Hydrogen cyanide (HCN), sometimes called prussic acid, is a chemical compound with the chemical formula HCN. It is a colorless, extremely poisonous and flammable liquid that boils slightly above room temperature, at 25.6 °C (78.1 °F). HCN is produced on an industrial scale and is a highly valuable precursor to many chemical compounds ranging from polymers to pharmaceuticals.
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||27.0253 g/mol|
|Appearance||Colorless liquid or gas|
|Odor||Oil of bitter almond|
|Density||0.6876 g cm−3|
|Melting point||−13.29 °C (8.08 °F; 259.86 K)|
|Boiling point||26 °C (79 °F; 299 K)|
|Solubility in ethanol||Miscible|
|Vapor pressure||100 kPa (25 °C)|
|75 μmol Pa−1 kg−1|
|Basicity (pKb)||4.79 (cyanide anion)|
Refractive index (nD)
|Viscosity||0.183 mPa·s (25 °C)|
Heat capacity (C)
|35.9 J K−1 mol−1 (gas)|
|201.8 J K−1 mol−1|
Std enthalpy of
|135.1 kJ mol−1|
|GHS Signal word||Danger|
|H225, H300, H310, H319, H330, H336, H370, H400, H410|
|P210, P261, P305+351+338|
|NFPA 704 (fire diamond)|
|Flash point||−17.8 °C (0.0 °F; 255.3 K)|
|538 °C (1,000 °F; 811 K)|
|Explosive limits||5.6% – 40.0%|
|Lethal dose or concentration (LD, LC):|
LC50 (median concentration)
|501 ppm (rat, 5 min)|
323 ppm (mouse, 5 min)
275 ppm (rat, 15 min)
170 ppm (rat, 30 min)
160 ppm (rat, 30 min)
323 ppm (rat, 5 min)
LCLo (lowest published)
|200 ppm (mammal, 5 min)|
36 ppm (mammal, 2 hr)
107 ppm (human, 10 min)
759 ppm (rabbit, 1 min)
759 ppm (cat, 1 min)
357 ppm (human, 2 min)
179 ppm (human, 1 hr)
|NIOSH (US health exposure limits):|
|TWA 10 ppm (11 mg/m3) [skin]|
|ST 4.7 ppm (5 mg/m3) [skin]|
IDLH (Immediate danger)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Structure and general properties
Hydrogen cyanide is weakly acidic with a pKa of 9.2. It partially ionizes in water solution to give the cyanide anion, CN−. A solution of hydrogen cyanide in water, represented as HCN, is called hydrocyanic acid. The salts of the cyanide anion are known as cyanides.
HCN has a faint bitter almond-like odor that some people are unable to detect owing to a recessive genetic trait. The volatile compound has been used as inhalation rodenticide and human poison, as well as for killing whales. Cyanide ions interfere with iron-containing respiratory enzymes.
History of discovery
Hydrogen cyanide was first isolated from a blue pigment (Prussian blue) which had been known since 1706, but whose structure was unknown. It is now known to be a coordination polymer with a complex structure and an empirical formula of hydrated ferric ferrocyanide. In 1752, the French chemist Pierre Macquer made the important step of showing that Prussian blue could be converted to an iron oxide plus a volatile component and that these could be used to reconstitute it. The new component was what is now known as hydrogen cyanide. Following Macquer's lead, it was first prepared from Prussian blue by the Swedish chemist Carl Wilhelm Scheele in 1782, and was eventually given the German name Blausäure (lit. "Blue acid") because of its acidic nature in water and its derivation from Prussian blue. In English, it became known popularly as prussic acid.
In 1787, the French chemist Claude Louis Berthollet showed that prussic acid did not contain oxygen, an important contribution to acid theory, which had hitherto postulated that acids must contain oxygen (hence the name of oxygen itself, which is derived from Greek elements that mean "acid-former" and are likewise calqued into German as Sauerstoff). In 1811, Joseph Louis Gay-Lussac prepared pure, liquified hydrogen cyanide. In 1815, Gay-Lussac deduced Prussic acid's chemical formula. The radical cyanide in hydrogen cyanide was given its name from cyan, not only an English word for a shade of blue but the Greek word for blue (Ancient Greek: κυανοῦς), again owing to its derivation from Prussian blue.
Production and synthesis
Hydrogen cyanide forms in at least limited amounts from many combinations of hydrogen, carbon, and ammonia. Hydrogen cyanide is currently produced in great quantities by several processes, as well as being a recovered waste product from the manufacture of acrylonitrile. In 2006 between 500 million and 1 billion pounds were produced in the US.
- 2 CH4 + 2 NH3 + 3 O2 → 2 HCN + 6 H2O
The energy needed for the reaction is provided by the partial oxidation of methane and ammonia.
- CH4 + NH3 → HCN + 3H2
- H+ + NaCN → HCN + Na+
This reaction is sometimes the basis of accidental poisonings because the acid converts a nonvolatile cyanide salt into the gaseous HCN.
Historical methods of production
The 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 obtainable from fruits that have a pit, such as cherries, apricots, apples, and bitter almonds, from which almond oil and flavoring are made. Many of these pits contain small amounts of cyanohydrins such as mandelonitrile and amygdalin, which slowly release hydrogen cyanide. One hundred grams of crushed apple seeds can yield about 70 mg of HCN. Some millipedes release hydrogen cyanide as a defense mechanism, as do certain insects, such as some burnet moths. Hydrogen cyanide is contained in the exhaust of vehicles, and in smoke from burning nitrogen-containing plastics. So-called "bitter" roots of the cassava plant may contain up to 1 gram of HCN per kilogram.
HCN on Titan
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. One of these measurements was in situ, where the Cassini spacecraft dipped between 1000–1100 km above Titan's surface to collect some atmospheric gas for mass spectrometry analysis. HCN likely formed in Titan's atmosphere through the reaction of photochemically produced methane and nitrogen radicals which proceeded through the H2CN intermediate, e.g., (CH3 + N → H2CN + H → HCN + H2).
HCN 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.
HCN in mammals
Some authors 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.
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.
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.
HCN is a constituent of tobacco smoke.
HCN and the origin of life
Hydrogen cyanide has been discussed as a precursor to amino acids and nucleic acids, and is proposed to have played a part in the origin of life. Although the relationship of these chemical reactions to the origin of life theory remains speculative, studies in this area have led to discoveries of new pathways to organic compounds derived from the condensation of HCN.
HCN in space
HCN has been detected in the interstellar medium and in the atmospheres of carbon stars. Since then, extensive studies have probed formation and destruction pathways of HCN in various environments and examined its use as a tracer for a variety of astronomical species and processes. HCN can be observed from ground-based telescopes through a number of atmospheric windows. The J=1→0, J=3→2, J= 4→3, and J=10→9 pure rotational transitions have all been observed.
HCN is formed in interstellar clouds through one of two major pathways: via a neutral-neutral reaction (CH2 + N → HCN + H) and via dissociative recombination (HCNH+ + e− → HCN + H). The dissociative recombination pathway is dominant by 30%; however, the HCNH+ must be in its linear form. Dissociative recombination with its structural isomer, H2NC+, exclusively produces hydrogen isocyanide (HNC).
HCN is destroyed in interstellar clouds through a number of mechanisms depending on the location in the cloud. In photon-dominated regions (PDRs), photodissociation dominates, producing CN (HCN + ν → CN + H). At further depths, photodissociation by cosmic rays dominate, producing CN (HCN + cr → CN + H). In the dark core, two competing mechanisms destroy it, forming HCN+ and HCNH+ (HCN + H+ → HCN+ + H; HCN + HCO+ → HCNH+ + CO). The reaction with HCO+ dominates by a factor of ~3.5. HCN has been used to analyze a variety of species and processes in the interstellar medium. It has been suggested as a tracer for dense molecular gas and as a tracer of stellar inflow in high-mass star-forming regions. Further, the HNC/HCN ratio has been shown to be an excellent method for distinguishing between PDRs and X-ray-dominated regions (XDRs).
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).
As a poison and chemical weapon
In World War One, hydrogen cyanide was used as a chemical weapon against the Central Powers by the French from 1916, and by the United States and Italy in 1918, but it was not found to be effective enough due to weather conditions. The gas is lighter than air and rapidly disperses up into the atmosphere; this is in contrast to denser agents such as phosgene or chlorine which tend to remain at ground level. Compared to such agents it must also be present in higher concentrations in order to be fatal. These properties combine to make its use in the field impractical. A hydrogen cyanide concentration in the range of 100–200 ppm in air will kill a human within 10 to 60 minutes. A hydrogen cyanide concentration of 2000 ppm (about 2380 mg/m3) will kill a human in about one minute. The toxicity is caused by the cyanide ion, which halts cellular respiration by acting 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. It is listed under Schedule 3 of the Chemical Weapons Convention as a potential weapon which has large-scale industrial uses, manufacturing plants in signatory countries which produce more than 30 metric tons per year must be declared to, and can be inspected by, the Organisation for the Prohibition of Chemical Weapons.
Hydrogen cyanide has been absorbed into a carrier for use as a pesticide. Perhaps the most infamous of these is Zyklon B (German: Cyclone B, with the B standing for Blausäure – prussic acid; also, to distinguish it from an earlier product later known as Zyklon A), it was used in Nazi German extermination camps during World War II to kill 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. The same product is currently made in the Czech Republic under the trademark "BLUE FUME". 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 potassium cyanide.
Under the name prussic acid, HCN has been used as a killing agent in whaling harpoons, although it proved quite dangerous to the crew deploying it, and thus it was quickly abandoned. From the middle of the 18th century it was used in a number of poisoning murders and suicides.
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