Hydrogen cyanide

Hydrogen cyanide

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Email this to a friend Hydrogen cyanide

Hydrogen cyanide

Hydrogen cyanide is a chemical compound with chemical formula HCN. A solution of hydrogen cyanide in water is called hydrocyanic acid. Hydrogen cyanide is a colorless, very poisonous, and highly volatile liquid that boils slightly above room temperature at 26 °C (78.8 °F). HCN has a faint, bitter, almond-like odor that some people are unable to detect due to a genetic trait.^[1] Hydrogen cyanide is weakly acidic and partly ionizes in solution to give the cyanide anion, CN^?. The salts of hydrogen cyanide are known as cyanides. HCN is a highly valuable precursor to many chemical compounds ranging from polymers to pharmaceuticals.

Hydrogen cyanide is a linear molecule, with a triple bond between carbon and nitrogen. It is a weak acid with a pK_a of 9.2. A minor isomer of HCN is HNC, hydrogen isocyanide.

Production and synthesis

Historical
Contemporary

Applications
Occurrence

HCN and the origin of life

Hydrogen cyanide as a poison and chemical weapon
Footnotes
References
External links

Production and synthesis

Historical

The first source for hydrogen cyanide was the reaction of acid on ferrocyanides. The rising demand due to the use of cyanides for mining operations in the 1890s was met by the Beilby process. George Thomas Beilby 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.

Contemporary

Hydrogen cyanide can be formed from virtually any combination of hydrogen, carbon, and nitrogen. Hydrogen cyanide is currently produced in large quantities by several processes, as well as being a recovered waste product from the manufacture of acrylonitrile.^[2] In the year 2000, 1.615 billion pounds (732,552 tons) were produced in the US.http://www.the-innovation-group.com/ChemProfiles/Hydrogen%20Cyanide.htm The most important process for the production of hydrogen cyanide 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:^[3]

2CH₄ + 2NH₃ + 3O₂ ? 2HCN + 6H₂O

The energy needed for the reaction is provided by the part 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:^[4]

CH₄ + NH₃ ? HCN + 3H₂

This reaction is akin to steam reforming, the reaction of methane and water.

In the Shawinigan Process, ammonia is passed over coke to give HCN. Carbon monoxide and ammonia at high temperature conditions:

CO + NH₃ ? HCN + H₂O

In another process, 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.

Applications

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, the precursor to Nylon 66.^[2]Ernst Gail, Stephen Gos, Rupprecht Kulzer, Jürgen Lorösch, Andreas Rubo, Manfred Sauer "Cyano Compounds, Inorganic" in Ullmann's Encyclopedia of Industrial Chemistry. 2004, Wiley-VCH Verlag, Weinheim. . Article Online Posting Date: January 15, 2004

Occurrence

HCN has been detected in the Interstellar medium.^[5] 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 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 (CAS#532-28-5) and amygdalin. Such molecules slowly release hydrogen cyanide.^[6]^[7]100 g of crushed apple seeds can yield about 10 mg of HCN. Some millipedes release hydrogen cyanide as a defense mechanism,^[8] as do certain insects such as some burnet moths. Hydrogen cyanide is contained in the exhaust of vehicles, in tobacco and wood smoke, and in smoke from burning nitrogen-containing plastics.

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. Leslie Orgel, among many researchers, has written extensively on the condensation of HCN.^[9] 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.^[10]

Hydrogen cyanide as a poison and chemical weapon

See also: cyanide poisoning

An HCN concentration of 300 mg/m³ in air will kill a human within a few minutes.^[11] The toxicity is caused by the cyanide ion, which prevents cellular respiration. Hydrogen cyanide (under the brand name Zyklon B) was most infamously employed by the Nazi regime in the mid-20th century.

Hydrogen cyanide is commonly listed amongst chemical warfare agents that cause general poisoning.^[12] 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 OPCW.

Hydrogen cyanide gas in air is explosive at concentrations over 5.6%, equivalent to 56,000 ppm^[13].

Footnotes

References

Institut national de recherche et de sécurité (1997). "Cyanure d'hydrogène et solutions aqueuses". Fiche toxicologique n° 4, Paris:INRS, 5pp. (PDF file, in French)