Boron trihydride, B₂H₆

Boron trihydride or boro-ethane, B₂H₆ has been mentioned that the compound B₄H₁₀ is very unstable. When kept over mercury at the ordinary temperature it decomposes with the formation of a gaseous hydride B₂H₆ and hydrogen. The rate of decomposition is greatly augmented by ultraviolet light, but not appreciably by sunlight. The change is practically complete at 100° after one hour.

Boron trihydride is a colourless gas possessing a repulsive odour which recalls that of B4H₁₀, and also that of hydrogen sulphide. Its vapour density corresponds with the molecular formula B₂H₆. The hydride melts at -169° and boils at -87° C. The vapour pressure of the liquid at various temperatures is as follows: -

Temp. °C	-130°	-120°	-112°	-110°	-100°	-90°	-87°
Vap. press, (in cms.)	4	10	22.4	25	40	65	76

Boron trihydride is fairly stable, but it slowly decomposes even at the ordinary temperature. When heated with a free flame, solid, but not liquid, hydrides are produced; when sparked, boron is deposited and non-volatile hydrides of unpleasant odour are produced. In the presence of air it gives rise to solids containing both boron and oxygen, and when exploded with oxygen it reacts thus: -

B₂H₆ + 3O₂ = B₂O₃ + 3H₂O.

It dissolves in carbon disulphide, and the solution decolorises bromine; it does not react with dry hydrogen chloride, but combines readily with dry ammonia.

Boron trihydride reacts with water to give boric acid and hydrogen: -

B₂H₆ + 6H₂O = 2H₃BO₃ + 6H₂.

Like the hydride B₄H₁₀, it gives a hypoborate with alkali hydroxide, but some hydrogen is evolved immediately the gas and. the hydroxide come into contact. The hypoborate slowly decomposes, and the final result may be expressed thus: -

B₂H₆ + 6NaOH = 2Na₃BO₃ + 6H₂.