Hydrogen cyanide explained


Structure and general properties

Hydrogen cyanide is a linear molecule, with a triple bond between carbon and nitrogen. The tautomer of HCN is HNC, hydrogen isocyanide.


HCN has a faint bitter almond-like odor that some people are unable to detect owing to a recessive genetic trait.[1] The volatile compound has been used as inhalation rodenticide and human poison, as well as for killing whales.[2] Cyanide ions interfere with iron-containing respiratory enzymes.

History of discovery

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. In 1752, the French chemist Pierre Macquer made the important step of showing that Prussian blue could be converted to an iron oxide plus a volatile component and that these could be used to reconstitute it.[3] The new component was what is now known as hydrogen cyanide. Following Macquer's lead, it was first prepared from Prussian blue by the Swedish chemist Carl Wilhelm Scheele in 1782,[4] and was eventually given the German name Blausäure (lit. "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,[5] an important contribution to acid theory, which had hitherto postulated that acids must contain oxygen[6] (hence the name of oxygen itself, which is derived from Greek elements that mean "acid-former" and are likewise calqued into German as Sauerstoff). In 1811, Joseph Louis Gay-Lussac prepared pure, liquified hydrogen cyanide.[7] In 1815, Gay-Lussac deduced Prussic acid's chemical formula.[8] The radical cyanide in hydrogen cyanide was given its name from cyan, not only an English word for a shade of blue but the Greek word for blue (Greek, Ancient (to 1453);: κύανος), again owing 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 2006 between 500 million and 1 billion pounds (between 230,000 and 450,000 t) were produced in the US.[9]

The most important process is the Andrussow oxidation invented by Leonid Andrussow at IG Farben in which methane and ammonia react in the presence of oxygen at about over a platinum catalyst:[10]

2 CH4 + 2 NH3 + 3 O2 → 2 HCN + 6 H2OThe energy needed for the reaction is provided by the partial oxidation of methane and ammonia.


CH4 + NH3 → HCN + 3H2This reaction is akin to steam reforming, the reaction of methane and water to give carbon monoxide and hydrogen.

In the Shawinigan Process, hydrocarbons, e.g. propane, are reacted with ammonia.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




HCN is used globally as a fumigant against many species of pest insect that infest food production facilities. Both its efficacy and method of application lead to very small amounts of the fumigant being used compared to other toxic substances used for the same purpose.[12] Using HCN as a fumigant also has minimal environmental impact compared to similar structural fumigant molecules such as sulfuryl flouride [13] and methyl bromide [14]


HCN is obtainable from fruits that have a pit, such as cherries, apricots, apples, and bitter almonds, 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.[15] [16] One hundred grams of crushed apple seeds can yield about 70 mg of HCN.[17] So-called "bitter" roots of the cassava plant may contain up to 1 gram of HCN per kilogram.[18] [19] Some millipedes, such as Harpaphe haydeniana, Desmoxytes purpurosea, and Apheloria release hydrogen cyanide as a defense mechanism,[20] as do certain insects, such as burnet moths and the larvae of Paropsisterna eucalyptus.[21] Hydrogen cyanide is contained in the exhaust of vehicles, and in smoke from burning nitrogen-containing plastics.

HCN on Titan

HCN has been measured in Titan's atmosphere by four instruments on the Cassini space probe, one instrument on Voyager, and one instrument on Earth.[22] One of these measurements was in situ, where the Cassini spacecraft dipped between above Titan's surface to collect atmospheric gas for mass spectrometry analysis.[23] HCN initially forms in Titan's atmosphere through the reaction of photochemically produced methane and nitrogen radicals which proceed through the H2CN intermediate, e.g., (CH3 + N → H2CN + H → HCN + H2).[24] [25] Ultraviolet radiation breaks HCN up into CN + H; however, CN is efficiently recycled back into HCN via the reaction CN + CH4 → HCN + CH3.

HCN on the young Earth

It has been postulated that carbon from a cascade of asteroids (known as the Late Heavy Bombardment), resulting from interaction of Jupiter and Saturn, blasted the surface of young Earth and reacted with nitrogen in Earth's atmosphere to form HCN.[26]

HCN in mammals





HCN is a constituent of tobacco smoke.[30]

HCN and the origin of life


HCN in space





In February 2016, it was announced that traces of hydrogen cyanide were found in the atmosphere of the hot Super-Earth 55 Cancri e with NASA's Hubble Space Telescope.[45]

As a poison and chemical weapon

See main article: Cyanide poisoning. In World War I, hydrogen cyanide was used by the French from 1916 as a chemical weapon against the Central Powers, and by the United States and Italy in 1918. It was not found to be effective enough due to weather conditions.[46] [47] The gas is lighter than air and rapidly disperses up into the atmosphere. Rapid dilution made its use in the field impractical. In contrast, denser agents such as phosgene or chlorine tended to remain at ground level and sank into the trenches of the Western Front's battlefields. Compared to such agents hydrogen cyanide had to be present in higher concentrations in order to be fatal.

A hydrogen cyanide concentration of 100–200 ppm in breathing air will kill a human within 10 to 60 minutes.[48] A hydrogen cyanide concentration of 2000 ppm (about 2380 mg/m3) will kill a human in about one minute.[48] The toxic effect is caused by the action of the cyanide ion, which halts cellular respiration. It acts as a non-competitive inhibitor for an enzyme in mitochondria called cytochrome c oxidase. As such, hydrogen cyanide is commonly listed among chemical weapons as a blood agent.[49]

The Chemical Weapons Convention lists it under Schedule 3 as a potential weapon which has large-scale industrial uses. Signatory countries must declare manufacturing plants that produce more than 30 metric tons per year, and allow inspection by the Organisation for the Prohibition of Chemical Weapons.

Perhaps its most infamous use is German: [[Zyklon B]] (German: Cyclone B, with the B standing for German: Blausäure – prussic acid; also, to distinguish it from an earlier product later known as Zyklon A),[50] it was used in Nazi German extermination camps during World War II to kill en masse as part of their Final Solution genocide program. Hydrogen cyanide was also used in the camps for delousing clothing in attempts to eradicate diseases carried by lice and other parasites. One of the original Czech producers continued making Zyklon B under the trademark "Uragan D2"[51] until recently.

Hydrogen cyanide was also the agent employed in judicial execution in some U.S. states, where it was produced during the execution by the action of sulfuric acid on sodium or potassium cyanide.

Under the name prussic acid, HCN has been used as a killing agent in whaling harpoons, although it proved quite dangerous to the crew deploying it, and was quickly abandoned.[2] From the middle of the 18th century it was used in a number of poisoning murders and suicides.[52]

Hydrogen cyanide gas in air is explosive at concentrations above 5.6%.[53] This concentration is far above a toxic level.

External links

Notes and References

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    Reprinted in Latin as: Book: https://books.google.com/books?id=BLo5AAAAcAAJ&pg=PA148 . De materia tingente caerulei berolinensis . The dark matter tingente caerulei berolinensis . Latin . Scheele CW, Hebenstreit EB . Schäfer GH . Opuscula Chemica et Physica . (Leipzig ("Lipsiae") (Germany) . Johann Godfried Müller . 1789 . 2 . 148–174 .
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