Editing Hydrogen cyanide

{{short description|Highly toxic chemical with the formula HCN}}
{{Redirect|Cyanane|a class of synthetic dyes|Cyanine}}
{{Redirect|Cyanide gas|other gaseous cyanides|Cyanogen chloride|and|Cyanogen fluoride|and|Cyanogen}}
{{Chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 476999282
| ImageFile3 = Hydrogen cyanide.jpg
| ImageFile1 = Hydrogen-cyanide-2D.svg
| ImageFile1_Ref = {{chemboximage|correct|??}}
| ImageFileL1 = Hydrogen-cyanide-3D-balls.svg
| ImageFileL1_Ref = {{chemboximage|correct|??}}
| ImageNameL1 = Ball and stick model of hydrogen cyanide
| ImageFileR1 = Hydrogen-cyanide-3D-vdW.svg
| ImageFileR1_Ref = {{chemboximage|correct|??}}
| ImageNameR1 = Spacefill model of hydrogen cyanide
| IUPACName = Formonitrile<ref name=pubchem/>
| SystematicName = Methanenitrile<ref name=pubchem/>
| OtherNames = {{unbulleted list|Formic anammonide|Hydridonitridocarbon<ref>{{Cite web |title = hydrogen cyanide (CHEBI:18407) | url = https://www.ebi.ac.uk/chebi/searchId.do?chebiId=18407 | work = Chemical Entities of Biological Interest | publisher = European Bioinformatics Institute | access-date = 2012-06-04 | location = UK | date = 18 October 2009 | at = Main}}</ref>|Hydrocyanic acid ''(aqueous)''|Prussic acid|Cyanane
}}
| Section1 = {{Chembox Identifiers
| CASNo = 74-90-8
| CASNo_Ref = {{cascite|correct|CAS}}
| PubChem = 768
| ChemSpiderID = 748
| ChemSpiderID_Ref = {{chemspidercite|changed|chemspider}}
| UNII = 2WTB3V159F
| UNII_Ref = {{fdacite|correct|FDA}}
| EINECS = 200-821-6
| UNNumber = 1051
| KEGG = C01326
| KEGG_Ref = {{keggcite|changed|kegg}}
| MeSHName = Hydrogen+Cyanide
| ChEBI = 18407
| ChEBI_Ref = {{ebicite|changed|EBI}}
| RTECS = MW6825000
| 3DMet = B00275
| SMILES = C#N
| StdInChI = 1S/CHN/c1-2/h1H
| StdInChI_Ref = {{stdinchicite|changed|chemspider}}
| StdInChIKey = LELOWRISYMNNSU-UHFFFAOYSA-N
| StdInChIKey_Ref = {{stdinchicite|changed|chemspider}}
}}
| Section2 = {{Chembox Properties
| Formula = HCN
| MolarMass = 27.0253 g/mol
| Appearance = Colorless liquid or gas
| Odor = [[Almond#Sweet and bitter almonds|bitter almond]]-like<ref>{{cite report | last1=Simeonova | first1=Fina Petrova | last2=Fishbein | first2=Lawrence | title=Hydrogen cyanide and cyanides : human health aspects | publisher=World Health Organization | date=2004 | issn=1020-6167 | url=https://iris.who.int/handle/10665/42942 |isbn=9241530618}}</ref>
| Density = 0.6876 g/cm<sup>3</sup><ref name=Haynes>{{cite book | editor= Haynes, William M. | year = 2011 |title = CRC Handbook of Chemistry and Physics | edition = 92nd | publisher = [[CRC Press]] | isbn = 978-1439855119| title-link = CRC Handbook of Chemistry and Physics }}</ref>
| MeltingPtC = -13.29
| MeltingPt_ref=<ref name=Haynes/>
| BoilingPtC = 26
| BoilingPt_ref=<ref name=Haynes/>{{rp|4.67}}
| Solubility = Miscible
| Solvent = [[ethanol]]
| SolubleOther = Miscible
| HenryConstant = 75 μmol Pa<sup>−1</sup> kg<sup>−1</sup>
| ConjugateAcid = [[Hydrocyanonium]]
| pKa = 9.21 (in water),
12.9 (in DMSO)<ref>{{Cite web| vauthors = Evans DA |title=pKa's of Inorganic and Oxo-Acids |url= http://ccc.chem.pitt.edu/wipf/MechOMs/evans_pKa_table.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://ccc.chem.pitt.edu/wipf/MechOMs/evans_pKa_table.pdf |archive-date=2022-10-09 |url-status=live |access-date=June 19, 2020}}</ref>
| ConjugateBase = [[Cyanide]]
| pKb = 4.79 (cyanide anion)
| RefractIndex = 1.2675<ref>{{ cite book | vauthors = Patnaik P | title = Handbook of Inorganic Chemicals | publisher = McGraw-Hill | year = 2002 | isbn = 978-0070494398 }}</ref>
| Viscosity = 0.183 mPa·s (25 °C)<ref name=Haynes/>{{rp|6.231}}
| VaporPressure = 100 kPa (25 °C)<ref name=Haynes/>{{rp|6.94}}
}}
| Section3 = {{Chembox Structure
| MolShape = Linear
| Dipole = 2.98 D
| PointGroup = C<sub>∞v</sub>
| CrystalStruct=tetragonal (>170 K)<br />orthorhombic (<170 K)<ref>{{cite journal | last1=Schulz | first1=Axel | last2=Surkau | first2=Jonas | title=Main group cyanides: from hydrogen cyanide to cyanido-complexes | journal=Reviews in Inorganic Chemistry | publisher=Walter de Gruyter GmbH | volume=43 | issue=1 | date=2022-09-21 | issn=0193-4929 | doi=10.1515/revic-2021-0044 | pages=49–188| doi-access=free }}</ref>
}}
| Section4 = {{Chembox Thermochemistry
| DeltaHf = 135.1 kJ mol<sup>−1</sup>
| DeltaHc =
| Entropy = 201.8 J K<sup>−1</sup> mol<sup>−1</sup>
| HeatCapacity = 35.9 J K<sup>−1</sup> mol<sup>−1</sup> (gas)<ref name=Haynes/>{{rp|5.19}}
}}
| Section5 = {{Chembox Hazards
| GHSPictograms = {{GHS flame}} {{GHS skull and crossbones}} {{GHS health hazard}} {{GHS environment}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|225|300+310+330|319|336|370|410}}
| PPhrases = {{P-phrases|210|261|305+351+338}}
| NFPA-H = 4
| NFPA-F = 4
| NFPA-R = 2
| FlashPtC = −17.8
| AutoignitionPtC = 538
| PEL = TWA 10 ppm (11 mg/m<sup>3</sup>) [skin]<ref name=PGCH>{{PGCH|0333}}</ref>
| ExploLimits = 5.6% – 40.0%<ref name=PGCH/>
| IDLH = 50 ppm<ref name=PGCH/>
| REL = ST 4.7 ppm (5 mg/m<sup>3</sup>) [skin]<ref name=PGCH/>
| LCLo = 200 ppm (mammal, 5 min)<br />36 ppm (mammal, 2 hr)<br />107 ppm (human, 10 min)<br />759 ppm (rabbit, 1 min)<br />759 ppm (cat, 1 min)<br />357 ppm (human, 2 min)<br />179 ppm (human, 1 hr)<ref name=IDLH>{{IDLH|74908|Hydrogen cyanide}}</ref>
| LC50 = 501 ppm (rat, 5 min)<br />323 ppm (mouse, 5 min)<br />275 ppm (rat, 15 min)<br />170 ppm (rat, 30 min)<br />160 ppm (rat, 30 min)<br />323 ppm (rat, 5 min)<ref name=IDLH/>
}}
| Section6 = {{Chembox Related
| OtherFunction_label = alkanenitriles
| OtherFunction = {{Unbulleted list|[[Hydrogen isocyanide]]|[[Isocyanic acid]]|[[Thiocyanic acid]]|[[Cyanogen iodide]]|[[Cyanogen bromide]]|[[Cyanogen chloride]]|[[Cyanogen fluoride]]|[[Acetonitrile]]|[[Aminoacetonitrile]]|[[Glycolonitrile]]|[[Cyanogen]]}}
}}
}}

'''Hydrogen cyanide''' (formerly known as '''prussic acid''') is a [[chemical compound]] with the [[chemical formula|formula]] HCN and [[structural formula]] {{chem2|H\sC\tN}}. It is a highly [[toxic]] and [[flammable]] liquid that [[boiling|boils]] slightly above [[room temperature]], at {{convert|25.6|°C}}. HCN is produced on an industrial scale and is a highly valued [[Precursor (chemistry)|precursor]] to many chemical compounds ranging from [[polymers]] to pharmaceuticals. Large-scale applications are for the production of [[potassium cyanide]] and [[adiponitrile]], used in mining and plastics, respectively.<ref name=Ullmann>{{ Ullmann |last1=Gail|first1=E.|last2=Gos|first2=S.|last3=Kulzer|first3=R.|last4=Lorösch |first4=J.|last5=Rubo|first5=A.|last6=Sauer|first6=M. | title = Cyano Compounds, Inorganic | doi = 10.1002/14356007.a08_159.pub2 }}</ref> It is more toxic than solid cyanide compounds due to its [[Volatility (chemistry)|volatile]] nature. A solution of hydrogen cyanide in [[water (molecule)|water]], represented as HCN([[aqueous|aq]]), is called ''hydrocyanic acid''. The [[Salt (chemistry)|salt]]s of the cyanide anion are known as [[cyanide]]s.

Whether hydrogen cyanide is an [[organic compound]] or not is a topic of debate among chemists, and opinions vary from author to author.{{examples|date=May 2025}} Traditionally, it is considered [[inorganic compound|inorganic]] by a significant{{vague|date=May 2025}} number{{quantify|date=May 2025}} of authors.{{who|date=May 2025}} Contrary to this view, it is considered organic by other authors,{{who|date=May 2025}} because hydrogen cyanide belongs to the class of organic compounds known as [[nitriles]] which have the formula {{chem2|R\sC\tN}}, where R is typically [[organyl group]] (e.g., [[alkyl]] or [[aryl]]) or [[hydrogen]].<ref>{{cite web | url=https://hmdb.ca/metabolites/HMDB0060292 | title=Human Metabolome Database: Showing metabocard for Hydrogen cyanide (HMDB0060292) }}</ref> In the case of hydrogen cyanide, the R group is hydrogen H, so the other names of hydrogen cyanide are methanenitrile and formonitrile.<ref name=pubchem>{{Cite PubChem|cid=768 | title=Hydrogen Cyanide }}</ref>

==Structure and general properties==
Hydrogen cyanide is a [[Linear molecular geometry|linear molecule]], with a [[triple bond]] between [[carbon]] and [[nitrogen]]. The [[tautomer]] of HCN is HNC, [[hydrogen isocyanide]].{{citation needed|date=September 2023}}

=== Smell ===
Much literature has historically claimed that hydrogen cyanide smells of [[almonds]] or [[Almond#Sweet and bitter almonds|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.<ref>{{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? }}</ref><ref>{{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? }}</ref> In an experiment to test what hydrogen cyanide smells like, the chemistry [[Youtuber]] Nigel Braun, a.k.a. [[NileRed]], described the smell as "''not at all like an almond''" but like "''weak bleach or chlorine''" or "''swimming pools''".<ref>{{cite web | url = https://www.youtube.com/watch?v=WYagO-nup6c | title = Does cyanide actually smell like almonds? | website = [[YouTube]] | date = 31 October 2020 }}</ref>

About half of people are unable to [[olfactory system|detect]] the smell of hydrogen cyanide owing to a recessive [[gene]]tic [[Trait (biological)|trait]].<ref>{{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 }}</ref>

The [[volatility (chemistry)|volatile]] compound has been used as inhalation [[rodenticide]] and human poison, as well as for killing whales.<ref name=irons>{{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}}</ref> Cyanide ions interfere with iron-containing respiratory enzymes.{{citation needed|date=September 2023}}

==Chemical properties==
Hydrogen cyanide is weakly [[acidic]] with a [[Acid dissociation constant|p''K''<sub>a</sub>]] of 9.2. It partially [[ionization|ionizes]] in [[Aqueous solution|water]] to give the [[cyanide]] anion, {{chem2|CN−}}.  HCN forms hydrogen bonds with its conjugate base, species such as {{chem2|(CN-)(HCN)_{n} }}.<ref>{{cite journal |doi=10.1002/anie.201915206 |title=Salts of HCN-Cyanide Aggregates: &#91;CN(HCN)<sub>2</sub>&#93;<sup>−</sup> and &#91;CN(HCN)<sub>3</sub>&#93;<sup>−</sup> |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 }}</ref>

Hydrogen cyanide reacts with [[alkene]]s to give nitriles.  The conversion, which is called [[hydrocyanation]], employs nickel complexes as catalysts.<ref>{{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}}</ref>
:{{chem2|RCH\dCH2 + HCN → RCH2\sCH2CN}}

Four molecules of HCN will tetramerize into [[diaminomaleonitrile]].<ref>{{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 }}</ref>

[[Metal cyanide]]s are typically prepared by [[salt metathesis]] from alkali metal cyanide salts, but [[mercuric cyanide]] is formed from aqueous hydrogen cyanide:<ref>{{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}}</ref>
:{{chem2|HgO  +  2 HCN  ->  Hg(CN)2  +  H2O}}

==History of discovery and naming==
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.<ref>{{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 }}</ref> 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,<ref>{{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 }}
<br /> 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 }}</ref> 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,<ref>{{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 }}
<br />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}}</ref> an important contribution to acid theory, which had hitherto postulated that acids must contain oxygen<ref>{{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 }}</ref> (hence the name of [[oxygen]] itself, which is derived from Greek elements that mean "acid-former" and are likewise [[calque]]d into German as ''Sauerstoff'').

In 1811, [[Joseph Louis Gay-Lussac]] prepared pure, liquified hydrogen cyanide,<ref>{{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}}</ref> and in 1815 he deduced Prussic acid's chemical formula.<ref>{{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}}</ref>

===Etymology===
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]]''.

==Production and synthesis==
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:<ref>{{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 }}</ref>
:{{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.<ref>{{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 }}</ref> Hydrogen cyanide is produced in large quantities by several processes and is a recovered waste product from the manufacture of [[acrylonitrile]].<ref name=Ullmann/>

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:<ref>{{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 }}</ref>
:{{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+}}<ref>{{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 }}</ref><ref>{{Cite PubChem|cid=26250|title=Potassium ferricyanide}}</ref>

===Historical methods of production===
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.

==Applications==
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 [[cyanohydrin]]s, 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 [[Nitrilotriacetic acid|NTA]]. Via the [[hydrocyanation]] process, HCN is added to [[butadiene]] to give [[adiponitrile]], a precursor to [[Nylon|Nylon-6,6]].<ref name=Ullmann/>

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.<ref>{{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}}</ref> Using HCN as a fumigant also has less environmental impact, compared to some other fumigants such as [[sulfuryl fluoride]],<ref>{{Cite web|url=https://news.mit.edu/2009/prinn-greenhouse-tt0311|title = New greenhouse gas identified|website=News.mit.edu| date=11 March 2009 }}</ref> and [[methyl bromide]].<ref>{{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}}</ref>

==Occurrence==
HCN is obtainable from [[fruit]]s that have a [[endocarp|pit]], such as [[cherry|cherries]], [[apricot]]s, [[apple]]s, and nuts such as [[bitter almonds]], from which almond oil and extract is made. Many of these pits contain small amounts of [[cyanohydrin]]s such as [[mandelonitrile]] and [[amygdalin]], which slowly release hydrogen cyanide.<ref>{{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 }}</ref><ref>{{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 }}</ref> One hundred grams of crushed apple seeds can yield about 70&nbsp;mg of HCN.<ref>{{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 }}</ref>  The roots of [[cassava]] plants contain [[cyanogenic glycosides]] such as [[linamarin]], which decompose into HCN in yields of up to 370&nbsp;mg per kilogram of fresh root.<ref>{{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 }}</ref> Some [[millipede]]s, such as ''[[Harpaphe haydeniana]]'', ''[[Desmoxytes purpurosea]]'', and ''[[Apheloria]]'' release hydrogen cyanide as a defense mechanism,<ref>{{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 }}</ref> as do certain insects, such as [[Zygaenidae|burnet moths]] and the larvae of ''[[Paropsisterna|Paropsisterna eucalyptus]]''.<ref>{{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 }}</ref> Hydrogen cyanide is contained in the exhaust of vehicles, and in smoke from burning nitrogen-containing [[plastic]]s.[[File:PIA18431-SaturnMoon-Titan-SouthPoleVortex-Cloud-20121129.jpg|thumb|left|The South Pole Vortex of Saturn's moon [[Titan (moon)|Titan]] is a giant swirling cloud of HCN (November 29, 2012)]]

=== On Titan ===
HCN has been measured in [[Atmosphere of Titan|Titan's atmosphere]] by four instruments on the [[Cassini–Huygens|Cassini space probe]], one instrument on [[Voyager 1|Voyager]], and one instrument on Earth.<ref>{{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}}</ref> 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.<ref>{{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}}</ref> HCN initially forms in Titan's atmosphere through the reaction of photochemically produced methane and nitrogen radicals which proceed through the H<sub>2</sub>CN intermediate, e.g., (CH<sub>3</sub> + N → H<sub>2</sub>CN + H → HCN + H<sub>2</sub>).<ref name="pearce2020">{{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 }}</ref><ref>{{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 }}</ref> Ultraviolet radiation breaks HCN up into CN + H; however, CN is efficiently recycled back into HCN via the reaction CN + CH<sub>4</sub> → HCN + CH<sub>3</sub>.<ref name="pearce2020" />

=== 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.<ref>{{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 }}</ref>

=== In mammals ===
Some authors{{who|date=December 2020}} have shown that [[neuron]]s can produce hydrogen cyanide upon activation of their [[opioid]] [[receptor (biochemistry)|receptors]] by endogenous or exogenous opioids. They have also shown that neuronal production of HCN activates [[NMDA receptor]]s 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]].<ref name="pmid9369328">{{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}}</ref>

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.<ref>{{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}}</ref>

[[Leukocyte]]s generate HCN during [[phagocytosis]], and can kill [[bacteria]], [[fungi]], and other pathogens by generating several different toxic chemicals, one of which is hydrogen cyanide.<ref name="pmid9369328" />

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.<ref>{{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 }}</ref>

HCN is a constituent of [[tobacco smoke]].<ref>{{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}}</ref>

===HCN and the origin of life===
As a precursor to amino acids and nucleic acids, hydrogen cyanide has been proposed to have played a part in the [[Abiogenesis|origin of life]]. Compounds of special interest are [[oligomer]]s of HCN including its trimer [[aminomalononitrile]] and tetramer [[diaminomaleonitrile]], which can be described as (HCN)3 and (HCN)4, respectively.<ref>{{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 }}</ref> 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]]).<ref>{{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}}</ref>

===In space===
{{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. <ref>{{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}}</ref>

HCN has been detected in the [[interstellar medium]]<ref name="Snyder, Lewis E.; Buhl, David 1971 L47">{{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}}</ref> and in the atmospheres of [[carbon star]]s.<ref>{{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 }}</ref> 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 [[Observation|observed]] from ground-based [[telescope]]s through a number of [[Atmosphere|atmospheric]] windows.<ref>{{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 }}</ref> The J=1→0, J=3→2, J= 4→3, and J=10→9 pure [[rotational transition]]s have all been observed.<ref name="Snyder, Lewis E.; Buhl, David 1971 L47" /><ref>{{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}}</ref><ref>{{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 }}</ref>

HCN is formed in [[Interstellar medium|interstellar]] clouds through one of two major pathways:<ref name="Boger, G. I. and Sternberg, A. 2005 302">{{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 }}</ref> via a neutral-neutral reaction (CH<sub>2</sub> + N → HCN + H) and via [[dissociative recombination]] (HCNH<sup>+</sup> + e<sup>−</sup> → HCN + H). The dissociative recombination pathway is dominant by 30%; however, the [[HCNH+|HCNH<sup>+</sup>]] must be in its linear form. Dissociative recombination with its structural isomer, H<sub>2</sub>NC<sup>+</sup>, exclusively produces [[hydrogen isocyanide]] (HNC).

HCN is destroyed in interstellar clouds through a number of mechanisms depending on the location in the cloud.<ref name="Boger, G. I. and Sternberg, A. 2005 302" /> In [[photon-dominated region]]s (PDRs), photodissociation dominates, producing [[Cyanide|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<sup>+</sup> and HCNH<sup>+</sup> (HCN + H<sup>+</sup> → HCN<sup>+</sup> + H; HCN + HCO<sup>+</sup> → HCNH<sup>+</sup> + CO). The reaction with HCO<sup>+</sup> 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<ref>{{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 }}</ref><ref>{{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 }}</ref> and as a tracer of stellar inflow in high-mass star-forming regions.<ref>{{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 }}</ref> Further, the HNC/HCN ratio has been shown to be an excellent method for distinguishing between PDRs and X-ray-dominated regions (XDRs).<ref>{{cite journal | vauthors = Loenen AF | journal = Proceedings IAU Symposium | year = 2007 | title = Molecular properties of (U)LIRGs: CO, HCN, HNC and HCO<sup>+</sup> | volume = 242 | pages = 462–466 |bibcode=2007IAUS..242..462L| doi = 10.1017/S1743921307013609 | arxiv = 0709.3423 | s2cid = 14398456}}</ref>

On 11 August 2014, astronomers released studies, using the [[Atacama Large Millimeter Array|Atacama Large Millimeter/Submillimeter Array (ALMA)]] for the first time, that detailed the distribution of HCN, [[Hydrogen isocyanide|HNC]], [[Formaldehyde|H<sub>2</sub>CO]], and [[dust]] inside the [[Coma (cometary)|comae]] of [[comet]]s [[C/2012 F6 (Lemmon)]] and [[Comet ISON|C/2012 S1 (ISON)]].<ref>{{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 }}</ref><ref>{{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 }}</ref>

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]].<ref>{{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}}</ref>

On 14 December 2023, astronomers reported the first time discovery, in the [[Plume (fluid dynamics)|plume]]s of [[Enceladus]], moon of the planet [[Saturn]], of hydrogen cyanide, a possible chemical essential for [[life]]<ref>{{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 }}</ref> as we know it, as well as other [[organic molecule]]s, some of which are yet to be better identified and understood. According to the researchers, "these [newly discovered] compounds could potentially support extant [[Microorganism|microbial communities]] or drive complex [[organic synthesis]] leading to the [[origin of life]]."<ref>{{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 }}</ref><ref>{{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 }}</ref>

==As a poison and chemical weapon==
{{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 [[Kingdom of Italy|Italy]] in 1918. It was not found to be effective enough due to weather conditions.<ref>Schnedlitz, Markus (2008) ''Chemische Kampfstoffe: Geschichte, Eigenschaften, Wirkung''. GRIN Verlag. p. 13. {{ISBN|3640233603}}.</ref><ref>[http://www.firstworldwar.com/weaponry/gas.htm Weapons of War - Poison Gas]. firstworldwar.com</ref> 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 [[trench warfare|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 [[Parts per million|ppm]] in breathing air will kill a human within 10 to 60 minutes.<ref name="cyanidecode1">[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.</ref> A hydrogen cyanide concentration of 2000 ppm (about 2380&nbsp;mg/m<sup>3</sup>) will kill a human in about one minute.<ref name="cyanidecode1"/> 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]].<ref>{{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}}</ref>

The [[Chemical Weapons Convention]] lists it under [[List of Schedule 3 substances (CWC)|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),<ref>{{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}}</ref> used in the [[Nazi Germany|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"<ref>{{cite web | url = https://www.draslovka.cz/about-us#products | title = Blue Fume  | publisher = Chemical Factory Draslovka a.s. | access-date = 2020-07-06 }}</ref> until around 2015.<ref>{{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 }}</ref>

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]].<ref>{{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 }}</ref>

Hydrogen cyanide was also the agent employed in judicial [[Capital punishment in the United States|execution]] in some [[U.S. state]]s, where it was produced during the execution by the action of [[sulfuric acid]] on [[sodium cyanide]] or [[potassium cyanide]].<ref>{{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 }}</ref>

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.<ref name=irons/> From the middle of the 18th century it was used in a number of poisoning murders and suicides.<ref>{{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}}</ref>

Hydrogen cyanide gas in air is explosive at concentrations above 5.6%.<ref>{{ 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 }}</ref>

==References==
{{Reflist}}

==External links==
* 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]

{{Molecules detected in outer space}}
{{Chemical warfare}}
{{Cyanides}}
{{Nitrogen compounds}}
{{Hydrogen compounds}}
{{Authority control}}

[[Category:Blood agents]]
[[Category:Cyanides]]
[[Category:Fumigants]]
[[Category:Hydrogen compounds]]
[[Category:Inorganic compounds]]
[[Category:Gaseous signaling molecules]]
[[Category:Soviet chemical weapons program]]
[[Category:Triatomic molecules]]