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Hydrogen cyanide

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Template:Chembox new Hydrogen cyanide is a chemical compound with chemical formula HCN. A solution of hydrogen cyanide in water is called hydrocyanic acid. Hydrogen cyanide is a colorless, extremely poisonous, and highly volatile liquid that boils slightly above room temperature at 26 °C (7.9 °F). HCN has a faint, bitter, almond-like odor that some people are unable to detect due to a genetic trait.[1] Hydrogen cyanide is weakly acidic and partly ionizes in solution to give the cyanide anion, CN–. The salts of hydrogen cyanide are known as cyanides. HCN is a highly valuable precursor to many chemical compounds ranging from polymers to pharmaceuticals.

Hydrogen cyanide is a linear molecule, with a triple bond between carbon and nitrogen. It is a weak acid with a pKa of 9.2. A minor isomer of HCN is HNC, hydrogen isocyanide.

Production and synthesis

Historical

The first source for hydrogen cyanide was the reaction of acid on ferrocyanides.[citation needed] The rising demand due to the use of cyanides for mining operations in the 1890s was met by the Beilby process. George Thomas Beilby 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.

Contemporary

Hydrogen cyanide can be formed from virtually any combination of hydrogen, carbon, and nitrogen. Hydrogen cyanide is currently produced in large quantities by several processes, as well as being a recovered waste product from the manufacture of acrylonitrile.[2] In the year 2000, 1.615 billion pounds (732,552 tons) were produced in the US.[1] The most important process for the production of hydrogen cyanide is the Andrussov oxidation invented by Leonid Andrussow at IG Farben in which methane and ammonia react in the presence of oxygen at about 1200 °C over a platinum catalyst:[3]

2CH4 + 2NH3 + 3O2 → 2HCN + 6H2O

The energy needed for the reaction is provided by the part oxidation of methane and ammonia.

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:[4]

CH4 + NH3 → HCN + 3H2

This reaction is akin to steam reforming, the reaction of methane and water.

In the Shawinigan Process, ammonia is passed over coke to give HCN. Carbon monoxide and ammonia at high temperature conditions:

CO + NH3 → HCN + H2O

In another process, practiced at BASF, formamide is heated and split into hydrogen cyanide and water:

CH(O)NH2 → HCN + H2O

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.

Applications

See also: sodium cyanide

HCN is the precursor to sodium cyanide and potassium cyanide, which are used mainly in mining. 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, the precursor to Nylon 66.[2] [5]

Occurrence

HCN is obtainable from fruits that have a pit, such as cherries, apricots, apples, and bitter almonds, from which almond oil and flavoring are made. Many of these pits contain small amounts of cyanohydrins such as mandelonitrile (CAS#532-28-5) and amygdalin. Such molecules slowly release hydrogen cyanide.[6][7]100 g of crushed apple seeds can yield about 10 mg of HCN.[citation needed] Some millipedes release hydrogen cyanide as a defense mechanism,[8] as do certain insects such as some burnet moths. Hydrogen cyanide is contained in the exhaust of vehicles, in tobacco and wood smoke, and in smoke from burning nitrogen-containing plastics.

HCN and the origin of life

Hydrogen cyanide has been discussed as a precursor to amino acids and nucleic acids. It is possible, for example, that HCN played a part in the origin of life. Leslie Orgel, among many researchers, has written extensively on the condensation of HCN.[9] Although the relationship of these chemical reactions to the origin of life remains speculative, studies in this area have led to discoveries of new pathways to organic compounds derived from condensation of HCN.[10]

HCN in space

See also: Astrochemistry

HCN has been detected in the Interstellar medium.[11] Since then, extensive studies have probed formation and destruction pathways of HCN in various environments and examined its use as a tracer for a variety of astronomical species and processes. HCN can be observed from ground-based telescopes through a number of atmospheric windows. The J=1→0, J=3→2, J= 4→3, and J=10→9 pure rotational transitions have all been observed.[12][13][14]


HCN is formed in interstellar clouds through one of two major pathways.[15]

Via a neutral-neutral reaction (CH2 + N → HCN + H) and via dissociative recombination (HCNH+ + e- → HCN + H). The dissociative recombination pathway is dominant by 30%, however, the HCNH+ must be in its linear form. Dissociative recombination with its structural isomer, H2NC+ produces hydrogen isocyanide (HNC), exclusively.

HCN is destroyed in interstellar clouds through a number of different mechanisms depending on the location in the cloud.[16] In the Photon-Dominated Region (PDR), photodissociation dominates, producing CN (HCN + ν → CN + H). At larger depths, photodissociation by cosmic rays dominate, producing CN (HCN + cr → CN + H). In the dark core, two competing mechanisms destroy it, forming HCN+ and HCNH+ (HCN + H+ → HCN+ + H; HCN + HCO+ → HCNH+ + CO). The reaction with HCO+ dominates by a factor of ~3.5. HCN has been used to analyze a variety of species and processes in the interstellar medium. It has been suggested as a tracer for dense molecular gas[17][18] and as a tracer of stellar inflow in high-mass star-forming regions[19]. Further, the HNC/HCN ratio has been shown to be an excellent method for distinguishing between PDRs and X-ray-dominated regions (XDRs).[20]

Hydrogen cyanide as a poison and chemical weapon

See also: cyanide poisoning

An HCN concentration of 300 mg/m3 in air will kill a human within a few minutes.[21] The toxicity is caused by the cyanide ion, which prevents cellular respiration. Hydrogen cyanide (under the brand name Zyklon B) was most infamously employed by the Nazi regime in the mid-20th century.

Hydrogen cyanide is commonly listed amongst chemical warfare agents that cause general poisoning.[22] As a substance listed under Schedule 3 of the Chemical Weapons Convention as a potential weapon which has large-scale industrial uses, manufacturing plants in signatory countries which produce more than 30 tonnes per year must be declared to, and can be inspected by, the Organisation for the Prohibition of Chemical Weapons.

Hydrogen cyanide gas in air is explosive at concentrations over 5.6%, equivalent to 56,000 ppm[23].

Footnotes

  1. ^ Online Mendelian Inheritance in Man, Cyanide, inability to smell
  2. ^ a b Ernst Gail, Stephen Gos, Rupprecht Kulzer, Jürgen Lorösch, Andreas Rubo, Manfred Sauer "Cyano Compounds, Inorganic" in Ullmann's Encyclopedia of Industrial Chemistry. 2004, Wiley-VCH Verlag, Weinheim. {{DOI: 10.1002/14356007.a08 159.pub2}}. Article Online Posting Date: January 15, 2004
  3. ^ L. Andrussow (1935). "The catalytic oxydation of ammonia-methane-mixtures to hydrogen cyanide". Angewandte Chemie. 48: 593–595.
  4. ^ F. Endter (1958). "Die technische Synthese von Cyanwasserstoff aus Methan und Ammoniak ohne Zusatz von Sauerstoff". Chemie Ingenieur Technik. 30 (5): 281–376. doi:10.1002/cite.330300506.
  5. ^ Ernst Gail, Stephen Gos, Rupprecht Kulzer, Jürgen Lorösch, Andreas Rubo, Manfred Sauer "Cyano Compounds, Inorganic" in Ullmann's Encyclopedia of Industrial Chemistry. 2004, Wiley-VCH Verlag, Weinheim. {{DOI: 10.1002/14356007.a08 159.pub2}}. Article Online Posting Date: January 15, 2004
  6. ^ J. Vetter (2000). "Plant cyanogenic glycosides". Toxicon. 38: 11–36. doi:10.1016/S0041-0101(99)00128-2.
  7. ^ D. A. Jones (1998). "Why are so many food plants cyanogenic?". Phytochemistry. 47: 155–162. doi:10.1016/S0031-9422(97)00425-1.
  8. ^ M. S. Blum, J. P. Woodring (1962). "Secretion of Benzaldehyde and Hydrogen Cyanide by the Millipede Pachydesmus crassicutis (Wood)". Science. 138: 512–513. doi:10.1126/science.138.3539.512. PMID 17753947.
  9. ^ Matthews, C. N. "The HCN World: Establishing Protein-Nucleic Augucid Life via Hydrogen Cyanide Polymers" Cellular Origin and Life in Extreme Habitats and Astrobiology (2004), 6 (Origins : Genesis, Evoluation and Diversity of Life), 121-135.
  10. ^ Al-Azmi, A.; Elassar, A.-Z. A.; Booth, B. L. "The Chemistry of Diaminomaleonitrile and its Utility in Heterocyclic Synthesis" Tetrahedron (2003), 59, 2749-2763. CODEN: TETRAB ISSN:0040-4020
  11. ^ Snyder, Lewis E.; Buhl, David (1971). "Observations of Radio Emission from Interstellar Hydrogen Cyanide". Astrophysical Journal. 163: L47.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  12. ^ Snyder, Lewis E.; Buhl, David (1971). "Observations of Radio Emission from Interstellar Hydrogen Cyanide". Astrophysical Journal. 163: L47.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  13. ^ Bieging, J. H.; et al. (2000). Astrophysical Journal. 543: 897. {{cite journal}}: Explicit use of et al. in: |author= (help); Missing or empty |title= (help)
  14. ^ Schilke, P. and Menten, K. M. (2003). Astrophysical Journal. 583: 446. {{cite journal}}: Missing or empty |title= (help)CS1 maint: multiple names: authors list (link)
  15. ^ Boger, G. I. and Sternberg, A. (2005). Astrophysical Journal. 632: 302. {{cite journal}}: Missing or empty |title= (help)CS1 maint: multiple names: authors list (link)
  16. ^ Boger, G. I. and Sternberg, A. (2005). Astrophysical Journal. 632: 302. {{cite journal}}: Missing or empty |title= (help)CS1 maint: multiple names: authors list (link)
  17. ^ Gao, Y. and Solomon, P. M. (2004). Astrophysical Journal. 606: 271. {{cite journal}}: Missing or empty |title= (help)CS1 maint: multiple names: authors list (link)
  18. ^ Gao, Y. and Solomon, P. M. (2004). Astrophysical Journal Supplements. 152: 63. {{cite journal}}: Missing or empty |title= (help)CS1 maint: multiple names: authors list (link)
  19. ^ Wu, J. and Evans, N. J. (2003). Astrophysical Journal. 592: L79. {{cite journal}}: Missing or empty |title= (help)CS1 maint: multiple names: authors list (link)
  20. ^ Loenen, A. F.; et al. (2007). Proceedings IAU Symposium. 202. {{cite journal}}: Explicit use of et al. in: |author= (help); Missing or empty |title= (help)
  21. ^ Hydrogen Cyanide
  22. ^ "Hydrogen Cyanide". Organisation for the Prohibition of Chemical Weapons. Retrieved 2006-10-07.
  23. ^ Documentation for Immediately Dangerous to Life or Health Concentrations (IDLHs) - 74908

References

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