hydrogen cyanide#As a poison and chemical weapon

Hydrogen cyanide is a linear molecule, with a triple bond between carbon and nitrogen. The tautomer of HCN is HNC, hydrogen isocyanide.{{citation needed|date=September 2023}}

Much literature has historically claimed that hydrogen cyanide smells of almonds or bitter almonds. However, there has been considerable confusion and disagreement over this, because the smell of household almond essence is due to benzaldehyde, which is released along with hydrogen cyanide from the breakdown of amygdalin present in some plant seeds, and thus is often mistaken for it.{{cite web | url = https://chemistry.stackexchange.com/questions/47204/how-do-people-know-hcn-smells-like-almonds | title = How do people know HCN smells like almonds? }}{{cite web | url = https://chemistry.stackexchange.com/questions/80564/do-almonds-smell-like-they-do-because-of-cyanide | title = Do almonds smell like they do because of cyanide? }}

About half of people are unable to detect the smell of hydrogen cyanide owing to a recessive genetic trait.{{cite web | work = Online Mendelian Inheritance in Man | url = https://www.ncbi.nlm.nih.gov/omim/304300 | title = Cyanide, inability to smell | access-date = 2010-03-31 }}

The volatile compound has been used as inhalation rodenticide and human poison, as well as for killing whales.{{Cite web| vauthors = Lytle T |title=Poison Harpoons|url=http://www.whalecraft.net/Poison_Irons.html|url-status=dead|archive-url=https://web.archive.org/web/20190215100154/http://www.whalecraft.net/Poison_Irons.html|archive-date=2019-02-15|website=Whalecraft.net}} Cyanide ions interfere with iron-containing respiratory enzymes.{{citation needed|date=September 2023}}

Hydrogen cyanide is weakly acidic with a pK_a of 9.2. It partially ionizes in water to give the cyanide anion, {{chem2|CN−}}. HCN forms hydrogen bonds with its conjugate base, species such as {{chem2|(CN-)(HCN)_{n} }}.{{cite journal |doi=10.1002/anie.201915206 |title=Salts of HCN-Cyanide Aggregates: [CN(HCN)₂]⁻ and [CN(HCN)₃]⁻ |date=2020 |last1=Bläsing |first1=Kevin |last2=Harloff |first2=Jörg |last3=Schulz |first3=Axel |last4=Stoffers |first4=Alrik |last5=Stoer |first5=Philip |last6=Villinger |first6=Alexander |journal=Angewandte Chemie International Edition |volume=59 |issue=26 |pages=10508–10513 |pmid=32027458 |pmc=7317722 }}

Hydrogen cyanide reacts with alkenes to give nitriles. The conversion, which is called hydrocyanation, employs nickel complexes as catalysts.{{cite book | last=Leeuwen | first=P. W. N. M. van | title=Homogeneous Catalysis: Understanding the Art | publisher=Kluwer Academic Publishers | publication-place=Dordrecht | date=2004 | isbn=1402019998 | oclc=54966334}}

:{{chem2|RCH\dCH2 + HCN → RCH2\sCH2CN}}

Four molecules of HCN will tetramerize into diaminomaleonitrile.{{cite journal |first1=J. P. |last1=Ferris|first2=R. A.|last2=Sanchez|title=Diaminomaleonitrile (Hydrogen Cyanide Tetramer) |journal=Organic Syntheses |date=1968 |volume=48 |page=60 |doi=10.15227/orgsyn.048.0060 }}

Metal cyanides are typically prepared by salt metathesis from alkali metal cyanide salts, but mercuric cyanide is formed from aqueous hydrogen cyanide:{{cite book|author1=F. Wagenknecht|author2=R. Juza|chapter=Mercury (II) cyanide|title=Handbook of Preparative Inorganic Chemistry|edition=2nd |editor=G. Brauer|publisher=Academic Press|year=1963|place=NY, NY|volume=2}}

:{{chem2|HgO + 2 HCN -> Hg(CN)2 + H2O}}

Hydrogen cyanide was first isolated in 1752 by French chemist Pierre Macquer who converted Prussian blue to an iron oxide plus a volatile component and found that these could be used to reconstitute it.{{cite journal | vauthors = Macquer PJ | author-link1 = Pierre Macquer | date = 1756 | url = http://gallica.bnf.fr/ark:/12148/bpt6k35505/f242 | title = Éxamen chymique de bleu de Prusse | trans-title = Chemical examination of Prussian blue | language= French | journal = Mémoires de l'Académie royale des Sciences | pages = 60–77 }} The new component was what is now known as hydrogen cyanide. It was subsequently prepared from Prussian blue by the Swedish chemist Carl Wilhelm Scheele in 1782,{{cite journal | vauthors = Scheele CW | date = 1782 | url = https://books.google.com/books?id=mHVJAAAAcAAJ&pg=PA264 | title = Försök, beträffande det färgande ämnet uti Berlinerblå | trans-title = Experiment concerning the coloring substance in Berlin blue | language = Swedish | journal = Kungliga Svenska Vetenskapsakademiens Handlingar (Royal Swedish Academy of Science's Proceedings | volume = 3 | pages = 264–275 }}

Reprinted in Latin as: {{cite book | chapter-url = https://books.google.com/books?id=BLo5AAAAcAAJ&pg=PA148 | chapter = De materia tingente caerulei berolinensis | trans-title = The dark matter tingente caerulei berolinensis | language = Latin | veditors = Scheele CW, Hebenstreit EB | translator = Schäfer GH | title = Opuscula Chemica et Physica | location = (Leipzig ("Lipsiae") (Germany) | publisher = Johann Godfried Müller | date = 1789 | volume = 2 | pages = 148–174 }} 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,{{cite journal | vauthors = Berthollet CL | date = 1789 | url = https://books.google.com/books?id=fC5EAAAAcAAJ&pg=PA148 | title = Mémoire sur l'acide prussique | trans-title = Memoir on prussic acid | language = French | journal = Mémoires de l'Académie Royale des Sciences | pages = 148–161 }}

Reprinted in: {{cite journal| vauthors = Berthollet CL |year=1789|url=http://gallica.bnf.fr/ark:/12148/bpt6k110315k/f40.image.langEN |title=Extrait d'un mémoire sur l'acide prussique|trans-title=Extract of a memoir on prussic acid|journal=Annales de Chimie|volume=1|pages=30–39}} an important contribution to acid theory, which had hitherto postulated that acids must contain oxygen{{cite news | vauthors = Newbold BT | title = Claude Louis Berthollet: A Great Chemist in the French Tradition | date = 1999-11-01 | newspaper = Canadian Chemical News | url = http://www.allbusiness.com/north-america/canada/370855-1.html | access-date = 2010-03-31 | archive-date = 2008-04-20 | archive-url = https://web.archive.org/web/20080420175823/http://www.allbusiness.com/north-america/canada/370855-1.html | url-status = dead }} (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,{{cite journal| vauthors = Gay-Lussac JL |year=1811 |url=https://books.google.com/books?id=uJs5AAAAcAAJ&pg=PA128 |title=Note sur l'acide prussique|trans-title=Note on prussic acid|journal=Annales de Chimie|volume=44|pages=128–133}} and in 1815 he deduced Prussic acid's chemical formula.{{cite journal| vauthors = Gay-Lussac JL |year=1815|url=https://books.google.com/books?id=m9s3AAAAMAAJ&pg=PA136|title=Recherche sur l'acide prussique|trans-title=Research on prussic acid|journal=Annales de Chimie|volume=95|pages=136–231}}

The word cyanide for the radical in hydrogen cyanide was derived from its French equivalent, cyanure, which Gay-Lussac constructed from the Ancient Greek word κύανος for dark blue enamel or lapis lazuli, again owing to the chemical’s derivation from Prussian blue. Incidentally, the Greek word is also the root of the English color name cyan.

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 {{cvt|1200|°C}} over a platinum catalyst:{{cite journal | vauthors = Andrussow L | title = The catalytic oxydation of ammonia-methane-mixtures to hydrogen cyanide | journal = Angewandte Chemie | year = 1935 | volume = 48 | issue = 37 | pages = 593–595 | doi = 10.1002/ange.19350483702 | bibcode = 1935AngCh..48..593A }}

:{{chem2|2 CH4 + 2 NH3 + 3 O2 → 2 HCN + 6 H2O}}

In 2006, between 500 million and 1 billion pounds (between 230,000 and 450,000 t) were produced in the US.{{cite web |url=http://cfpub.epa.gov/iursearch/2006_iur_companyinfo.cfm?chemid=6177&outchem=both |title=Non-confidential 2006 IUR Records by Chemical, including Manufacturing, Processing and Use Information |website=EPA |archive-url=https://web.archive.org/web/20130510000856/http://cfpub.epa.gov/iursearch/2006_iur_companyinfo.cfm?chemid=6177&outchem=both |accessdate=2013-01-31|archive-date=2013-05-10 }} Hydrogen cyanide is produced in large quantities by several processes and is a recovered waste product from the manufacture of acrylonitrile.

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:{{cite journal | vauthors = Endter F | title = Die technische Synthese von Cyanwasserstoff aus Methan und Ammoniak ohne Zusatz von Sauerstoff | journal = Chemie Ingenieur Technik | year = 1958 | volume = 30 | issue = 5 | pages = 305–310 | doi = 10.1002/cite.330300506 }}

:{{chem2|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, 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:

:{{chem2|H+ + CN- → HCN}}

This reaction is sometimes the basis of accidental poisonings because the acid converts a nonvolatile cyanide salt into the gaseous HCN.

Hydrogen cyanide could be obtained from potassium ferricyanide and acid:

:{{chem2|6 H+ + [Fe(CN)6]3− → 6 HCN + Fe3+}}{{cite web | url = http://www.labchem.com/tools/msds/msds/LC19040.pdf | title = MSDS for potassium ferricyanide | access-date = 2023-04-17 | archive-date = 2016-04-18 | archive-url = https://web.archive.org/web/20160418075117/http://www.labchem.com/tools/msds/msds/LC19040.pdf | url-status = dead }}{{Cite PubChem|cid=26250|title=Potassium ferricyanide}}

The large 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-6,6.

HCN is used globally as a fumigant against many species of pest insects 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.{{Cite web|url=http://www.fao.org/3/X5042E/x5042E0k.htm#Fumigation%20of%20large%20structures|title = Manual of fumigation for insect control – Space fumigation at atmospheric pressure (Cont.)|website=Food and Agriculture Organization}} Using HCN as a fumigant also has less environmental impact, compared to some other fumigants such as sulfuryl fluoride,{{Cite web|url=https://news.mit.edu/2009/prinn-greenhouse-tt0311|title = New greenhouse gas identified|website=News.mit.edu| date=11 March 2009 }} and methyl bromide.{{cite web|url=https://csl.noaa.gov/assessments/ozone/1994/chapters/chapter10.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://csl.noaa.gov/assessments/ozone/1994/chapters/chapter10.pdf |archive-date=2022-10-09 |url-status=live|title=Chapter 10 : Methyl Bromide|website=Csl.noaa.gov}}

HCN is obtainable from fruits that have a pit, such as cherries, apricots, apples, and nuts such as bitter almonds, from which almond oil and extract is made. Many of these pits contain small amounts of cyanohydrins such as mandelonitrile and amygdalin, which slowly release hydrogen cyanide.{{cite journal | vauthors = Vetter J | title = Plant cyanogenic glycosides | journal = Toxicon | volume = 38 | issue = 1 | pages = 11–36 | date = January 2000 | pmid = 10669009 | doi = 10.1016/S0041-0101(99)00128-2| bibcode = 2000Txcn...38...11V }}{{cite journal | vauthors = Jones DA | title = Why are so many food plants cyanogenic? | journal = Phytochemistry | volume = 47 | issue = 2 | pages = 155–162 | date = January 1998 | pmid = 9431670 | doi = 10.1016/S0031-9422(97)00425-1 | bibcode = 1998PChem..47..155J }} One hundred grams of crushed apple seeds can yield about 70 mg of HCN.{{cite web |url=http://www.thenakedscientists.com/HTML/index.php?id=31&tx_naksciquestions_pi1%5BshowUid%5D=2737&cHash=69220df3a3 |title=Are Apple Cores Poisonous? |access-date=6 March 2014 |url-status=dead |archive-url=https://web.archive.org/web/20140306130316/http://www.thenakedscientists.com/HTML/index.php?id=31&tx_naksciquestions_pi1%5BshowUid%5D=2737&cHash=69220df3a3 |publisher=The Naked Scientists|date=26 September 2010|archive-date=6 March 2014 }} The roots of cassava plants contain cyanogenic glycosides such as linamarin, which decompose into HCN in yields of up to 370 mg per kilogram of fresh root.{{cite journal | vauthors = Aregheore EM, Agunbiade OO | title = The toxic effects of cassava (manihot esculenta grantz) diets on humans: a review | journal = Veterinary and Human Toxicology | volume = 33 | issue = 3 | pages = 274–275 | date = June 1991 | pmid = 1650055 }} Some millipedes, such as Harpaphe haydeniana, Desmoxytes purpurosea, and Apheloria release hydrogen cyanide as a defense mechanism,{{cite journal | vauthors = Blum MS, Woodring JP | title = Secretion of Benzaldehyde and Hydrogen Cyanide by the Millipede Pachydesmus crassicutis (Wood) | journal = Science | volume = 138 | issue = 3539 | pages = 512–513 | date = October 1962 | pmid = 17753947 | doi = 10.1126/science.138.3539.512 | s2cid = 40193390 | bibcode = 1962Sci...138..512B }} as do certain insects, such as burnet moths and the larvae of Paropsisterna eucalyptus.{{cite journal | vauthors = Zagrobelny M, de Castro ÉC, Møller BL, Bak S | title = Cyanogenesis in Arthropods: From Chemical Warfare to Nuptial Gifts | journal = Insects | volume = 9 | issue = 2 | pages = 51 | date = May 2018 | pmid = 29751568 | pmc = 6023451 | doi = 10.3390/insects9020051 | doi-access = free }} Hydrogen cyanide is contained in the exhaust of vehicles, and in smoke from burning nitrogen-containing plastics.File:PIA18431-SaturnMoon-Titan-SouthPoleVortex-Cloud-20121129.jpg is a giant swirling cloud of HCN (November 29, 2012)]]

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.{{Cite journal| vauthors = Loison JC, Hébrard E, Dobrijevic M, Hickson KM, Caralp F, Hue V, Gronoff G, Venot O, Bénilan Y | display-authors = 6 |date=February 2015|title=The neutral photochemistry of nitriles, amines and imines in the atmosphere of Titan|journal=Icarus|volume=247|pages=218–247|doi=10.1016/j.icarus.2014.09.039|bibcode=2015Icar..247..218L|url=https://lirias.kuleuven.be/handle/123456789/486735}} One of these measurements was in situ, where the Cassini spacecraft dipped between {{cvt|1000 and 1100|km}} above Titan's surface to collect atmospheric gas for mass spectrometry analysis.{{Cite journal| vauthors = Magee BA, Waite JH, Mandt KE, Westlake J, Bell J, Gell DA |date=December 2009|title=INMS-derived composition of Titan's upper atmosphere: Analysis methods and model comparison|journal=Planetary and Space Science|volume=57|issue=14–15|pages=1895–1916|doi=10.1016/j.pss.2009.06.016|bibcode=2009P&SS...57.1895M}} 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).{{cite journal | vauthors = Pearce BK, Molaverdikhani K, Pudritz RE, Henning T, Hébrard E |title=HCN Production in Titan's Atmosphere: Coupling Quantum Chemistry and Disequilibrium Atmospheric Modeling |journal=Astrophysical Journal |year=2020 |volume=901 |issue=2 |page=110 |doi=10.3847/1538-4357/abae5c |arxiv=2008.04312 |bibcode=2020ApJ...901..110P |s2cid=221095540 |doi-access=free }}{{cite journal | vauthors = Pearce BK, Ayers PW, Pudritz RE | title = A Consistent Reduced Network for HCN Chemistry in Early Earth and Titan Atmospheres: Quantum Calculations of Reaction Rate Coefficients | journal = The Journal of Physical Chemistry A | volume = 123 | issue = 9 | pages = 1861–1873 | date = March 2019 | pmid = 30721064 | doi = 10.1021/acs.jpca.8b11323 | arxiv = 1902.05574 | s2cid = 73442008 | bibcode = 2019JPCA..123.1861P }} Ultraviolet radiation breaks HCN up into CN + H; however, CN is efficiently recycled back into HCN via the reaction CN + CH₄ → HCN + CH₃.

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.{{cite news |url=https://www.nytimes.com/2015/05/05/science/making-sense-of-the-chemistry-that-led-to-life-on-earth.html |title=Making Sense of the Chemistry That Led to Life on Earth |access-date=5 May 2015 |newspaper=The New York Times |date=2015-05-04 | vauthors = Wade N }}

Some authors{{who|date=December 2020}} have shown that neurons can produce hydrogen cyanide upon activation of their opioid receptors by endogenous or exogenous opioids. They have also shown that neuronal production of HCN activates NMDA receptors and plays a role in signal transduction between neuronal cells (neurotransmission). Moreover, increased endogenous neuronal HCN production under opioids was seemingly needed for adequate opioid analgesia, as analgesic action of opioids was attenuated by HCN scavengers. They considered endogenous HCN to be a neuromodulator.{{cite journal | vauthors = Borowitz JL, Gunasekar PG, Isom GE | title = Hydrogen cyanide generation by mu-opiate receptor activation: possible neuromodulatory role of endogenous cyanide | journal = Brain Research | volume = 768 | issue = 1–2 | pages = 294–300 | date = September 1997 | pmid = 9369328 | doi = 10.1016/S0006-8993(97)00659-8 | s2cid = 12277593}}

It has also been shown that, while stimulating muscarinic cholinergic receptors in cultured pheochromocytoma cells increases HCN production, in a living organism (in vivo) muscarinic cholinergic stimulation actually decreases HCN production.{{cite journal | vauthors = Gunasekar PG, Prabhakaran K, Li L, Zhang L, Isom GE, Borowitz JL | title = Receptor mechanisms mediating cyanide generation in PC12 cells and rat brain | journal = Neuroscience Research | volume = 49 | issue = 1 | pages = 13–18 | date = May 2004 | pmid = 15099699 | doi = 10.1016/j.neures.2004.01.006 | s2cid = 29850349}}

Leukocytes generate HCN during phagocytosis, and can kill bacteria, fungi, and other pathogens by generating several different toxic chemicals, one of which is hydrogen cyanide.

The vasodilatation caused by sodium nitroprusside has been shown to be mediated not only by NO generation, but also by endogenous cyanide generation, which adds not only toxicity, but also some additional antihypertensive efficacy compared to nitroglycerine and other non-cyanogenic nitrates which do not cause blood cyanide levels to rise.{{cite journal | vauthors = Smith RP, Kruszyna H | title = Toxicology of some inorganic antihypertensive anions | journal = Federation Proceedings | volume = 35 | issue = 1 | pages = 69–72 | date = January 1976 | pmid = 1245233 }}

HCN is a constituent of tobacco smoke.{{cite journal | vauthors = Talhout R, Schulz T, Florek E, van Benthem J, Wester P, Opperhuizen A | title = Hazardous compounds in tobacco smoke | journal = International Journal of Environmental Research and Public Health | volume = 8 | issue = 2 | pages = 613–628 | date = February 2011 | pmid = 21556207 | pmc = 3084482 | doi = 10.3390/ijerph8020613 | doi-access = free}}

As a precursor to amino acids and nucleic acids, hydrogen cyanide has been proposed to have played a part in the origin of life. Compounds of special interest are oligomers of HCN including its trimer aminomalononitrile and tetramer diaminomaleonitrile, which can be described as (HCN)3 and (HCN)4, respectively.{{cite journal |doi=10.3390/life3030421|doi-access=free|title=Simple Organics and Biomonomers Identified in HCN Polymers: An Overview|year=2013|last1=Ruiz-Bermejo|first1=Marta|last2=Zorzano|first2=María-Paz|last3=Osuna-Esteban|first3=Susana|journal=Life|volume=3|issue=3|pages=421–448|pmid=25369814|pmc=4187177|bibcode=2013Life....3..421R }} Although the relationship of these chemical reactions to the origin of life theory remains speculative, studies in this area uncovered new pathways to organic compounds derived from the condensation of HCN (e.g. Adenine).{{cite journal | vauthors = Al-Azmi A, Elassar AZ, Booth BL | title = The Chemistry of Diaminomaleonitrile and its Utility in Heterocyclic Synthesis | journal = Tetrahedron | year = 2003 | volume = 59 | issue = 16 | pages = 2749–2763 | doi = 10.1016/S0040-4020(03)00153-4}}

{{see also|Astrochemistry}}

Because hydrogen cyanide is a precursor to nucleic acids, which are critical for terrestrial life, astronomers are incentivized to search for derivatives of HCN. {{Cite AV media |url=https://www.ted.com/talks/karin_oberg_the_galactic_recipe_for_a_living_planet/transcript?subtitle=en |title=The galactic recipe for a living planet |date=2020-04-10 |last=Öberg |first=Karin |language=en |access-date=2024-12-24 |via=www.ted.com}}

HCN has been detected in the interstellar medium{{cite journal | title = Observations of Radio Emission from Interstellar Hydrogen Cyanide | vauthors = Snyder LE, Buhl D | journal = Astrophysical Journal | year = 1971 | volume = 163 | pages = L47–L52 | doi = 10.1086/180664 | bibcode=1971ApJ...163L..47S}} and in the atmospheres of carbon stars.{{cite book | vauthors = Jørgensen UG | title=Molecules in Astrophysics: Probes and Processes | chapter=Cool Star Models | volume=178 | series=International Astronomical Union Symposia. Molecules in Astrophysics: Probes and Processes | veditors = van Dishoeck EF | publisher=Springer Science & Business Media | year=1997 | isbn=978-0792345381 | page=446 | chapter-url=https://books.google.com/books?id=VW50otz5v8sC&pg=PA446 }} 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.{{cite journal | vauthors = Treffers RR, Larson HP, Fink U, Gautier TN | title = Upper limits to trace constituents in Jupiter's atmosphere from an analysis of its 5-μm spectrum | journal = Icarus | year = 1978 | volume = 34 | issue = 2 | pages = 331–343 | doi = 10.1016/0019-1035(78)90171-9 | bibcode = 1978Icar...34..331T }} The J=1→0, J=3→2, J= 4→3, and J=10→9 pure rotational transitions have all been observed.{{cite journal | vauthors = Bieging JH, Shaked S, Gensheimer PD | title = Submillimeter- and Millimeter-Wavelength Observations of SiO and HCN in Circumstellar Envelopes of AGB Stars | journal = Astrophysical Journal | year = 2000 | volume = 543 | issue = 2 | pages = 897–921 | doi = 10.1086/317129 | bibcode = 2000ApJ...543..897B | doi-access = free}}{{cite journal | title = Detection of a Second, Strong Sub-millimeter HCN Laser Line toward Carbon Stars | vauthors = Schilke P, Menten KM | journal = Astrophysical Journal | year = 2003 | volume = 583 | issue = 1 | pages = 446–450 | doi = 10.1086/345099 | bibcode = 2003ApJ...583..446S | s2cid = 122549795| doi-access = free }}

HCN is formed in interstellar clouds through one of two major pathways:{{cite journal | title = CN and HCN in Dense Interstellar Clouds | vauthors = Boger GI, Sternberg A | journal = Astrophysical Journal | year = 2005 | volume = 632 | issue = 1 | pages = 302–315 | doi = 10.1086/432864 | bibcode = 2005ApJ...632..302B | arxiv = astro-ph/0506535 |s2cid=118958200 }} 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{{cite journal | vauthors = Gao Y, Solomon PM | title = The Star Formation Rate and Dense Molecular Gas in Galaxies | journal = Astrophysical Journal | year = 2004 | volume = 606 | issue = 1 | pages = 271–290 | doi = 10.1086/382999 | bibcode=2004ApJ...606..271G | arxiv = astro-ph/0310339 |s2cid=11335358 }}{{cite journal | vauthors = Gao Y, olomon PM | title = HCN Survey of Normal Spiral, Infrared-luminous, and Ultraluminous Galaxies | journal = Astrophysical Journal Supplement Series | year = 2004 | volume = 152 |issue=1 | pages = 63–80 | doi = 10.1086/383003 | bibcode = 2004ApJS..152...63G | arxiv = astro-ph/0310341 |s2cid=9135663 }} and as a tracer of stellar inflow in high-mass star-forming regions.{{cite journal | vauthors = Wu J, Evans NJ | title = Indications of Inflow Motions in Regions Forming Massive Stars | journal = Astrophysical Journal | year = 2003 | volume = 592 | issue = 2 | pages = L79–L82 | doi = 10.1086/377679 | bibcode = 2003ApJ...592L..79W | arxiv = astro-ph/0306543 |s2cid=8016228 }} Further, the HNC/HCN ratio has been shown to be an excellent method for distinguishing between PDRs and X-ray-dominated regions (XDRs).{{cite journal | vauthors = Loenen AF | journal = Proceedings IAU Symposium | year = 2007 | title = Molecular properties of (U)LIRGs: CO, HCN, HNC and HCO⁺ | volume = 242 | pages = 462–466 |bibcode=2007IAUS..242..462L| doi = 10.1017/S1743921307013609 | arxiv = 0709.3423 | s2cid = 14398456}}

On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H₂CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).{{cite web | vauthors = Zubritsky E, Neal-Jones N |title=Release 14-038 – NASA's 3-D Study of Comets Reveals Chemical Factory at Work |url=http://www.nasa.gov/press/2014/august/goddard/nasa-s-3-d-study-of-comets-reveals-chemical-factory-at-work |date=11 August 2014 |work=NASA |access-date=12 August 2014 }}{{cite journal | vauthors = Cordiner MA, Remijan AJ, Boissier J, Milam SN, Mumma MJ, Charnley SB, Paganini L, Villanueva G, Bockelée-Morvan D, Kuan YJ, Chuang YL | display-authors = 6 |title=Mapping the Release of Volatiles in the Inner Comae of Comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON) Using the Atacama Large Millimeter/Submillimeter Array |date=11 August 2014 |journal=The Astrophysical Journal |volume=792 |pages=L2 |issue=1 |doi=10.1088/2041-8205/792/1/L2 |bibcode= 2014ApJ...792L...2C |arxiv=1408.2458|s2cid=26277035 }}

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.{{cite web|url=https://phys.org/news/2016-02-super-earth-atmosphere.html|title=First detection of super-earth atmosphere|publisher=ESA/Hubble Information Centre|date=February 16, 2016}}

On 14 December 2023, astronomers reported the first time discovery, in the plumes of Enceladus, moon of the planet Saturn, of hydrogen cyanide, a possible chemical essential for life{{cite news |last=Green |first=Jaime |title=What Is Life? - The answer matters in space exploration. But we still don't really know. |url=https://www.theatlantic.com/science/archive/2023/12/defining-life-existentialism-scientific-theory/676238/ |date=5 December 2023 |work=The Atlantic |url-status=live |archiveurl=https://archive.today/20231205121742/https://www.theatlantic.com/science/archive/2023/12/defining-life-existentialism-scientific-theory/676238/ |archivedate=5 December 2023 |accessdate=15 December 2023 }} as we know it, as well as other organic molecules, some of which are yet to be better identified and understood. According to the researchers, "these [newly discovered] compounds could potentially support extant microbial communities or drive complex organic synthesis leading to the origin of life."{{cite news |last=Chang |first=Kenneth |title=Poison Gas Hints at Potential for Life on an Ocean Moon of Saturn - A researcher who has studied the icy world said "the prospects for the development of life are getting better and better on Enceladus." |url=https://www.nytimes.com/2023/12/14/science/enceladus-moon-cyanide-life-saturn.html |date=14 December 2023 |work=The New York Times |url-status=live |archiveurl=https://archive.today/20231214210144/https://www.nytimes.com/2023/12/14/science/enceladus-moon-cyanide-life-saturn.html |archivedate=14 December 2023 |accessdate=15 December 2023 }}{{cite journal |author=Peter, Jonah S. |display-authors=et al. |title=Detection of HCN and diverse redox chemistry in the plume of Enceladus |url=https://www.nature.com/articles/s41550-023-02160-0 |date=14 December 2023 |journal=Nature Astronomy |volume=8 |issue=2 |pages=164–173 |doi=10.1038/s41550-023-02160-0 |arxiv=2301.05259 |bibcode=2024NatAs...8..164P |s2cid=255825649 |url-status=live |archiveurl=https://archive.today/20231215144349/https://www.nature.com/articles/s41550-023-02160-0 |archivedate=15 December 2023 |accessdate=15 December 2023 }}

{{main|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.Schnedlitz, Markus (2008) Chemische Kampfstoffe: Geschichte, Eigenschaften, Wirkung. GRIN Verlag. p. 13. {{ISBN|3640233603}}.[http://www.firstworldwar.com/weaponry/gas.htm Weapons of War - Poison Gas]. firstworldwar.com 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.[http://www.cyanidecode.org/cyanide_environmental.php Environmental and Health Effects] {{Webarchive|url=https://web.archive.org/web/20121130094124/http://www.cyanidecode.org/cyanide_environmental.php |date=2012-11-30 }}. Cyanidecode.org. Retrieved on 2012-06-02. A hydrogen cyanide concentration of 2000 ppm (about 2380 mg/m³) will kill a human in about one minute. 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.{{cite web | url = http://www.opcw.org/about-chemical-weapons/types-of-chemical-agent/blood-agents/hydrogen-cyanide/ | title = Hydrogen Cyanide | publisher = Organisation for the Prohibition of Chemical Weapons | access-date = 2009-01-14}}

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 {{lang|de|Zyklon B}} (German: Cyclone B, with the B standing for {{lang|de|Blausäure}} – prussic acid; also, to distinguish it from an earlier product later known as Zyklon A),{{cite book |last1=Van Pelt |first1=Robert Jan |url=https://archive.org/details/auschwitz1270top00dwor/page/443 |title=Auschwitz, 1270 to the present |last2=Dwork |first2=Debórah |publisher=Norton |year=1996 |isbn=9780300067552 |page=[https://archive.org/details/auschwitz1270top00dwor/page/443 443] |author-link=Robert Jan van Pelt |author2-link= |url-access=registration}} used in the Nazi German extermination camps of Majdanek and Auschwitz-Birkenau during World War II to kill Jews and other persecuted minorities 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"{{cite web | url = https://www.draslovka.cz/about-us#products | title = Blue Fume | publisher = Chemical Factory Draslovka a.s. | access-date = 2020-07-06 }} until around 2015.{{Cite web |date=2015-07-17 |title=Uragan D2 |url=http://www.draslovka.cz/uragan-d2 |access-date=2022-10-19 |archive-url=https://web.archive.org/web/20150717224853/http://www.draslovka.cz/uragan-d2 |archive-date=2015-07-17 }}

During World War II, the US considered using it, along with cyanogen chloride, as part of Operation Downfall, the planned invasion of Japan, but President Harry Truman decided against it, instead using the atomic bombs developed by the secret Manhattan Project.{{cite web |url=https://www.youtube.com/watch?v=8CFPaSH84ZU |title=How would have WW2 gone if the US had not used nuclear bombs on Japan? |author=Binkov's Battlegrounds |date=April 27, 2022 |website=YouTube.Com |publisher= |access-date=June 23, 2022 }}

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 cyanide or potassium cyanide.{{cite news | url = https://www.theguardian.com/us-news/2021/may/28/arizona-gas-chamber-executions-documents | title = Arizona 'refurbishes' its gas chamber to prepare for executions, documents reveal | work = The Guardian | date = 28 May 2021 | access-date = 2022-06-14 | last1 = Pilkington | first1 = Ed }}

Under the name prussic acid, HCN has been used as a killing agent in whaling harpoons, though it was quickly abandoned for being dangerous to the crew. From the middle of the 18th century it was used in a number of poisoning murders and suicides.{{cite web |title=The Poison Garden website |url=http://thepoisongarden.co.uk/atoz/prunus_laurocerasus.htm |url-status=dead |archive-url=https://web.archive.org/web/20200210022050/http://thepoisongarden.co.uk/atoz/prunus_laurocerasus.htm |access-date=18 October 2014 |archive-date=10 February 2020 |website=Thepoisongarden.co.uk}}

Hydrogen cyanide gas in air is explosive at concentrations above 5.6%.{{ cite web | url = https://www.cdc.gov/niosh/idlh/74908.html | title = Documentation for Immediately Dangerous to Life or Health Concentrations (IDLHs) – 74908 | date = 2 November 2018 | publisher = NIOSH }}

{{Reflist}}

• Institut national de recherche et de sécurité (1997). "[https://web.archive.org/web/20060220084315/http://www.inrs.fr/inrs-pub/inrs01.nsf/inrs01_ftox_view/860430FE710FCFD7C1256CE8004F67CB/$File/ft4.pdf Cyanure d'hydrogène et solutions aqueuses]". Fiche toxicologique n° 4, Paris:INRS, 5pp. (PDF file, in French)
• [http://www.inchem.org/documents/icsc/icsc/eics0492.htm International Chemical Safety Card 0492]
• [http://www.inchem.org/documents/cicads/cicads/cicad61.htm Hydrogen cyanide and cyanides] (CICAD 61)
• [https://web.archive.org/web/20060517035532/http://www.npi.gov.au/database/substance-info/profiles/29.html National Pollutant Inventory: Cyanide compounds fact sheet]
• [https://www.cdc.gov/niosh/npg/npgd0333.html NIOSH Pocket Guide to Chemical Hazards]
• [https://web.archive.org/web/20110607130633/http://www.hpa.org.uk/infections/topics_az/deliberate_release/chemicals/cyanide.pdf#search=%22%22dicobalt%20edetate%22%22 Department of health review]
• [https://www.aqua-calc.com/page/density-table/substance/hydrogen-blank-cyanide-coma-and-blank-gas Density of Hydrogen Cyanide gas]

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