Hydrogen cyanide

Hydrogen cyanide
General
Systematic name	Hydrogen cyanide
Other names	Hydrocyanic acid prussic acid, formonitrile formic anammonide carbon hydride nitride cyclon
Molecular formula	HCN
SMILES	C#N
Molar mass	27.03 g/mol
Appearance	Colorless gas or pale blue highly volatile liquid
CAS number
Properties
Density and phase	0.687 g/cm³, liquid.
Solubility in water	Completely miscible.
Melting point	-13.4°C (259.75 K)
Boiling point	26°C (299.15 K)
Acidity (pK_a)	9.2 to 9.3
Structure
Molecular shape	Linear
Dipole moment	2.98 D
Hazards
MSDS	External MSDS
Main hazards	Highly toxic, highly flammable.
NFPA 704
Flash point	−17.78°C
Autoignition	538°C
R/S statement	R: , , , , . S: , , , , , , , , .
RTECS number	MW6825000
Supplementary data page
Structure and properties	n, ε_r, 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) Chemical infobox

Hydrogen cyanide is a chemical compound with chemical formula HCN. 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 ionizes to become 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.

Production and synthesis

Hydrogen cyanide is produced in large quantities by two processes. In the Degussa process, ammonia and methane react at 1200 °C over a platinum catalyst:

CH₄ + NH₃ → HCN + 3 H₂

This reaction is akin to steam reforming, the reaction of methane and water. In the Andrussov process, oxygen is added

CH₄ + NH₃ + 1.5 O₂ → HCN + 3 H₂O

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

H⁺ + NaCN → HCN + Na⁺

This reaction is the basis of accidental poisonings because the acid converts the nonvolatile salt into the gaseous HCN.

Occurrence and applications

Cyanide 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.

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 are made, also contain hydrogen cyanide. This hydrogen cyanide is contained in cyanohydrins such as mandelonitrile, and can be released by chemical means. 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.

Hydrogen cyanide as a chemical weapon

Most military sources list hydrogen cyanide among the chemical warfare agents causing general poisoning. 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 ^*. A form of hydrogen cyanide known as Zyklon B was used during World War II in the Nazi gas chambers of Auschwitz and Majdanek.

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

Reactions

hydrogen cyanide + ketone or aldehyde → cyanohydrin

Source

Institut national de recherche et de sécurité (1997). "Cyanure d'hydrogène et solutions aqueuses". Fiche toxicologique n° 4, Paris:INRS, 5pp. (PDF file, in French)

External links

Inorganic compounds | Cyanides | Hydrogen compounds | Chemical weapons | Fumigants

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