|Jmol-3D images||Image 1|
|Molar mass||27.03 g mol−1|
|Appearance||Very pale, blue, transparent liquid or colorless gas|
|Odor||Oil of bitter almond|
|Density||0.687 g mL−1|
-14--12 °C, 259-261 K, 7-10 °F
25-26 °C, 298.5-299.5 K, 78-79 °F
|Solubility in water||Miscible|
|Solubility in ethanol||Miscible|
|kH||75 μmol Pa−1 kg−1|
|Refractive index (nD)||1.2675 |
|Viscosity||201 μPa s|
|Dipole moment||2.98 D|
|Std enthalpy of
|-4.999 MJ kg−1|
|Std enthalpy of
|642 kJ mol−1|
|113.01 J K−1 mol−1|
|Specific heat capacity, C||71.00 kJ K−1 mol−1 (at 27 °C)|
|EU classification||F+ T+ N|
|R-phrases||R12, R26/27/28, R50/53|
|S-phrases||(S1/2), S16, S36/37, S38, S45, S53, S59, S61|
|Flash point||−17.8 °C|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Hydrogen cyanide (HCN) (with the alternate archaic name of prussic acid) is an inorganic 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 partially 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, 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.
History of discovery 
Hydrogen cyanide was first isolated from a blue pigment (Prussian blue) which had been known from 1704 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 iron oxide plus a volatile component and that these could be used to reconstitute it. The new component was what we now know as hydrogen cyanide. Following Macquer's lead, it was first prepared from Prussian blue by the Swedish chemist Carl Wilhelm Scheele in 1782, 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, an important contribution to acid theory, which had thitherto postulated that acids must contain oxygen (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. In 1815 Gay-Lussac deduced Prussic acid's chemical formula. 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, 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 were produced in the US.
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 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.
- CH4 + NH3 → HCN + 3H2
- 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.
HCN is the precursor to sodium cyanide and potassium cyanide, which are used mainly in gold and silver mining and for the electroplating of those metals. 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 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. One hundred grams of crushed apple seeds can yield about 70 mg of HCN. Some millipedes release hydrogen cyanide as a defense mechanism, 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. 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. For example, HCN is proposed to have played a part in the origin of life. Although the relationship of these chemical reactions to the origin of life theory remains speculative, studies in this area have led to discoveries of new pathways to organic compounds derived from the condensation of HCN.
HCN in space 
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+, exclusively produces hydrogen isocyanide (HNC).
HCN is destroyed in interstellar clouds through a number of mechanisms depending on the location in the cloud. In photon-dominated regions (PDRs), 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 
A hydrogen cyanide concentration of 300 mg/m3 in air will kill a human within 10–60 minutes. A hydrogen cyanide 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 has been absorbed into a carrier for use as a pesticide. Under IG Farben's brand name Zyklon B (German >Cyclone B, with the B standing for Blausäure - "Prussic Acid"), it was used in the Nazi concentration camp murders during the Holocaust. 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. During the First World War, USA and Italy used hydrogen cyanide against the Central Powers in 1918. France had used it in combat already in 1916, but was ineffective due to physical conditions.
Hydrogen cyanide gas in air is explosive at concentrations over 5.6%. This is far above its toxicity level.
- "Hydrogen Cyanide - Compound Summary". PubChem Compound. USA: National Center for Biotechnology Information. 16 September 2004. Identification. Retrieved 2012-06-04.
- "hydrogen cyanide (CHEBI:18407)". Chemical Entities of Biological Interest. UK: European Bioinformatics Institute. 18 October 2009. Main. Retrieved 2012-06-04.
- Perrin, D. D. (1982). Ionisation Constants of Inorganic Acids and Bases in Aqueous Solution (2nd ed.). Oxford: Pergamon Press.
- Patnaik, P. (2002). Handbook of Inorganic Chemicals. McGraw-Hill. ISBN 0-07-049439-8.
- Maxwell, G. R. (2004). Synthetic Nitrogen Products: A Practical Guide to the Products and Processes. Springer. p. 348. ISBN 978-0-306-48225-0. Retrieved 2012-06-03.
- Gail, E.; Gos, S.; Kulzer, R.; Lorösch, J.; Rubo, A.; Sauer, M. (2005), "Cyano Compounds, Inorganic", Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH, doi:10.1002/14356007.a08_159.pub2
- "Cyanide, inability to smell". Online Mendelian Inheritance in Man. Retrieved 2010-03-31.
- Pierre-Joseph Macquer (presented: 1752 ; published: 1756) "Éxamen chymique de bleu de Prusse" (Chemical examination of Prussian blue), Mémoires de l'Académie royale des Sciences , pages 60-77.
- Carl W. Scheele (1782) "Försök, beträffande det färgande ämnet uti Berlinerblå" (Experiment concerning the coloring substance in Berlin blue), Kungliga Svenska Vetenskapsakademiens handlingar (Royal Swedish Academy of Science's Proceedings), 3: 264-275 (in Swedish).
- Reprinted in Latin as: "De materia tingente caerulei berolinensis" in: Carl Wilhelm Scheele with Ernst Benjamin Gottlieb Hebenstreit (ed.) and Gottfried Heinrich Schäfer (trans.), Opuscula Chemica et Physica (Leipzig ("Lipsiae"), (Germany): Johann Godfried Müller, 1789), vol. 2, pages 148-174.
- Berthollet (presented: 1787 ; published: 1789)
- Reprinted in: Berthollet (1789) "Extrait d'un mémoire sur l'acide prussique" (Extract of a memoir on prussic acid), Annales de chimie 1: 30-39.
- Newbold, B. T. (1999-11-01). "Claude Louis Berthollet: A Great Chemist in the French Tradition". Canadian Chemical News. Retrieved 2010-03-31.
- Gay-Lussac (1811) "Note sur l'acide prussique" (Note on prussic acid), Annales de chimie, 44: 128 - 133.
- Gay-Lussac (1815) "Recherche sur l'acide prussique" (Research on prussic acid) Annales de Chimie 95: 136-231.
- . EPA. Retrieved on 2013-01-31.
- Andrussow, L. (1935). "The catalytic oxydation of ammonia-methane-mixtures to hydrogen cyanide". Angewandte Chemie 48 (37): 593–595. doi:10.1002/ange.19350483702.
- Endter, F. (1958). "Die technische Synthese von Cyanwasserstoff aus Methan und Ammoniak ohne Zusatz von Sauerstoff". Chemie Ingenieur Technik 30 (5): 305–310. doi:10.1002/cite.330300506.
- Vetter, J. (2000). "Plant cyanogenic glycosides". Toxicon 38 (1): 11–36. doi:10.1016/S0041-0101(99)00128-2. PMID 10669009.
- Jones, D. A. (1998). "Why are so many food plants cyanogenic?". Phytochemistry 47 (2): 155–162. doi:10.1016/S0031-9422(97)00425-1. PMID 9431670.
- "Are Apple Cores Poisonous? The Naked Scientists September 2010". Retrieved 6 September 2012.
- Blum, M. S.; Woodring, J. P. (1962). "Secretion of Benzaldehyde and Hydrogen Cyanide by the Millipede Pachydesmus crassicutis (Wood)". Science 138 (3539): 512–513. Bibcode:1962Sci...138..512B. doi:10.1126/science.138.3539.512. PMID 17753947.
- Aregheore, E. M.; Agunbiade, O. O. (1991). "The toxic effects of cassava (manihot esculenta grantz) diets on humans: a review". Veterinary and Human Toxicology 33 (3): 274–275. PMID 1650055.
- White, W. L. B.; Arias-Garzon, D. I.; McMahon, J. M.; Sayre, R. T. (1998). "Cyanogenesis in Cassava, The Role of Hydroxynitrile Lyase in Root Cyanide Production". Plant Physiology 116 (4): 1219–1225. doi:10.1104/pp.116.4.1219. PMC 35028. PMID 9536038.
- Matthews, C. N. (2004). "The HCN World: Establishing Protein - Nucleic Acid Life via Hydrogen Cyanide Polymers". Origins: Genesis, Evolution and Diversity of Life. Cellular Origin and Life in Extreme Habitats and Astrobiology 6. pp. 121–135. doi:10.1007/1-4020-2522-X_8. ISBN 978-1-4020-2522-8.
- Al-Azmi, A.; Elassar, A.-Z. A.; Booth, B. L. (2003). "The Chemistry of Diaminomaleonitrile and its Utility in Heterocyclic Synthesis". Tetrahedron 59 (16): 2749–2763. doi:10.1016/S0040-4020(03)00153-4.
- Snyder, L. E.; Buhl, D. (1971). "Observations of Radio Emission from Interstellar Hydrogen Cyanide" (pdf). Astrophysical Journal 163: L47–L52. Bibcode:1971ApJ...163L..47S. doi:10.1086/180664.
- Treffers, R.; Larson, H. P.; Fink, U.; Gautier, T. N. (1978). "Upper limits to trace constituents in Jupiter's atmosphere from an analysis of its 5-μm spectrum". Icarus 34 (2): 331–343. Bibcode:1978Icar...34..331T. doi:10.1016/0019-1035(78)90171-9.
- Bieging, J. H.; Shaked, S.; Gensheimer, P. D. (2000). "Submillimeter‐ and Millimeter‐Wavelength Observations of SiO and HCN in Circumstellar Envelopes of AGB Stars" (pdf). Astrophysical Journal 543 (2): 897–921. Bibcode:2000ApJ...543..897B. doi:10.1086/317129.
- Schilke, P.; Menten, K. M. (2003). "Detection of a Second, Strong Sub-millimeter HCN Laser Line toward Carbon Stars" (pdf). Astrophysical Journal 583 (1): 446–450. Bibcode:2003ApJ...583..446S. doi:10.1086/345099.
- Boger, G. I.; Sternberg, A. (2005). "CN and HCN in Dense Interstellar Clouds" (pdf). Astrophysical Journal 632 (1): 302–315. arXiv:astro-ph/0506535. Bibcode:2005ApJ...632..302B. doi:10.1086/432864.
- Gao, Y.; Solomon, P. M. (2004). "The Star Formation Rate and Dense Molecular Gas in Galaxies" (pdf). Astrophysical Journal 606 (1): 271–290. arXiv:astro-ph/0310339. Bibcode:2004ApJ...606..271G. doi:10.1086/382999.
- Gao, Y.; Solomon, P. M. (2004). "HCN Survey of Normal Spiral, Infrared‐luminous, and Ultraluminous Galaxies" (pdf). Astrophysical Journal Supplements 152: 63–80. arXiv:astro-ph/0310341. Bibcode:2004ApJS..152...63G. doi:10.1086/383003.
- Wu, J.; Evans, N. J. (2003). "Indications of Inflow Motions in Regions Forming Massive Stars" (pdf). Astrophysical Journal 592 (2): L79–L82. arXiv:astro-ph/0306543. Bibcode:2003ApJ...592L..79W. doi:10.1086/377679.
- Loenen, A. F. (2007). Proceedings IAU Symposium 202.
- Environmental and Health Effects. Cyanidecode.org. Retrieved on 2012-06-02.
- Dwork, D.; van Pelt, R. J. (1996). Auschwitz, 1270 to the present. Norton. p. 443. ISBN 0-393-03933-1.
- "Hydrogen Cyanide". Organisation for the Prohibition of Chemical Weapons. Retrieved 2009-01-14.
- Markus Schnedlitz (2008) Chemische Kampfstoffe: Geschichte, Eigenschaften, Wirkung. GRIN Verlag, ISBN 364023360-3, p. 13
- Poison Hand Darted Harpoons and Lances.
- "Documentation for Immediately Dangerous to Life or Health Concentrations (IDLHs) – 74908". NIOSH.
- 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)
- International Chemical Safety Card 0492
- Hydrogen cyanide and cyanides (CICAD 61)
- National Pollutant Inventory: Cyanide compounds fact sheet
- NIOSH Pocket Guide to Chemical Hazards
- Department of health review
- OSHA: HCN Health Guidelines