Hydrogen cyanide (with the historical common name of Prussic acid) is a chemical compound with chemical formula HCN. Hydrogen cyanide 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 some people are unable 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.
Hydrogen cyanide forms in at least limited amounts from many combinations of hydrogen, carbon, and ammonia. Hydrogen cyanide is currently produced in great quantities by several processes, as well as being a recovered waste product from the manufacture of acrylonitrile. In the year 2000, 732,552 tons were produced in the US. The most important process is the Andrussov oxidation invented by Leonid Andrussow at IG Farben in which methane and ammonia react in the presence of oxygen at about 1200 °C over a platinum catalyst:
- 2 CH4 + 2 NH3 + 3 O2 → 2 HCN + 6 H2O
The energy needed for the reaction is provided by the partial oxidation of methane and ammonia.
Of lesser importance is the Degussa process (BMA process) in which no oxygen is added and the energy must be transferred indirectly through the reactor wall:
- CH4 + NH3 → HCN + 3H2
This reaction is akin to steam reforming, the reaction of methane and water to give carbon monoxide and hydrogen.
In the Shawinigan Process, ammonia and natural gas are passed over coke. As practiced at BASF, formamide is heated and split into hydrogen cyanide and water:
- CH(O)NH2 → HCN + H2O
In the laboratory, small amounts of HCN are produced by the addition of acids to cyanide salts of alkali metals:
- H+ + NaCN → HCN + Na+
This reaction is sometimes the basis of accidental poisonings because the acid converts a nonvolatile cyanide salt into the gaseous HCN.
HCN is the precursor to sodium cyanide and potassium cyanide, which are used mainly in mining. Via the intermediacy of cyanohydrins, a variety of useful organic compounds are prepared from HCN including the monomer methyl methacrylate, from acetone, the amino acid methionine, via the Strecker synthesis, and the chelating agents EDTA and NTA. Via the hydrocyanation process, HCN is added to butadiene to give adiponitrile, a precursor to Nylon 66
Hydrogen cyanide as a poison and chemical weapon
A hydrogen cyanide concentration of 300 mg/m3 in air will kill a human within about 10 minutes. It is estimated that hydrogen cyanide at a concentration of 3500 ppm (about 3200 mg/m3) will kill a human in about 1 minute. The toxicity is caused by the cyanide ion, which halts cellular respiration by inhibiting an enzyme in mitochondria called cytochrome c oxidase.
Hydrogen cyanide absorbed into a carrier for use as a pesticide (under IG Farben's brand name Cyclone B, or in German Zyklon B, with the B standing for Blausäure) was employed by Nazi Germany in the mid-20th century in extermination camps. The same product is currently made in the Czech Republic under the trademark "Uragan D2." Hydrogen cyanide is also the agent used in gas chambers employed in judicial execution in some U.S. states, where it is produced during the execution by the action of sulfuric acid on an egg-sized mass of potassium cyanide.
Hydrogen cyanide is commonly listed amongst chemical warfare agents known as blood agents. As a substance listed under Schedule 3 of the Chemical Weapons Convention as a potential weapon which has large-scale industrial uses, manufacturing plants in signatory countries which produce more than 30 tonnes per year must be declared to, and can be inspected by, the Organisation for the Prohibition of Chemical Weapons.
Under the name prussic acid, HCN has been used as a killing agent in whaling harpoons.
Hydrogen cyanide gas in air is explosive at concentrations over 5.6%, equivalent to 56000 ppm
