Hydrogen cyanide (also called prussic acid) is a chemical compound with the formula HCN and structural formula H−C≡N. It is a highly toxic and flammable liquid that boils slightly above room temperature, at 25.6 °C (78.1 °F). HCN is produced on an industrial scale and is a highly valued precursor to many chemical compounds ranging from polymers to pharmaceuticals. Large-scale applications are for the production of potassium cyanide and adiponitrile, used in mining and plastics, respectively. It is more toxic than solid cyanide compounds due to its volatile nature. A solution of hydrogen cyanide in water, represented as HCN(aq), is called hydrocyanic acid. The salts of the cyanide anion are known as cyanides. Whether hydrogen cyanide is an organic compound or not is a topic of debate among chemists. It is traditionally considered inorganic, but can also be considered a nitrile, giving rise to its alternative names of methanenitrile and formonitrile.
Structure and general properties Hydrogen cyanide is a linear molecule, with a triple bond between carbon and nitrogen. The C-N bond length is 115 picometers. The isomer of HCN is HNC, hydrogen isocyanide.
Odor About half of people are unable to detect the odor of hydrogen cyanide owing to a recessive genetic trait. For those that can detect it, the odor has been described as bitter almond-like.
Chemical properties Hydrogen cyanide is weakly acidic with a pKa of 9.2. It partially ionizes in water to give the cyanide anion, CN−. HCN forms hydrogen bonds with its conjugate base, species such as (CN−)(HCN)n. In one of its principal uses, HCN reacts with alkenes to give nitriles. The conversion, which is called hydrocyanation, employs nickel complexes as catalysts.
RCH=CH2 + HCN → RCH2−CH2CN HCN is unstable with respect to self-condensation. Four molecules of HCN will tetramerize into diaminomaleonitrile. This and similar reactions have attracted attention with regard to their possible relevance to the origin of life. Metal cyanides are typically prepared by salt metathesis from alkali metal cyanide salts, but mercuric cyanide is formed from aqueous hydrogen cyanide:
HgO + 2 HCN → Hg(CN)2 + H2O
History of discovery and naming Hydrogen cyanide was first isolated in 1752 by French chemist Pierre Macquer who converted Prussian blue to an iron oxide plus a volatile component and found that these could be used to reconstitute it. The new component was what is now known as hydrogen cyanide. It was subsequently prepared from Prussian blue by the Swedish chemist Carl Wilhelm Scheele in 1782, and was eventually given the German name Blausäure (lit. "Blue acid") because of its acidic nature in water and its derivation from Prussian blue. In English, it became known popularly as prussic acid. In 1787, the French chemist Claude Louis Berthollet showed that prussic acid did not contain oxygen, an important contribution to acid theory, which had hitherto postulated that acids must contain oxygen (hence the name of oxygen itself, which is derived from Greek elements that mean "acid-former" and are likewise calqued into German as Sauerstoff) and into Slavic languages, such as kyslík into Czech, or кислород into Russian. In 1811, Joseph Louis Gay-Lussac prepared pure, liquified hydrogen cyanide, and in 1815 he deduced prussic acid's chemical formula.
Etymology The word cyanide for the radical in hydrogen cyanide was derived from its French equivalent, cyanure, which Gay-Lussac constructed from the Ancient Greek word κύανος for dark blue enamel or lapis lazuli, again owing to the chemical’s derivation from Prussian blue. This Greek word is also the root of the English color name cyan.
Production and synthesis The most important process is the Andrussow oxidation invented by Leonid Andrussow at IG Farben in which methane and ammonia react in the presence of oxygen at about 1,200 °C (2,190 °F) over a platinum catalyst:
2 CH4 + 2 NH3 + 3 O2 → 2 HCN + 6 H2O In 2006, between 500 million and 1 billion pounds (between 230,000 and 450,000 t) were produced in the US. Hydrogen cyanide is produced in large quantities by several processes and is a recovered waste product from the manufacture of acrylonitrile. Of lesser importance is the Degussa process (BMA process) in which no oxygen is added and the energy must be transferred indirectly through the reactor wall:
CH4 + NH3 → HCN + 3 H2 This reaction is akin to steam reforming, the reaction of methane and water to give carbon monoxide and hydrogen. In the Shawinigan Process, hydrocarbons, e.g. propane, are reacted with ammonia. 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 is sometimes the basis of accidental poisonings because the acid converts a nonvolatile cyanide salt into the gaseous HCN. Hydrogen cyanide could be obtained from potassium ferricyanide and acid:
6 H+ + [Fe(CN)6]3− → 6 HCN + Fe3+
Historical methods of production The large demand for cyanides for mining operations in the 1890s was met by George Thomas Beilby, who patented a method to produce hydrogen cyanide by passing ammonia over glowing coal in 1892. This method was used until Hamilton Castner in 1894 developed a synthesis starting from coal, ammonia, and sodium yielding sodium cyanide, which reacts with acid to form gaseous HCN.
Applications HCN is the precursor to sodium cyanide and potassium cyanide, which are used mainly in gold and silver mining and for the electroplating of those metals. Via the intermediacy of cyanohydrins, a variety of useful organic compounds are prepared from HCN including the monomer methyl methacrylate, from acetone, the amino acid methionine, via the Strecker synthesis, and the chelating agents EDTA and NTA. Via the hydrocyanation process, HCN is added to butadiene to give adiponitrile, a precursor to Nylon-6,6. HCN is used globally as a fumigant against many species of pest insects that infest food production facilities. Both its efficacy and method of application lead to very small amounts of the fumigant being used compared to other toxic substances used for the same purpose. Using HCN as a fumigant also has less environmental impact, compared to some other fumigants such as sulfuryl fluoride, and methyl bromide.
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