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Hydrogen cyanide

Related subjects: Chemical compounds

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Hydrogen cyanide
CAS number 74-90-8
RTECS number MW6825000
Jmol-3D images Image 1
Molecular formula HCN
Molar mass 27.03 g/mol
Appearance Colorless gas or pale blue
highly volatile liquid
Density 0.687 g/cm³, liquid.
Melting point

-13.4°C (259.75 K, 7.88°F)

Boiling point

26°C (299.15 K, 78.8°F)

Solubility in water Completely miscible.
Acidity (pKa) 9.2 - 9.3
R-phrases R12, R26, R27, R28, R32.
S-phrases (S1), (S2), S7, S9, S13, S16,
S28, S29, S45.
Main hazards Highly toxic, highly flammable.
NFPA 704
NFPA 704.svg
Flash point −17.78 °C
Related compounds
Related compounds Cyanogen
Cyanogen chloride
trimethylsilyl cyanide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Hydrogen cyanide is a chemical compound with chemical formula HCN. A solution of hydrogen cyanide in water is called hydrocyanic acid. Hydrogen cyanide is a colorless, very poisonous, and highly volatile liquid that boils slightly above room temperature at 26 °C (78.8 °F). HCN has a faint, bitter, almond-like odour that some people are unable to detect due to a genetic trait. Hydrogen cyanide is weakly acidic and partly ionizes in solution to give the cyanide anion, CN. The salts of hydrogen cyanide are known as cyanides. HCN is a highly valuable precursor to many chemical compounds ranging from polymers to pharmaceuticals.

Production and synthesis

Hydrogen cyanide is currently produced in large quantities by three processes. In the year 2000, 1.615 billion pounds (732,552 tons) were produced in the US. The most important process for the production of hydrogen cyanide is the Andrussov oxidation invented by Leonid Andrussow in which methane and ammonia react in the presence of oxygen at about 1200 °C over a platinum catalyst:

CH4 + NH3 + 1.5O2 → HCN + 3H2O

The energy needed for the reaction is provided by the part oxidation of methane and ammonia.

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:

CH4 + NH3 → HCN + 3H2

This reaction is akin to steam reforming, the reaction of methane and water. In another process, practiced at BASF, formamide is heated and split into hydrogen cyanide and water:

CH(O)NH2 → HCN + H2O

In the laboratory, small amounts of HCN are produced by the addition of acids to cyanide salts of alkali metals:

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.


HCN adds to ketones and aldehydes to give cyanohydrins. Amino acids are prepared by this reaction; the essential amino acid methionine is manufactured by this route.The cyanohydrin of acetone is a precursor to methyl methacrylate.

In hydrocyanation, HCN adds to alkenes to give nitriles. This reaction is employed to manufacture adiponitrile, the precursor to Nylon 66.

Occurrence and applications

Cyanide is used in tempering steel, dyeing, explosives, engraving, the production of acrylic resin plastic, and other organic chemical products (eg: historically: formic acid). The less toxic ethyl acetate (C4H8O2) has now largely replaced the use of cyanide in insect killing jars. Cyanide is also being used for capital punishment.

Fruits that have a pit, such as cherries and apricots, bitter almonds and apples, from which almond oil and flavoring are made, contain small amounts of cyanohydrins such as mandelonitrile (CAS#532-28-5). Such molecules slowly release hydrogen cyanide. 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, in tobacco and wood smoke, and in smoke from burning nitrogen-containing plastics.

100 g of crushed apple seeds can yield 219 mg of Amygdalin which can generate ~10 mg of HCN.

HCN and the origin of life

Hydrogen cyanide has been discussed as a precursor to amino acids and nucleic acids. It is possible, for example, that HCN played a part in the origin of life. Leslie Orgel, among many researchers, has written extensively on the condensation of HCN. Although the relationship of these chemical reactions to the origin of life remains speculative, studies in this area have led to discoveries of new pathways to organic compounds derived from condensation of HCN.

Hydrogen cyanide as a chemical weapon

An HCN concentration of 300 mg/m3 in air will kill a human within a few minutes. The toxicity is caused by the cyanide ion, which prevents cellular respiration. Hydrogen cyanide (under the brand name Zyklon B) was perhaps most infamously employed by the Nazi regime in mid-20th century. This is also the poison that Adolf Hitler used to commit suicide on April 30th, 1945. More recent examples include the usage of this gas in gas chambers.

Hydrogen cyanide is commonly listed amongst chemical warfare agents which cause general poisoning. As a substance 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 tonnes per year must be declared to, and can be inspected by, the OPCW.

Although there have been no verified instances of this compound being used as a weapon, it has been reported that hydrogen cyanide may have been employed by Iraq in the war against Iran and against the Kurds in northern Iraq during the 1980s.

In 1995 a device was discovered in a restroom in the Kayabacho Tokyo subway station consisting of bags of sodium cyanide and sulfuric acid with a remote controlled motor to rupture them in what was believed to be an attempt to produce toxic amounts of hydrogen cyanide gas by the Aum Shinrikyo cult . In 2003, Al Qaeda reportedly planned to attack the New York City Subway using hydrogen cyanide gas but aborted the attack for unknown reasons.

Hydrogen cyanide gas in air is explosive at concentrations over 5.6%, equivalent to 56,000  ppm.

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