# Hydrogen cyanide

Hydrogen cyanide (formerly known as prussic acid) is a chemical compound with the formula HCN and structural formula H−C≡N. It is a highly toxic 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 valued 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. It is more toxic than solid cyanide compounds due to its volatile nature. A solution of hydrogen cyanide in water, represented as HCN(aq), is called hydrocyanic acid. The salts of the cyanide anion are known as cyanides.
Whether hydrogen cyanide is an organic compound or not is a topic of debate among chemists, and opinions vary from author to author. Traditionally, it is considered inorganic by a significant number of authors. Contrary to this view, it is considered organic by other authors, because hydrogen cyanide belongs to the class of organic compounds known as nitriles which have the formula R−C≡N, where R is typically organyl group (e.g., alkyl or aryl) or hydrogen. In the case of hydrogen cyanide, the R group is hydrogen H, so the other names of hydrogen cyanide are methanenitrile and formonitrile.

## Structure and general properties

Hydrogen cyanide is a linear molecule, with a triple bond between carbon and nitrogen. The tautomer of HCN is HNC, hydrogen isocyanide.

### Smell

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. In an experiment to test what hydrogen cyanide smells like, the chemistry Youtuber NileRed described the smell as "not at all like an almond" but like "weak bleach or chlorine" or "swimming pools".
About half of people are unable to detect the smell of hydrogen cyanide 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.

## Chemical properties

Hydrogen cyanide is weakly acidic with a pK$_a$ of 9.2. It partially ionizes in water to give the cyanide anion, CN$^−$. HCN forms hydrogen bonds with its conjugate base, species such as (CN$^−$)(HCN)$_n$.
Hydrogen cyanide reacts with alkenes to give nitriles. The conversion, which is called hydrocyanation, employs nickel complexes as catalysts.
    RCH=CH$_2$ + HCN → RCH$_2$−CH$_2$CN
Four molecules of HCN will tetramerize into diaminomaleonitrile.
Metal cyanides are typically prepared by salt metathesis from alkali metal cyanide salts, but mercuric cyanide is formed from aqueous hydrogen cyanide:
    HgO + 2 HCN → Hg(CN)$_2$ + H$_2$O

## 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. 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, 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, and in 1815 he deduced Prussic acid's chemical formula.

### 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 1,200 °C (2,190 °F) over a platinum catalyst:
    2 CH$_4$ + 2 NH$_3$ + 3 O$_2$ → 2 HCN + 6 H$_2$O
In 2006, between 500 million and 1 billion pounds (between 230,000 and 450,000 t) were produced in the US. 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:
    CH$_4$ + NH$_3$ → HCN + 3H$_2$
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:
    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:
    6 H$^+$ + [Fe(CN)$_6$]−3 → 6 HCN + Fe+3

### 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 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. Using HCN as a fumigant also has less environmental impact, compared to some other fumigants such as sulfuryl fluoride, and methyl bromide.

## Occurrence

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. One hundred grams of crushed apple seeds can yield about 70 mg of HCN. 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. Some millipedes, such as Harpaphe haydeniana, Desmoxytes purpurosea, and Apheloria release hydrogen cyanide as a defense mechanism, as do certain insects, such as burnet moths and the larvae of Paropsisterna eucalyptus. Hydrogen cyanide is contained in the exhaust of vehicles, and in smoke from burning nitrogen-containing plastics.

### 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 1,000 and 1,100 km (620 and 680 mi) above Titan's surface to collect atmospheric gas for mass spectrometry analysis. HCN initially forms in Titan's atmosphere through the reaction of photochemically produced methane and nitrogen radicals which proceed through the H$_2$CN intermediate, e.g., (CH$_3$ + N → H$_2$CN + H → HCN + H$_2$). Ultraviolet radiation breaks HCN up into CN + H; however, CN is efficiently recycled back into HCN via the reaction CN + CH$_4$ → HCN + CH$_3$.

### 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.

### 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.
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.
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 (e.g. Adenine). Because amino acids are critical for life as we know it, and that hydrogen cyanide is a precursor for these molecules, astronomers who search for life on planets beyond Earth use spectroscopy to detect molecules like hydrogen cyanide after confirming suitable temperatures and the presence of water.

### 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 (CH$_2$ + 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, H$_2$NC$^+$, 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, H$_2$CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).
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.
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 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."

## As a poison and chemical weapon

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. 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. A hydrogen cyanide concentration of 2000 ppm (about 2380 mg/m$^3$) 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.
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 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), 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" until around 2015.
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.
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.
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 it was quickly abandoned. From the middle of the 18th century it was used in a number of poisoning murders and suicides.
Hydrogen cyanide gas in air is explosive at concentrations above 5.6%.