Hydrogen cyanide (also calledprussic acid) is achemical compound with theformula HCN andstructural formulaH−C≡N. It is a highlytoxic andflammable liquid thatboils slightly aboveroom temperature, at 25.6 °C (78.1 °F). HCN is produced on an industrial scale and is a highly valuedprecursor to many chemical compounds ranging frompolymers to pharmaceuticals. Large-scale applications are for the production ofpotassium cyanide andadiponitrile, used in mining and plastics, respectively.[10] It is more toxic than solid cyanide compounds due to itsvolatile nature. A solution of hydrogen cyanide inwater, represented as HCN(aq), is calledhydrocyanic acid. Thesalts of the cyanide anion are known ascyanides.
Whether hydrogen cyanide is anorganic compound or not is a topic of debate among chemists. It is traditionally consideredinorganic, but can also be considered anitrile,[11] giving rise to its alternative names of methanenitrile and formonitrile.[2]
About half of people are unable todetect the odor of hydrogen cyanide owing to a recessivegenetictrait.[14] For those that can detect it, the odor has been described as bitter almond-like.
Hydrogen cyanide is weaklyacidic with apKa of 9.2. It partiallyionizes inwater to give thecyanide anion,CN−. HCN forms hydrogen bonds with its conjugate base, species such as(CN−)(HCN)n.[15]
Hydrogen cyanide reacts withalkenes to give nitriles. The conversion, which is calledhydrocyanation, employs nickel complexes as catalysts.[16]
Hydrogen cyanide was first isolated in 1752 by French chemistPierre Macquer who convertedPrussian blue to aniron oxide plus a volatile component and found that these could be used to reconstitute it.[19] The new component was what is now known as hydrogen cyanide. It was subsequently prepared from Prussian blue by the Swedish chemistCarl Wilhelm Scheele in 1782,[20] and was eventually given the German nameBlausäure (lit. "Blue acid") because of its acidic nature in water and its derivation from Prussian blue. In English, it became known popularly asprussic acid.
In 1787, the French chemistClaude Louis Berthollet showed that prussic acid did not contain oxygen,[21] an important contribution to acid theory, which had hitherto postulated that acids must contain oxygen[22] (hence the name ofoxygen itself, which is derived from Greek elements that mean "acid-former" and are likewisecalqued into German asSauerstoff).
In 1811,Joseph Louis Gay-Lussac prepared pure, liquified hydrogen cyanide,[23] and in 1815 he deduced Prussic acid's chemical formula.[24]
The wordcyanide for the radical in hydrogen cyanide was derived from its French equivalent,cyanure, which Gay-Lussac constructed from the Ancient Greek wordκύανος for dark blue enamel orlapis lazuli, again owing to the chemical’s derivation from Prussian blue. Incidentally, the Greek word is also the root of the English color namecyan.
In 2006, between 500 million and 1 billion pounds (between 230,000 and 450,000 t) were produced in the US.[26] Hydrogen cyanide is produced in large quantities by several processes and is a recovered waste product from the manufacture ofacrylonitrile.[10]
Of lesser importance is theDegussa process (BMA process) in which no oxygen is added and the energy must be transferred indirectly through the reactor wall:[27]
The large demand for cyanides for mining operations in the 1890s was met byGeorge Thomas Beilby, who patented a method to produce hydrogen cyanide by passingammonia over glowingcoal in 1892. This method was used untilHamilton Castner in 1894 developed a synthesis starting from coal, ammonia, andsodium yieldingsodium cyanide, which reacts with acid to form gaseous HCN.
HCN is used globally as afumigant 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.[30] Using HCN as a fumigant also has less environmental impact, compared to some other fumigants such assulfuryl fluoride,[31] andmethyl bromide.[32]
HCN has been measured inTitan's atmosphere by four instruments on theCassini space probe, one instrument onVoyager, and one instrument on Earth.[39] One of these measurements wasin situ, where the Cassini spacecraft dipped between 1,000 and 1,100 km (620 and 680 mi) above Titan's surface to collect atmospheric gas formass spectrometry analysis.[40] 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).[41][42] Ultraviolet radiation breaks HCN up into CN + H; however, CN is efficiently recycled back into HCN via the reaction CN + CH4 → HCN + CH3.[41]
It has been postulated that carbon from a cascade of asteroids (known as theLate 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.[43]
Some authors[who?] have shown thatneurons can produce hydrogen cyanide upon activation of theiropioidreceptors by endogenous or exogenous opioids. They have also shown that neuronal production of HCN activatesNMDA receptors and plays a role insignal transduction between neuronal cells (neurotransmission). Moreover, increased endogenous neuronal HCN production under opioids was seemingly needed for adequate opioidanalgesia, as analgesic action of opioids was attenuated by HCN scavengers. They considered endogenous HCN to be aneuromodulator.[44]
It has also been shown that, while stimulatingmuscariniccholinergic receptors in culturedpheochromocytoma cellsincreases HCN production, in a living organism (in vivo) muscarinic cholinergic stimulation actuallydecreases HCN production.[45]
Leukocytes generate HCN duringphagocytosis, and can killbacteria,fungi, and other pathogens by generating several different toxic chemicals, one of which is hydrogen cyanide.[44]
Thevasodilatation caused bysodium 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 tonitroglycerine and other non-cyanogenic nitrates which do not cause blood cyanide levels to rise.[46]
As a precursor to amino acids and nucleic acids, hydrogen cyanide has been proposed to have played a part in theorigin of life. Compounds of special interest areoligomers of HCN including its trimeraminomalononitrile and tetramerdiaminomaleonitrile, which can be described as (HCN)3 and (HCN)4, respectively.[48] 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).[49]
Because hydrogen cyanide is a precursor to nucleic acids, which are critical for terrestrial life,astronomers are incentivized to search for derivatives of HCN.[50]
HCN has been detected in theinterstellar medium[51] and in the atmospheres ofcarbon stars.[52] 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 beobserved from ground-basedtelescopes through a number ofatmospheric windows.[53] The J=1→0, J=3→2, J= 4→3, and J=10→9 purerotational transitions have all been observed.[51][54][55]
HCN is formed ininterstellar clouds through one of two major pathways:[56] via a neutral-neutral reaction (CH2 + N → HCN + H) and viadissociative recombination (HCNH+ + e− → HCN + H). The dissociative recombination pathway is dominant by 30%; however, theHCNH+ must be in its linear form. Dissociative recombination with its structural isomer, H2NC+, exclusively produceshydrogen isocyanide (HNC).
HCN is destroyed in interstellar clouds through a number of mechanisms depending on the location in the cloud.[56] Inphoton-dominated regions (PDRs), photodissociation dominates, producingCN (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[57][58] and as a tracer of stellar inflow in high-mass star-forming regions.[59] Further, the HNC/HCN ratio has been shown to be an excellent method for distinguishing between PDRs and X-ray-dominated regions (XDRs).[60]
On 14 December 2023, astronomers reported the first time discovery, in theplumes ofEnceladus, moon of the planetSaturn, of hydrogen cyanide, a possible chemical essential forlife[64] as we know it, as well as otherorganic molecules, some of which are yet to be better identified and understood. According to the researchers, "these [newly discovered] compounds could potentially support extantmicrobial communities or drive complexorganic synthesis leading to theorigin of life."[65][66]
InWorld War I, hydrogen cyanide was used by the French from 1916 as a chemical weapon against theCentral Powers, and by the United States andItaly in 1918. It was not found to be effective enough due to weather conditions.[67][68] 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 asphosgene orchlorine tended to remain at ground level and sank into thetrenches of the Western Front's battlefields. Compared to such agents, hydrogen cyanide had to be present in higher concentrations in order to be fatal. To increase gas persistence, it was mixed with smoke producing compounds. For example, French composition calledVincennite combined 50% hydrogen cyanide with 30%arsenic trichloride and 15%stannic chloride for smoke production, plus 5%chloroform.[69]
A hydrogen cyanide concentration of 100–200ppm in breathing air will kill a human within 10 to 60 minutes.[70] A hydrogen cyanide concentration of 2000 ppm (about 2380 mg/m3) will kill a human in about one minute.[70] The toxic effect is caused by the action of the cyanide ion, which haltscellular respiration. It acts as anon-competitive inhibitor for an enzyme in mitochondria calledcytochrome c oxidase. As such, hydrogen cyanide is commonly listed amongchemical weapons as ablood agent.[71]
Perhaps its most infamous use isZyklon B (German:Cyclone B, with theB standing forBlausäure – prussic acid; also, to distinguish it from an earlier product later known as Zyklon A),[72] used in theNazi Germanextermination camps ofMajdanek andAuschwitz-Birkenau duringWorld War II to kill Jews and other persecuted minoritiesen masse as part of theirFinal 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"[73] until around 2015.[74]
Under the nameprussic acid, HCN has been used as a killing agent inwhaling harpoons, though it was quickly abandoned for being dangerous to the crew.[77] From the middle of the 18th century it was used in a number of poisoning murders and suicides.[78]
Hydrogen cyanide gas in air is explosive at concentrations above 5.6%.[79]
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