Boron halides, purification

Born tribromide is used as received. It is photosensitive, but can be purified by storage over elemental mercury, followed by storage in the dark. The use of the boron halides BF, and BCl, results in much lower yields and dilTiculties with purification, and they are therefore considered to be unsatisfactory for this reaction. 

Purification. For those applications that require high purity such as semiconductor components and some nuclear graphites, the material is heat-treated in a halogen atmosphere. This treatment can remove impurities such as aluminum, boron, calcium, iron, silicon, vanadium and titanium to less than 0.5 ppm.f °l The halogen reacts with the metal to form a volatile halide which diffuses out of the graphite. The duration of the treatment increases with increcising cross section of the graphite part. 

For convenience in their purification and characterization, boronic acids are often best handled as ester derivatives, in which the two hydroxyl groups are masked. Likewise, transformation of the hydroxyl groups into other substituents such as halides may also provide the increased reactivity necessary for several synthetic applications. The next sections describe the most popular classes of boronic acid derivatives. 

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