Boroethane, B₂H₆

The action of chlorine and bromine on this hydride of boron has been studied by Stock, Kuss, and Priess. Chlorine causes explosions at ordinary temperatures, but bromine reacts slowly, even in the light; at 100°, however, the change is complete in a few hours. One half of the halogen used up is recovered as halogen hydride, i.e. the reaction is one of substitution, and boroethane behaves as a saturated compound. The maximum valency of boron towards hydrogen is accordingly four.

Excess of halogen leads to the production of the unimolecular halides, BX₃, and not to B₂X₆. In order to trace the mechanism of these changes, the action of the halogens on excess of the boron hydride was studied and the following conclusions reached. The initial products are such products as B₂H₄X₂ and B₂H₃X₃, which speedily decompose, without the elimination of halogen hydride, yielding B₂H₅X and B₂H₆ on the one hand, and BX₃ on the other. Thus, of the various halogenated derivatives theoretically possible, a mixture of the extremes is produced. No evidence of the existence of BHX₂, BH₂X, B₂HX₅, and B₂H₂X₄ could be obtained.

Boroethane monochloride, B₂H₅Cl, is a spontaneously inflammable gas. The monobromide, B₂H₅Br, is a colourless gas with an irritating odour. It melts at 104° and boils at c. 10° C. The vapour pressure is as follows: -

Temp. °C.	-80°	-70°	-60°	-50°	-40°	-30°	-20°	-10°	-5°
Vap. press, in mms.	3	9	16	30	53	95	162	255	335

It burns with a pale green flame and fumes in the air owing to its reaction with water: -

B₂H₅Br + 3H₂O = B₂O₃ + HBr + 5H₂.

It immediately reacts with potassium hydroxide to form the hypoborate KOBH₃. It does not react with sodium, however, to any extent, so that the hydride B₄H₁₀ cannot be thus produced.