Boron Chloride and Bromide

Large quantities of boron chloride are best prepared by the chlorination of a mixture of boron oxide (or borax) and carbon. Smaller amounts may be obtained (1) by the chlorination of amorphous boron or metallic borides or (2) by the reaction between boron oxide and phosphorus (V) chloride in a sealed tube. A more convenient laboratory method for the preparation of boron chloride makes use of the reaction between boron fluoride and aluminum chloride. For the preparation of boron bromide, the reaction of boron fluoride and aluminum bromide is recommended as much less troublesome than other methods. Potassium tetrafluoborate may be used in place of the boron fluoride in each of these syntheses. 

Apparatus. The reaction is carried out most conveniently in an all-glass apparatus constructed as shown in Fig. 8. A 1-1. round-bottomed flask is sealed to the bottom of a 500-ml. distilling flask by means of a 25-cm. length of 30-mm. tubing. The tube for the introduction of the boron fluoride passes through a one-hole cork stopper in the distilling flask and extends to the middle of the larger flask. The smaller flask serves as a condenser to prevent 

The product collects in the U-tube, which is sealed off after completion of the reaction and its contents distilled through a good column. The yield is 47 g. (80 per cent). 

Boron Bromide. One hundred thirty-three and one-half grams of aluminum bromide (0.5 mol) (synthesis 7) is distilled into the reaction flask and treated with boron fluoride as described for boron chloride. It is desirable to reflux the aluminum bromide gently in the stream of boron fluoride. The contents of the flask soon solidify. When this happens the mass is heated strongly in boron fluoride to distill all of the produfct over into the U-tube where it is condensed. Small amoimts of bromine in the product are readily removed by shaking with mercury. Yield, 87.7 g. (70 per cent). This product is satisfactory for most purposes, but may be redistilled through a good column to effect further purification. 

Modification. One hundred thirty-three grams (1 mol) of aluminum chloride (or 267 g. of aluminum bromide) and 62 g. (0.5 mol) of potassium tetrafluoborate are placed in the apparatus described above (or in an ordinary distilling flask) and heated by means of an oil bath for 4 hours, with the temperature slowly increasing from 150 to 175. The product is condensed by liquid nitrogen and then fractionated. Yield BCU, 30 g. (0.26 mol) (52 per cent based on KBF4) BBrs, 37.6 g. (30 per cent). 

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Boron chloride and bromide melt at —107° and —46°, respectively, to give colorless liquids of high refractive index and large coefficients of thermal expansion. Boron chloride boils at 12.5° and the bromide, at 90.8°. At 0° the... 

Boron chloride and bromide, synthesis 6 Tellurium(IV) chloride, synthesis 36... 

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