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Group 13: Boron-Compounds

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Group 13: Boron-Compounds - Lesson Summary

Some important compounds of boron are borax, orthoboric acid and diborane.

Borax (decahydrate of sodium tetra borate) is the most important boron compound. Borax is used as a flux in soldering and as a cleaning agent in leather industry and laundry.

Borax dissolves in water and the resulting solution is alkaline as sodium hydroxide is produced.

When borax is heated it loses its water of crystallisation and swells. When the anhydrous sodium tetra borate is further heated, it becomes a transparent liquid.

The "borax bead test" is a qualitative way to test for the presence of transition metal ions.

Orthoboric acid is another important compound of boron. Its chemical formula is H3BO3. It is also a white, crystalline solid; it is sparingly soluble in cool water, but highly soluble in hot water.

Diborane (B2H6) is another important compound of boron. Boranes are a class of compounds containing only boron and hydrogen. Diborane is a colourless, highly toxic and extremely reactive.

Diborane is prepared by the Oxidisation of sodium borohydride with iodine.

The reaction of diborane in oxygen is extremely exothermic, releasing 1976 kJ of energy for each mole of diborane.

Boranes were once considered for use as solid fuels for rockets because of their exothermic reaction with oxygen.

Diborane is an electron deficient molecule. The two boron atoms and the four terminal hydrogen atoms of the molecule are all in the same plane. These four terminal B -H bonds are regular 2-centered- 2 electron bonds.

The bridging hydrogen atoms lie above and below this plane. The two bridges B-H-B bonds are unusual three centered two electron bonds. The boron atoms in diborane undergo sp³ hybridisation. The overlapping of a vacant sp³ hybrid orbital of one boron atom and sp³ hybrid orbital of another boron atom containing one electron with the pure s -orbital of bridging hydrogen containing one electron results in the "banana bond" .Similarly other banana bond is formed on other side .

Thus the two banana bonds formed lie above and below the plane of the boron atoms. The remaining two sp³ hybrid orbitals of each boron atom overlap with terminal hydrogen atoms to form normal sigma bonds, results in the formation of four terminal hydrogen bonds.

Sodium borohydride and lithium borohydride can be prepared by reacting the metal hydride with diborane.

Borohydrides are good reducing agents. Thus, widely used in organic synthesis, as well as to prepare other metal borohydrides.

       2LiH + B2H6 → 2Li+[BH4]-

Boron has a number of applications, because it is a hard solid with a high melting point, low density and low electrical conductivity.

Boron fibres are used to make bullet-proof vests and light-weight composite material for aeroplanes.

The boron-10 isotope has a high ability to absorb neutrons. Therefore, metal borides are used as control rods and protective shields in nuclear power plants.

Borax and boric acid are used in the manufacture of heat-resistant glass, such as Pyrex. They are also used to make glass wool and fibreglass.

Borax is used as a flux for soldering metals, for heat-, scratch- and stain-resistant glazed coatings for ceramics, and in the manufacture of medicinal soaps.

Aluminium is a bright, silvery white metal. It has a high tensile strength. It also has high thermal and electrical conductivity. Aluminium forms a number of alloys with various metals, including copper, manganese, magnesium, silicon and zinc.

Aluminium and its alloys are readily shaped into pipes, tubing, rods, wires or foils. Hence aluminium and its alloys are widely used in packing, cookware, utensil making, construction, aeroplane and transportation industry.


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