Hydrogen cyanide

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Hydrogen cyanide
IUPAC name Hydrogen cyanide
Other names Hydrocyanic acid
prussic acid,
formic anammonide
carbon hydride nitride
CAS number [74-90-8]
RTECS number MW6825000
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

25.6°C (299 K, 78°F)

Solubility in water Completely miscible.
Acidity (pKa) 9.2 - 9.3
Molecular shape Linear
Dipole moment 2.98 D
Main hazards Highly toxic, highly flammable.
NFPA 704

NFPA 704.svg

R-phrases R12, R26, R27, R28, R32.
S-phrases (S1), S2, S7, S9, S13, S16,
S28, S29, S45.
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)

Hydrogen cyanide is a chemical compound with the formula HCN. It is a colorless, very poisonous, and highly volatile liquid that boils slightly above room temperature. The gas is explosive in air, at concentrations above 5.6 percent. A solution of hydrogen cyanide in water is called hydrocyanic acid, and its salts are called cyanides.

Given its toxicity, hydrogen cyanide is listed as a chemical warfare agent. It was the active ingredient in Zyklon B, an insecticide used by the Nazi regime for the mass murder of Jews in the gas chambers of Auschwitz, Majdanek, and other extermination camps.

On the other hand, HCN is a highly valuable precursor for the synthesis of many chemical compounds, ranging from polymers to pharmaceuticals. In the year 2000, an estimated 1,615 million pounds (732,552 tons) of this substance was produced in the United States.[1]


Some fruits that have a pit (such as cherries and apricots), bitter almonds, and apple seeds contain small amounts of cyanohydrins such as mandelonitrile (CAS#532-28-5). Such molecules slowly decompose to release hydrogen cyanide.[2][3]

Some millipedes release hydrogen cyanide as a defense mechanism.[4] So 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.

Role in the origin of life?

Some have proposed that hydrogen cyanide may have played a part in the origin of life, in terms of being a possible precursor to amino acids and nucleic acids. Leslie Orgel, among other researchers, has written extensively on the condensation reactions 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 the condensation of HCN.[5]


Hydrogen cyanide has a faint, bitter, almond-like odor. Some people, however, are unable to detect the odor due to a genetic trait.[6]

The boiling point of HCN is 26 °C (78.8 °F). In air, HCN gas is explosive at concentrations over 5.6 percent, equivalent to 56,000 parts per million (ppm).[7]

Hydrogen cyanide is weakly acidic and partly ionizes in solution to give the cyanide anion, CN.


Currently, hydrogen cyanide is produced in large quantities by three processes. The most important of these processes is called Andrussov oxidation, invented by Leonid Andrussow. In this method, methane and ammonia react in the presence of oxygen at about 1200 °C over a platinum catalyst:[8]

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 energy is transferred indirectly through the reactor wall:[9]

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 has sometimes been the basis of accidental poisonings because the acid converts a nonvolatile cyanide salt into the gaseous HCN.


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

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


Hydrogen cyanide is a valuable starting material for the synthesis of various organic compounds. Its main use is in the production of adiponitrile, which is then used to manufacture nylon. A second important application is for the synthesis of methyl methacrylate, which in turn is needed for the production of polymethyl methacrylate (PMMA) resins. In addition, HCN is used in tempering steel, dyeing, explosives, and engraving.

The less toxic ethyl acetate (C4H8O2) has now largely replaced the use of cyanide in insect killing jars. However, cyanide is being used for capital punishment.

As a chemical weapon

An HCN concentration of 300 parts per million in air will kill a human within a few minutes.[10] The toxicity is caused by the cyanide ion, which prevents cellular respiration.

Zyklon B was an insecticide that releases gaseous hydrogen cyanide upon exposure to air. It gained notoriety for its use by the Nazi regime in the mid-twentieth century as a method of mass murder, in the gas chambers of Auschwitz and Majdanek during the Holocaust.[11][12][13]

Hydrogen cyanide is commonly listed amongst chemical warfare agents that cause general poisoning.[14] It is listed under Schedule 3 of the Chemical Weapons Convention as a potential weapon with large-scale industrial uses. As such, manufacturing plants in signatory countries that produce more than 30 metric tons per year must be declared to, and can be inspected by, the Organization for the Prohibition of Chemical Weapons (OPCW).

Although there have been no verified instances of this compound being used as a weapon in warfare, 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.[15]

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.[16] In 2003, Al Qaeda reportedly planned to attack the New York City Subway using hydrogen cyanide gas but aborted the attack for unknown reasons.[17]

See also


  1. Hydrogen Cyanide Retrieved February 22, 2008.
  2. J. Vetter, 2000, Plant cyanogenic glycosides, Toxicon 38:11-36.
  3. D.A. Jones, 1998, Why are so many food plants cyanogenic? Phytochemistry 47:155-162.
  4. M.S. Blum and J.P. Woodring, 1962, Secretion of Benzaldehyde and Hydrogen Cyanide by the Millipede Pachydesmus crassicutis (Wood), Science. 138:512-13.
  5. A. Al-Azmi, A.Z. Elassar, and B.L. Booth, 2003, The Chemistry of Diaminomaleonitrile and its Utility in Heterocyclic Synthesis, Tetrahedron 59:2749-2763.
  6. Cyanide, inability to smell Online Mendelian Inheritance in Man. Retrieved February 22, 2008.
  7. Hydrogen cyanide Centers for Disease Control and Prevention. Retrieved February 22, 2008.
  8. L. Andrussow, 1935, The catalytic oxidation of ammonia-methane-mixtures to hydrogen cyanide, Angewandte Chemie 48:593-595.
  9. F. Endter, 1958, Die technische Synthese von Cyanwasserstoff aus Methan und Ammoniak ohne Zusatz von Sauerstoff, Chemie Ingenieur Technik 30:5:281-376.
  10. Occupational Safety and Health Guideline for Hydrogen Cyanide OSHA.
  11. Zyklon B The Nizkor Project. Retrieved December 21, 2010.
  12. Holocaust Timeline. The History Place. Retrieved December 21, 2010.
  13. Holocaust Encyclopedia: Gassing Operations. United States Holocaust Memorial Museum. Retrieved December 21, 2010.
  14. Hydrogen Cyanide Organization for the Prohibition of Chemical Weapons.
  15. Facts About Cyanide Centers for Disease Control and Prevention. Retrieved February 22, 2008.
  16. Chronology of Aum Shinrikyo's CBW Activities James Martin Center for Nonproliferation Studies. Retrieved February 22, 2008.
  17. Ron Suskind, 2006, The Untold Story of al-Qaeda's Plot to Attack the Subway Time. Retrieved February 22, 2008.

ISBN links support NWE through referral fees

  • Brown Jr., Theodore L., H. Eugene LeMay, Bruce Edward Bursten, and Julia R. Burdge. 2002. Chemistry: The Central Science, 9th ed. Upper Saddle River, NJ: Prentice Hall. ISBN 0130669970
  • Chang, Raymond. 2006. Chemistry, 9th ed. New York, NY: McGraw-Hill Science/Engineering/Math. ISBN 0073221031
  • Cotton, F. Albert, and Geoffrey Wilkinson. 1980. Advanced Inorganic Chemistry, 4th ed. New York, NY: Wiley. ISBN 0-471-02775-8
  • McMurry, J., and R.C. Fay. 2004. Chemistry, 4th ed. Upper Saddle River, NJ: Prentice Hall. ISBN 0131402080

External links

All links retrieved January 22, 2018.

Agents of Chemical Warfare
Blood agents: Cyanogen chloride (CK) – Hydrogen cyanide (AC)
Blister agents: Lewisite (L) – Sulfur mustard gas (HD, H, HT, HL, HQ) – Nitrogen mustard gas (HN1, HN2, HN3)
Nerve agents: G-Agents: Tabun (GA) – Sarin (GB) – Soman (GD) – Cyclosarin (GF) | V-Agents: VE – VG – VM – VX
Pulmonary agents: Chlorine – Chloropicrin (PS) – Phosgene (CG) – Diphosgene (DP)
Incapacitating agents: Agent 15 (BZ) – KOLOKOL-1
Riot control agents: Pepper spray (OC) – CS gas – CN gas (mace) – CR gas


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