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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 (78.8 °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 salt of hydrogen cyanide are known as cyanides.

HCN has a faint, bitter, almond-like odor that some people are unable to detect due 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.

History of discovery

Hydrogen cyanide was first isolated from a blue dye (Prussian blue) which had been known from 1704 but had a structure which was unknown. It is now known to be a coordination polymer with a complex structure and an empirical formula of hydrated ferric ferrocyanide. In 1752, the French chemist Pierre Macquer made the important step of showing that Prussian blue could be converted to iron oxide plus a volatile component and that these could be used to reconstitute the dye. The new component was what we now know as hydrogen cyanide. Following Macquer's lead, it was first isolated from Prussian blue in pure form and characterized about 1783 by the Swedish chemist Carl Wilhelm Scheele, and was eventually given the German name Blausäure (literally "Blue acid") because of its acidic nature in water and its derivation from Prussian blue. In English it became known popularly as Prussic acid.

In 1787 the French chemist Claude Louis Berthollet showed that Prussic acid did not contain oxygen, an important contribution to acid theory, which had hitherto postulated that acids must contain oxygen (hence the name of oxygen itself, which refers to acid-former). In 1815 Joseph Louis Gay-Lussac deduced Prussic acid's chemical formula. The radical cyanide in hydrogen cyanide was given its name from the Greek word for blue, again due to its derivation from Prussian blue.

Production and synthesis

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 BASFmarker, 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.

Historical methods of production

The demand for cyanides for mining operations in the 1890s was met by George Thomas Beilby, who patented a method to produce hydrogen cyanide by passing ammonia over glowing coal in 1892. This method was used until Hamilton Castner in 1894 developed a synthesis starting from coal, ammonia, and sodium yielding sodium cyanide, which reacts with acid to form gaseous HCN.


See also: sodium cyanide
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.


HCN is obtainable from fruits that have a pit, such as cherries, apricots, apples, and bitter almond, from which almond oil and flavoring are made. Many of these pits contain small amounts of cyanohydrins such as mandelonitrile and amygdalin, which slowly release hydrogen cyanide. 100 g of crushed apple seeds can yield about 10 mg of HCN.Some millipedes release hydrogen cyanide as a defense mechanism, as do certain insects such as some burnet moth. Hydrogen cyanide is contained in the exhaust of vehicles, in tobacco and wood smoke, and in smoke from burning nitrogen-containing plastics. So-called "bitter" roots of the cassava plant may contain up to 1 gram of HCN per kilogram.

HCN and the origin of life

Hydrogen cyanide has been discussed as a precursor to amino acids and nucleic acids. It is possible, for example, that HCN played a part in the origin of life. Although the relationship of these chemical reactions to the origin of life remains speculative, studies in this area have led to discoveries of new pathways to organic compounds derived from condensation of HCN.

HCN in space

See also: Astrochemistry
HCN has been detected in the interstellar medium. Since then, extensive studies have probed formation and destruction pathways of HCN in various environments and examined its use as a tracer for a variety of astronomical species and processes. HCN can be observed from ground-based telescopes through a number of atmospheric windows. The J=1→0, J=3→2, J= 4→3, and J=10→9 pure rotational transitions have all been observed.

HCN is formed in Interstellar clouds through one of two major pathways: via a neutral-neutral reaction (CH2 + N → HCN + H) and via dissociative recombination (HCNH+ + e- → HCN + H). The dissociative recombination pathway is dominant by 30%; however, the HCNH+ must be in its linear form. Dissociative recombination with its structural isomer, H2NC+ produces hydrogen isocyanide (HNC), exclusively.

HCN is destroyed in interstellar clouds through a number of mechanisms depending on the location in the cloud. In the Photon-Dominated Region (PDR), photodissociation dominates, producing CN (HCN + ν → CN + H). At further depths, photodissociation by cosmic rays dominate, producing CN (HCN + cr → CN + H). In the dark core, two competing mechanisms destroy it, forming HCN+ and HCNH+ (HCN + H+ → HCN+ + H; HCN + HCO+ → HCNH+ + CO). The reaction with HCO+ dominates by a factor of ~3.5. HCN has been used to analyze a variety of species and processes in the interstellar medium. It has been suggested as a tracer for dense molecular gas and as a tracer of stellar inflow in high-mass star-forming regions. Further, the HNC/HCN ratio has been shown to be an excellent method for distinguishing between PDRs and X-ray-dominated regions (XDRs).

Hydrogen cyanide as a poison and chemical weapon

See also: cyanide poisoning
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 most infamously employed by Nazi Germany in the mid-20th century in concentration and death camps. The same product is currently made in the Czech Republic under the trademark "Uragan D1." 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 that cause general poisoning and skin blisters. 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 Weaponsmarker.

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 56,000 ppm/> Documentation for Immediately Dangerous to Life or Health Concentrations (IDLHs) - 74908.


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  4. The Innovation Group
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