Cyanogen Bromide, CNBr

Cyanogen Bromide (Bromocyanogen), CNBr, resembles Cyanogen Chloride, CNCl in its manner of preparation, being formed by the action of bromine on hydrocyanic acid or a cyanide. It forms transparent prisms by sublimation, which pass into a cubical form; it melts at 52° C., and boils at 61.3° C.; its vapour density is 3.607; it is poisonous, and its vapour is pungent and affects the eyes. The heat of formation of CNBr from its elements is -37,000 calories, and from (CN)₂ and Br₂ +40,000 calories. It easily polymerises to (CNBr)₃.

In dilute aqueous solution cyanogen bromide shows no evidence of ionisation. Hydrogen sulphide reacts quantitatively with it, thus:

CN-Br + H₂S = HCN + HBr + S;

and the following reactions are also characteristic:

2KOH + CNBr = KBr + KCNO + H₂O
Na₂SO₃ + CNBr + H₂O = NaBr + NaCN + H₂SO₄
HI + CNBr = IBr + HCN; IBr + HI = HBr + I₂
K₂S + CNBr = KBr + KSCN.

A theory of the mechanism of these reactions is given by Dixon and Taylor.