Hydrogen cyanide (with the historical common name of Prussic acid) is a chemical compound with chemical formula HCN. It is a colorless, extremely poisonous liquid that boils slightly above room temperature at 26 °C (79 °F). 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 partly ionizes in water solution to give the cyanide anion, CN–. A solution of hydrogen cyanide in water is called hydrocyanic acid. The salts of hydrogen cyanide are known as cyanides.
HCN has a faint, bitter, burnt almond-like odor that only some people are able to detect owing to a genetic trait. The volatile compound has been used as inhalation rodenticide and human poison. Cyanide ions interfere with iron-containing respiratory enzymes.
HCN is produced on an industrial scale and is a highly valuable precursor to many chemical compounds ranging from polymers to pharmaceuticals.