Hydrogen cyanide

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Hydrogen cyanide
IUPAC names
Formonitrile (substitutive)
Hydridonitridocarbon (additive)
Other names
Carbon hydride nitride
Formic anammonide
Hydrocyanic acid
Prussic acid
Jmol-3D images Image
Molar mass 27.0253 g/mol
Appearance Colorless volatile liquid
Odor Bitter almond-like
Density 0.687 g/cm3
Melting point 13.4 °C (56.1 °F; 286.5 K)
Boiling point 25.6 °C (78.1 °F; 298.8 K)
Solubility Reacts with amines, hydrogen peroxide
Miscible with ethanol
Slightly soluble in diethyl ether
Vapor pressure 630 mmHg (at 20 °C)
Acidity (pKa) 9.21
113.01 J·K−1·mol−1
109.9 kJ·mol−1
Safety data sheet Matheson
Flash point −17.8 °C
Related compounds
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Hydrogen cyanide, also known as prussic acid, is a chemical compound with the chemical formula HCN. It is a colorless, extremely poisonous and flammable liquid that boils just above room temperature, at 25.6 °C.

The exact classification of HCN as organic or inorganic remains unresolved. Going by the C-H bond definition, HCN would be organic, but in aqueous solution it dissociates into H+ and CN-, and since the cyanide ion is a pseudohalogen, this makes it inorganic. Hydrogen cyanide is considered a borderline chemical compound.



Hydrogen cyanide reacts with sodium hydroxide to yield sodium cyanide.

HCN + NaOH → NaCN + H2O

Hydrogen cyanide produces the nucleophilic cyanide ion CN, which can attack the carbonyl carbon of an aldehyde or ketone, forming a hydroxyl nitrile or cyanohydrin.



Hydrogen cyanide is a highly toxic colorless liquid with a bitter almond smell. It is miscible with water and ethanol, and slightly soluble in diethyl ether. HCN melts at -13.4 °C and boils at 25.6 °C. Its density is 0.687 g/cm3.


Anhydrous hydrogen cyanide is only available to large chemical entities and cannot be purchased by individuals or small companies.

HCN is is listed on schedule 3 of the Chemical Weapons Convention, meaning all transactions are monitored.

HCN absorbed by diatomaceous earth is somewhat easier to come by: it is sometimes sold as an industrial-strength pesticide. The most well known brand is Uragan D2 (Czech Republic). Another, very infamous brand under which this product was sold was Zyklon-B (the agent used to kill people in Nazi gas chambers).


Due to its extreme toxicity, HCN production is generally avoided and instead sodium cyanide or other cyanide salts are used for reactions where a cyanide group is required. The procedures below should only performed if you have proper installations and experience.

Adding any strong acid to sodium cyanide will liberate hydrogen cyanide, which will rapidly evaporate due to its low boiling point.

Adding a dilute acid to Prussian blue and heating the flask will also yield hydrogen cyanide gas.

The thermal decomposition of amides in the presence of a catalyst, such as formamide will yield hydrogen cyanide gas.

The combustion of nitrile containing plastic materials will give off very impure hydrogen cyanide gas.


Seriously, if you really want to make experiments with cyanides, it's MUCH safer to use cyanide salts. Experimenting with hydrogen cyanide is not worth the risk.



Hydrogen cyanide is lethal. Air concentrations of 100–200 ppm in air will kill a human within 10-60 minutes, while 2000 ppm (about 2380 mg/m3) will kill a human in about 1 minute. Since it boils slightly above room temperature, open containers are a big no-no, and metal cylinders are instead employed, which severely limit its evaporation.

Contact of HCN with bases and amines may cause violent polymerization and even explosion.

Hydrogen cyanide will also attack certain plastic and rubber materials.

In case of poisoning, glucose, methylene blue or sodium thiosulfate can be used as antidotes.


DO NOT STORE HCN! EVER! If you absolutely want to, you can make your own Uragan D2 and store that.


Hydrogen cyanide can be neutralized with a variety of chemicals, such as hydrogen peroxide, sodium hypochlorite (bleach), which will convert it to the less toxic isocyanate species. Sodium thiosulfate will convert it to thiocyanate, which is less toxic. These two products can be further oxidized to nitrogen, carbon dioxide and water with a strong oxidizer.


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