Halides elemental boron

Boron(Bor in Ger Rus, Bore in Fr, Boro in Ital Span), B, at wt 10.82 occurs in two forms blk monoclinic crysts yel or brn amor powd mp 204( > bp 3930°, d 2.33(cryst), 1.73(amor) sol in nitric and sulfuric acids and in most molten metals insdl in w, ale, ether solns of caustic alkalies. The element boron was isolated in 1808 by Gay-Lussac Thenard who reduced boron oxide(B2C>3) with K by heating it in an iron tube. Sir Humphrey Davey independently obtd the same product and called the element "boron . This element is not found free in nature, but occurs in the form of borates boric acid. It can be prepd by the reduction of the oxide, borates or boron halides... 

Coupling of aromatic heterocycles goes well. The 2-position of a pyridine is very electrophilic and not at all nucleophilic (Chapter 43) but couplings at this position are fine with either the halide or the boronic acid in that position. Clearly, it is a mistake to see either of these substituents as contributing a nucleophilic carbon . It is better to see the reaction as a coupling of two equal partners and the two substituents (halide and boronic acid) as a control element to ensure cross-coupling and prevent dimerization. In the second example potassium te t-butoxide was crucial as weaker and less hindered bases gave poor yields. 

Elemental boron reacts directly with fluorine and chlorine but for practical purposes the halides are obtained via the BF3 route from boric acid ... 

We have excluded from our classification halides and boron. For the last element, this is perhaps a mistake, because, as mentioned above, it may be one of the components in amorphous alloys in future developments. 

The melting and boiling points of the aluminium halides, in contrast to the boron compounds, are irregular. It might reasonably be expected that aluminium, being a more metallic element than boron, would form an ionic fluoride and indeed the fact that it remains solid until 1564 K. when it sublimes, would tend to confirm this, although it should not be concluded that the fluoride is, therefore, wholly ionic. The crystal structure is such that each aluminium has a coordination number of six, being surrounded by six fluoride ions. 

The tribromide and triodide of both boron and aluminium can be made by the direct combination of the elements although better methods are known for each halide. The properties of each halide closely resemble that of the chloride. 

The boron halides are made either by direct reaction of the elements at a high temperature or from boron oxide. The most important is boron trifluoride, BF, an industrial catalyst produced by the reaction between boron oxide, calcium fluoride, and sulfuric acid ... 

The hydrogen reduction of the halides of nonmetallic elements, such as silicon (Reaction 8) and boron, is an essential process in the production of semiconductors and high-strength fibers. 

Reactions between anionic species containing one or more group-IIIB elements (particularly boron) and complexes of transition-metal halides are used to produce an immense number of ionic boron-containing compounds. For this reason a strong selection factor must be made. 

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