Hydrogen cyanide is a linear molecule, with a triple bond between carbon and nitrogen. A minor tautomer of HCN is HNC, hydrogen isocyanide.
Hydrogen cyanide is weakly acidic with a pKa of 9.2. It partially ionizes in water solution to give the cyanide anion, CN−. A solution of hydrogen cyanide in water, represented as HCN, is called hydrocyanic acid. The salts of the cyanide anion are known as cyanides.
HCN has a faint bitter almond-like odor that some people are unable to detect owing to a recessive genetic trait.[13] The volatile compound has been used as inhalation rodenticide and human poison, as well as for killing whales.[14] Cyanide ions interfere with iron-containing respiratory enzymes.
Hydrogen cyanide was first isolated from a blue pigment (Prussian blue) which had been known since 1706, but whose structure was unknown. It is now known to be a coordination polymer with a complex structure and an empirical formula of hydrated ferric ferrocyanide.