Boron hydrides

The simplest of the hydrides is diborane, B₂H₆. It is prepared by the reduction of boron trichloride in either by lithium aluminium hydride. This is a general method for the preparation of non-metallic hydrides.

4BCl₃ + 3LiAlH₄ 2B₂H₆ + 3LiCl + 3AlCl₃

Diborane is an inflammable gas which is immediately decomposed by water:



It burns in air with a green flame, and is an extremely reactive substance; in the laboratory it must be handled in special apparatus such that it comes into contact only with glass and mercury. Special taps must be used since diborane attacks tap-grease.

Boron trioxide or boric oxide, B₂O₃

Boric oxide is the anhydride of boric acid, H₃BO₃; it is prepared by heating the latter:

2H₃BO₃ B₂O₃ + 3H₂O

The acid is re-formed slowly when the oxide is added to water.

Boric oxide is dimorphous, existing as a glassy solid or a crystalline solid. It is an acidic oxide in that, when fused with metal oxides or hydroxides, it forms borates; some of these have characteristic colours (borax bead test in analysis).

Boric acid, H₃BO₃

It may be prepared by adding excess sulphuric or hydrochloric acids to a hot saturated solution of borax; the boric acid separates on cooling, and can be purified by recrystallization, since it is only moderately soluble in cold water.

Boric acid is properly called orthoboric acid, to distinguish it from metaboric acid, HBO₂, which is formed when orthoboric acid is heated to about 100˚C. Boric acid is also commonly known as ‘boracic acid’ and has been used as a mild antiseptic.

Sodium tetraborate, borax, Na₂B₄O₇.10H₂O

Sodium tetraborate is obtained by dissolving the mineral kernite in water and crystallizing the solution.

On heating borax, it loses water and finally forms a glassy mass of the anhydrous salt. This fused ‘glasses’ will dissolve metal oxides to give coloured borates (borax bead test).

Borax is alkaline in aqueous solution, since it is hydrolysed, and it can be titrated against hydrochloric acid using methyl red as indicator:

B₄O₇^2- + 2H⁺ + 5H₂O 4H₃BO₃

Boron nitride, BN

Boron nitride is chemically unreactive, and can be prepared by allowing ammonia to react with boron trichloride. The first product is boron amide which decomposes on heating to give the nitride:



B(NH₂)₃ BN + 2NH₃

Boron nitride is chemically unreactive, and can be melted at 3000˚C by heating under pressure. It is a covalent compound, but the lack of volatility is due to the formation of ‘giant molecules’ as in graphite or diamond. The bond B-N is isoeletronic with C-C.

Boron trifluoride, BF₃

This colourless, reactive gas is a covalent compound prepared by heating boron trioxide and fluorspar with concentrated sulphuric acid:

B₂O₃ + 3CaF₂ + 3H₂SO₄ 2BF₃ + 3CaSO₄ + 3H₂O

Tetrafluoroboric acid, HBF₄

Tetrafluoroboric acid is a strong acid and the tetrafluoroborates are stable salts; the anion (BF₄)-, is very similar in size and shape (tetrahedral) to the perchlorate ion, ClO₄^-, and hence tetrafluoroborates are often isomorphous with perchlorates, e.g. KBF₄ and KClO₄.

Boron trichloride, BCl₃

Like boron trifluoride, this covalent compound is a gas at ordinary temperature (b.p. 12˚C). It reacts vigorously with water but does not form an ion BCl₄^- (although salts of this anion are known); instead, hydrochloric acid is obtained (cf. phosphorus trichloride).

BCl₃ + 3H₂O H₃BO₃ + HCl

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