Boron Francisco Javier Cervigon Ruckauer

Boron

OBTAINING AND ISOLATION OF BORON

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BORON COMPOUNDS: BORANES

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BORON COMPOUNDS: BORIDES

Borides involve a numerous group of binary compounds of boron which present main characteristics of all being very hard, and showing high melting temperatures and being chemically resistant. As a result, they are becoming increasingly important as materials that can be used for such purposes as rocket nose cones. In addition, they show an electrical conductivity which often exceeds those of the parent metals which coupled with their inertness to chemical attack lead to their application as electrodes in industrial processes. Moreover, the high absorption cross-section of boron-10 for thermal neutrons makes these compounds suitable in nuclear applications such as neutron shields and control rods.

These compounds present complex varieties of stoichiometries and structures, in fact, the most important of them, boron carbide, although it has the empirical formula B4C, the structure is better represented as B12C3 because it consists of B12 icosahedra, as in the element itself, with carbon atoms linking all the neighboring icosahedra.

One preparative method is the reduction of diboron trioxide with carbon:

2B2O3(s)+7C(s)→ΔB4C(s)+6CO(g)

In addition, boron carbide is one of the hardest substances available, second only to diamond and for instance, its fibers have enormous tensile strength and are used in bulletproof clothing.

BORON COMPOUNDS: HALIDES

The most interesting halides are boron trifluoride and boron trichloride. Therefore, boron trifluoride is relevant in the context of bonding; in fact, it is the prototypical Lewis acid, boron trichloride illustrating the high chemical reactivity of most nonmetal chlorides compared to ionic chlorides. 

1. Boron trifluoride, contrary to the previously observed behavior in the BH3, does not dimerise: it remains as the simple trigonal planar compound BF3. This species is also an electron-deficient molecule, however, it presents strong covalent bonds (B-F bond energy of 613 kJ/mol−1 versus C-F bond energy of 485 kJ/mol−1). To understand the high stability of this particular bond, we need to consider that the boron atom has an empty 2pzorbital that parallels to the full 2pz orbital of each fluorine atom. As a consequence, it can be postulated to that, in addition to the σ bonding, a delocalized π system involving the empty p orbital on the boron and one full p orbital on each of the fluorine atoms exist. In addition, boron trifluoride behaves as a powerful Lewis acid by using the vacant 2pz orbital. For instance, the reaction between boron trifluoride and ammonia is a classic illustration of this behavior, where the nitrogen lone pair acts as the electron pair donor: 

BF3(g)+:NH3(g)⇄F3B:NH3(s)

2. Boron trichloride is one of the most representative examples of the high chemical reactivity found in most nonmetal chlorides compared to ionic chlorides. This species is a typical small covalently bonded molecule that is susceptible to protolysis by mild proton sources such as water, alcohols, and even amines. The rapid hydrolysis of BCl3 to give boric acid, B(OH)3 illustrates a representative example. This reaction together with metathesis reactions, is very useful in preparative chemistry, and in general terms, can be extended to the bromide and iodide halides (Figure 8): 

BCl3(g)+3H2O(l)⇄B(OH)3(aq)+3HCl(aq)

Fig. 4.1 The reactions of boron-halogen compounds (X = halogen].

USES AND APPLICATIONS OF BORON

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Francisco Javier Cervigon Ruckauer