Hydrogen cyanide

Hydrogen cyanide is a highly toxic and volatile chemical compound with the formula HCN. Known historically as prussic acid, it has significant relevance in industrial chemistry, environmental science and toxicology. Its ability to act as a precursor to a wide array of chemical products makes it an important industrial feedstock, though its extreme toxicity requires stringent handling and regulatory control.

Structure and General Properties

Hydrogen cyanide consists of a linear chain of hydrogen, carbon and nitrogen atoms, with a characteristic carbon–nitrogen triple bond that imparts notable reactivity. It exists mainly as a colourless liquid or gas that boils slightly above room temperature. In aqueous form, the compound is referred to as hydrocyanic acid. The cyanide anion (CN⁻), formed through partial dissociation of HCN in water, constitutes the basis of a broad family of salts known as cyanides.
There is longstanding discussion regarding whether hydrogen cyanide should be classified as an organic or inorganic compound. Although traditionally placed within inorganic chemistry, its structural similarity to organic nitriles has led many authors to regard it as an organic molecule. Nitriles possess the general formula R–C≡N, where the R group may be an organic substituent or hydrogen. In the case of hydrogen cyanide, hydrogen acts as the substituent, leading to alternative systematic names such as methanenitrile and formonitrile.
A tautomeric form of hydrogen cyanide is hydrogen isocyanide (HNC), although this species is significantly less stable. Hydrogen cyanide is weakly acidic, exhibiting limited dissociation in solution. It forms hydrogen-bonded complexes with its conjugate base and can undergo further reactions, including tetramerisation to produce diaminomaleonitrile.

Odour, Toxicity and Biological Effects

Hydrogen cyanide has been widely reported in literature as possessing an almond-like odour, although the reliability of this description remains questionable. Almond essence contains benzaldehyde, a compound released alongside HCN during the breakdown of certain plant glycosides, and is often confused with the odour of pure hydrogen cyanide. Experimental observations have described the odour as more akin to mild chlorine or a swimming pool. Furthermore, approximately half of the human population cannot detect hydrogen cyanide due to a recessive genetic trait affecting olfactory perception.
The compound’s high volatility renders it more dangerous than many solid cyanides. Cyanide ions disrupt cellular respiration by inhibiting iron-containing enzymes, particularly cytochrome c oxidase. Hydrogen cyanide has historically been used as an inhalation poison, a rodenticide, and, controversially, in whaling. In mammals, small quantities of hydrogen cyanide may be produced endogenously: studies indicate that neuronal activation of opioid receptors can stimulate HCN production, which in turn may modulate signal transduction via NMDA receptors.

Chemical Reactivity

Hydrogen cyanide participates readily in addition reactions with unsaturated compounds. A prominent example is hydrocyanation, in which HCN adds to alkenes to generate nitriles; this transformation underpins industrial synthesis of adiponitrile using nickel-based catalysts. In aqueous medium, HCN dissociates to form cyanide ions, and metal cyanides may be obtained through appropriate salt metathesis reactions. Some cyanides, such as mercuric cyanide, are directly formed from reactions involving aqueous hydrogen cyanide.

Historical Background and Naming

The first isolation of hydrogen cyanide occurred in 1752 when Pierre Macquer decomposed Prussian blue into iron oxide and a volatile component capable of reconstituting the pigment. In 1782, Carl Wilhelm Scheele prepared the substance and recognised its acidic properties. Its association with Prussian blue led to the German name Blausäure, meaning “blue acid”, and the corresponding English term “prussic acid”.
A significant milestone occurred in 1787 when Claude Louis Berthollet demonstrated that the acid did not contain oxygen, an important discovery at a time when acids were believed to require oxygen. In 1811, Joseph Louis Gay-Lussac produced pure liquid hydrogen cyanide and deduced its empirical formula in 1815. Gay-Lussac also coined the term cyanure, derived from the Greek kyanos meaning “dark blue”, which later became “cyanide” in English and provided the root of the modern colour name cyan.

Industrial Production and Synthesis

Large-scale manufacture of hydrogen cyanide primarily follows the Andrussow process, developed by Leonid Andrussow. In this method, methane and ammonia react with oxygen at high temperature over a platinum catalyst. During the early twenty-first century, hundreds of thousands of tonnes were produced annually in the United States alone. HCN is also obtained as a by-product during the manufacture of acrylonitrile, an important monomer in plastics production.
Alternative production routes include the Degussa (BMA) process, which operates without added oxygen and uses indirect heat transfer, resembling steam reforming reactions. The Shawinigan process employs hydrocarbons such as propane in reaction with ammonia. Laboratory-scale production typically involves acidification of cyanide salts, though this method carries risk because it releases gaseous HCN.
Historically, hydrogen cyanide was produced to meet the growing demand for cyanide compounds during the late nineteenth century. George Thomas Beilby patented a method using ammonia passed over heated coal in 1892, later superseded by Hamilton Castner’s process involving coal, ammonia and sodium to generate sodium cyanide.

Industrial and Chemical Applications

Hydrogen cyanide is an essential precursor in the manufacture of sodium and potassium cyanide, both of which are widely used in gold and silver extraction as well as in electroplating. Through cyanohydrin chemistry, HCN contributes to the synthesis of several commercially important products. These include methyl methacrylate from acetone, methionine through the Strecker synthesis, and major chelating agents such as EDTA and nitrilotriacetic acid. Hydrocyanation of butadiene yields adiponitrile, a key monomer in the production of nylon.
Beyond organic synthesis, hydrogen cyanide is used globally as a fumigant for pest control in food storage and processing facilities. Its relatively low dosage requirements and lower environmental persistence make it a competitive alternative to other fumigants such as methyl bromide and sulphuryl fluoride.

Natural Occurrence

Hydrogen cyanide occurs naturally in a variety of plant and animal systems. The seeds and kernels of fruits such as cherries, apricots and apples often contain cyanogenic glycosides—compounds including amygdalin and mandelonitrile—that release hydrogen cyanide upon enzymatic breakdown. Bitter almonds, from which almond oil is produced, contain particularly high levels of such compounds. Cassava roots also contain cyanogenic glycosides such as linamarin, yielding substantial amounts of hydrogen cyanide unless properly processed.
Several animal species utilise hydrogen cyanide as a defence mechanism. This includes millipedes such as Harpaphe haydeniana and Desmoxytes purpurosea, as well as certain insects, including members of the Zygaenidae family. Hydrogen cyanide can also be generated during combustion of nitrogen-containing materials and is a component of vehicle exhaust.

Extraterrestrial and Prebiotic Occurrence

Hydrogen cyanide has been detected in the atmosphere of Titan, Saturn’s largest moon, using instruments aboard the Cassini–Huygens mission, Voyager 1 and terrestrial observatories. On Titan, HCN forms through photochemical reactions involving methane and nitrogen radicals and plays a role in the moon’s complex atmospheric chemistry. Ultraviolet radiation can break HCN into reactive fragments, although efficient recycling processes regenerate the molecule.
On early Earth, hydrogen cyanide may have been produced during the Late Heavy Bombardment, when carbon-containing meteorites interacted with atmospheric nitrogen. HCN is considered a plausible precursor in prebiotic chemistry, contributing to pathways leading to amino acids and nucleic acid components.