 5N KOH solution isadded with constant stirring until the color of methyl orange changes.Crystalline KBF4 precipitates out at the same time. The motherliquor and subsequent water washings are decanted and the crystalsdried under vacuum. The yield is 90%.

PROPERTIES:

White, crystalline salt, nonhygroscopic.M.p. 530C, d% 2.505. Solubility in water (20C) 0.45; (100C)

6.3 g./lOO ml. Dimorphous: rhombic-bipyramidal and cubicstructures (trans, temp. 276-280C).

REFERENCES:

D. Vorlander, J. Hollatz and J. Fischer. Ber. dtsch. chem.Ges. 6jj, 535 (1932).

Potassium HydroxyfluoroborateKBFSOH

2 KHF2 + H3BO3 = KBF3OH + KF + 7 H2O156.22 61.84 123.96 58.1 36

Technical grade KHF3 (100 g.) is dissolved in 250 ml. of watercontained in a polyethylene beaker. The KgSiFs and the undissolvedKHF3 are filtered off after several hours of standing; the clearsolution is placed in an ice-cold water bath and 40 g. of boric acidis added with stirring. Rapid dissolution occurs. Small crystalsseparate from the solution within an hour. They are suction-filtered on a fritted glass filter, washed with a small amount ofice-cold water and with 95% methanol solution and acetone. Thesalt is then dried at 120C.

PROPERTIES:

Melts without decomposition. Less soluble in water than KBF4.Yields no precipitate with nitron acetate; hydrolyzed by KOH

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