Hydrogen cyanide

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Hydrogen cyanide
Missing image
Chemical structure of hydrogen cyanide

Other names Hydrocyanic acid
Prussic acid
formic anammonide
carbon hydride nitride
Molecular formula HCN
Molar mass 27.03 g/mol
Appearance colorless gas or pale blue
highly volatile liquid
CAS number [74-90-8]
EINECS number 200-821-6
Density and phase 0.687 g/cm3, liquid
Solubility in water completely miscible
Solubility in alcohol miscible
Solubility in ether slightly soluble
Melting point −13.4°C (260 K)
Boiling point 26°C (299 K)
Acidity (pKa) 9.2 to 9.3
Molecular shape linear
Dipole moment  ? D
Safety data
EU classification Highly flammable (F+)
Very toxic (T+)
Dangerous for
the environment (N)
R-phrases R12, R26, R50/53
S-phrases S1/2, S7/9, S16, S36/37
S38, S45, S60, S61
PEL (US, OSHA) 11 mg/m3
IDLH (US, NIOSH) 55 mg/m3
Flash point −17.78°C
Autoignition temperature 538°C
Explosive limits in air 5.6–40.0 vol%
RTECS number MW6825000
Supplementary data page
Structure & properties n, εr, etc.
Thermodynamic data Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Related compounds
Other anions Hydrogen fluoride
Other cations Sodium cyanide
Potassium cyanide
Related compounds Cyanogen
Cyanogen chloride
Except where noted otherwise, data are given for
materials in their standard state (at 25°C, 100 kPa)
Infobox disclaimer and references

Hydrogen cyanide is a chemical compound with chemical formula H-C≡N. A solution of hydrogen cyanide in water is called hydrocyanic acid or prussic acid. Pure hydrogen cyanide is a colorless, very poisonous, and highly volatile liquid that boils slightly above room temperature at 26 C, thereby generating hydrogen cyanide gas. Hydrogen cyanide has a faint, bitter, almond-like odor that some people are unable to smell due to a genetic trait. Hydrogen cyanide is weakly acidic and partly converts to the cyanide ion CN in aqueous solution, resulting in a colorless volatile liquid with the typical hydrogen cyanide odor. The salts of hydrogen cyanide are known as cyanides.

Hydrogen cyanide is produced in large quantities all over the world by the chemical industry where it is used in tempering steel, dyeing, explosives, engraving, the production of acrylic resin plastic, and other organic chemical products. Its use in insect killing jars has now largely been replaced by less toxic ethyl acetate. It can be produced by reacting a cyanide salt with a strong acid, or directly from ammonia and carbon monoxide.

Fruits that have a pit, such as cherries or apricots, often contain small quantities of hydrogen cyanide in the pit. Bitter almonds, from which almond oil and flavoring is made, also contain hydrogen cyanide. Some millipedes release hydrogen cyanide as a defense mechanism. Hydrogen cyanide is contained in the exhaust of vehicles, in tobacco smoke, and in the smoke of burning nitrogen-containing plastics.

An HCN concentration of 300 parts per million of air will kill a human within a few minutes. The toxicity is caused by the cyanide ion. The mechanism of this toxicity, and the uses of the poison, are described on the cyanide page. Hydrogen cyanide (under the brand name Zyklon B) was perhaps most infamously employed by the Nazi regime in Germany as a method of mass-execution. Hydrogen cyanide is now listed under schedule 3 of the Chemical Weapons Convention.

Hydrogen cyanide forms a foul tasting compound when it combines with tobacco smoke. For this reason, some chemists choose to have a lit cigarette in their mouth while they are working with it, as they receive an early warning against possible cyanide poisoning.

Hydrogen cyanide gas in air is explosive at concentrations over 56,000 ppm.


  1. hydrogen cyanide + ketone or aldehydecyanohydrin

External links

es:Cianuro de hidrgeno fr:Cyanure d'hydrogne nl:Blauwzuur ja:シアン化水素 pl:Cyjanowodr pt:cido ciandrico ru:Синильная кислота sv:Vtecyanid


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