Silicon Tetrabromide, SiBr₄

Silicon Tetrabromide (Tetrabromosilicane), SiBr₄, first obtained by Serullas, is prepared similarly to silicon tetrachloride by passing bromine vapour over an intimate mixture of silica and carbon or the crude silicon of Gattermann; hydrogen bromide may be substituted for bromine, when silicobromoform accompanies the tetrabromide. An alternative method consists in causing silicon tetrachloride to react with carbon tetrabromide, when exchange of halogen takes place.

Silicon tetrabromide is a heavy, colourless liquid which evolves dense white fumes in moist air; its density at 0° C. is 2.8128 (Pierre) or 2.82 (Reynolds); its boiling-point has been found by different observers to lie between 148° C. and 154° C. The most recent, and probably the most accurate, determination is that of Blix, who found 150.8° C. at 751.4 mm. pressure. Solid silicon tetrabromide is a pearly crystalline mass which according to Blix melts at 5° C., but according to other observers between -12° C. and -15° C. The solid is heavier than the liquid.

The heats of formation are as follow:

Si (cryst.) + 4Br (liq.) = SiBr₄ (liq.) + 71,000 calories.
Si (cryst.) + 4Br (vap.) = SiBr₄ (vap.) + 85,800

Water reacts vigorously with silicon tetrabromide, hydrolysing it with the formation of silicic and hydrobromic acids; litharge produces some lead bromide and lead silicate; ammonia forms the white, amorphous, additive compound SiBr₄.7NH₃, which is decomposed by water; phosphine, organic bases, and thio- and alkylated ureas also form additive compounds with silicon tetrabromide.