cyanide

The cyanide ion, CN⁻.
From the top:
1. Valence-bond structure
2. Space-filling model
3. Electrostatic potential surface
4. "Carbon lone pair" HOMO/LUMO

A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom.^[1] Cyanides most commonly refer to salts of the anion CN⁻, which is isoelectronic with carbon monoxide and with molecular nitrogen.^[2][3] Most cyanides are highly toxic.^[4]

In organic chemistry compounds containing a -C≡N group are known as nitriles, and compounds that contain the -N≡C group are known as isocyanides. Organic nitriles and isocyanides are far less toxic because they do not release cyanide ions easily.

The dye Prussian blue was first accidentally made (it is presumed) around 1706, from substances containing iron and carbon and nitrogen, and the (then unknown) cyanide was formed during the manufacture of the dye. An iron-containing compound was found in Prussian blue and named "ferrocyanide", meaning "blue substance with iron", from Latin ferrum = "iron" and Greek kyanos = "(dark) blue". When ferrocyanide was analyzed, removing the iron from the compound and from its name left "cyanide".^{[clarification needed]}

Nomenclature and etymology

In IUPAC nomenclature, organic compounds that have a –C≡N functional group are called nitriles. Thus, nitriles are organic compounds.^[5][6] An example of a nitrile is CH₃CN, acetonitrile, also known as methyl cyanide.

In inorganic chemistry, salts containing the C≡N^- ion are referred to as cyanides. Nitriles usually do not release cyanide ions.

A functional group with a hydroxyl and cyanide bonded to the same carbon is called cyanohydrin, and cyanohydridins are hydrolyzed into hydrogen cyanide and a carbonyl compound (ketone or aldehyde).

The word "cyanide" was extracted from "ferrocyanide", a cyanide derivative of iron. The name "ferrocyanide" was invented as meaning "blue substance with iron", as ferrocyanides were first discovered as components of the intensely colored dye Prussian blue. Kyanos is Greek for "(dark) blue".^[7]

Occurrence

Cyanides are produced by certain bacteria, fungi, and algae and are found in a number of plants. Cyanides are found, although in small amounts, in certain seeds and fruit stones, e.g., those of apple, mango, peach, and bitter almonds.^[8] In plants, cyanides are usually bound to sugar molecules in the form of cyanogenic glycosides and defend the plant against herbivores. Cassava roots (also called manioc), an important potato-like food grown in tropical countries (and the base from which tapioca is made), also contain cyanogenic glycosides.^[9][10]

The cyanide radical CN· has been identified in interstellar space.^[11]

Hydrogen cyanide is produced by the combustion or pyrolysis of certain materials under oxygen-deficient conditions. For example, it can be detected in the exhaust of internal combustion engines and tobacco smoke. Certain plastics, especially those derived from acrylonitrile, release hydrogen cyanide when heated or burnt.^[12]

Coordination chemistry

The cyanide anion is a potent ligand for many transition metals.^[13] The very high affinities of metals for this anion can be attributed to its negative charge, compactness, and ability to engage in π-bonding. Well-known complexes include:

• hexacyanides [M(CN)₆]³⁻ (M = Ti, V, Cr, Mn, Fe, Co), which are octahedral in shape;
• the tetracyanides, [M(CN)₄]²⁻ (M = Ni, Pd, Pt), which are square planar in their geometry;
• the dicyanides [M(CN)₂]⁻ (M = Cu, Ag, Au), which are linear in geometry.

The deep-blue pigment Prussian blue, used in the making of blueprints, is derived from iron cyanide complexes (hence the name cyanide, from cyan, a shade of blue). Prussian blue can produce hydrogen cyanide when exposed to strong acids.

Certain enzymes, for example, the hydrogenases, contain cyanide ligands attached to iron in their active sites. The biosynthesis of cyanide in the [NiFe]-hydrogenases proceeds from carbamoylphosphate, which converts to cysteinyl thiocyanate, the CN⁻ donor.^[14]

Organic derivatives

Because of the cyanide anion's high nucleophilicity, cyano groups are readily introduced into organic molecules by displacement of a halide group (e.g., the chloride on methyl chloride). In general, organic cyanides are called nitriles. Thus, CH₃CN can be called methyl cyanide but more commonly is referred to as acetonitrile. In organic synthesis, cyanide is a C-1 synthon; i.e., it can be used to lengthen a carbon chain by one, while retaining the ability to be functionalized.

RX + CN⁻ → RCN + X⁻ (nucleophilic substitution) followed by

1. RCN + 2 H₂O → RCOOH + NH₃ (hydrolysis under reflux with mineral acid catalyst), or
2. 2 RCN + LiAlH₄ + (second step) 4 H₂O → 2 RCH₂NH₂ + LiAl(OH)₄ (under reflux in dry ether, followed by addition of H₂O)

Manufacture

The principal process used to manufacture cyanides is the Andrussow process in which gaseous hydrogen cyanide is produced from methane and ammonia in the presence of oxygen and a platinum catalyst.^[15][16]

2 CH₄ + 2 NH₃ + 3 O₂ → 2 HCN + 6 H₂O

Gaseous hydrogen cyanide may be dissolved in aqueous sodium hydroxide solution to produce sodium cyanide.

Toxicity

Many cyanides are highly toxic. The cyanide anion is an inhibitor of the enzyme cytochrome c oxidase (also known as aa₃) in the fourth complex of the electron transport chain (found in the membrane of the mitochondria of eukaryotic cells). It attaches to the iron within this protein. The binding of cyanide to this cytochrome prevents transport of electrons from cytochrome c oxidase to oxygen. As a result, the electron transport chain is disrupted, meaning that the cell can no longer aerobically produce ATP for energy. Tissues that depend highly on aerobic respiration, such as the central nervous system and the heart, are particularly affected. This is an example of histotoxic hypoxia.

The most hazardous compound is hydrogen cyanide, which, because it is a gas at ambient temperatures and pressure, can be inhaled. A supplied air respirator must be worn when working with hydrogen cyanide. Hydrogen cyanide is produced when a solution containing a labile cyanide is acidified because HCN is a weak acid; alkaline solutions are safer to use because they do not evolve hydrogen cyanide. Hydrogen cyanide may be produced in the combustion of polyurethanes; for this reason, polyurethanes are not recommended for use in domestic and aircraft furniture. Oral ingestion of a small quantity — typically 200 mg — of solid cyanide or cyanide solution, and airborne exposure of 270 ppm may lead to death within minutes.^[17]

Organic nitriles, which do not readily release cyanide ions, have low toxicities. Compounds such as trimethylsilyl cyanide (CH₃)₃SiCN readily release HCN or the cyanide ion upon contact with water.

Antidote

Hydroxocobalamin reacts with cyanide to form cyanocobalamin, which can be eliminated by the kidneys. This method has the advantage of avoiding the formation of methemoglobin (see below). This antidote kit is sold under the brand name Cyanokit and was approved by the FDA in 2006.^[18]

An older cyanide antidote kit included administration of three substances: amyl nitrite pearls (inhalation) and sodium nitrite and sodium thiosulfate (infusion). The goal of the antidote is to generate a large pool of ferric iron to compete with cytochrome a₃ (part of the electron transport chain necessary for cellular respiration/energy production) for cyanide. The nitrites oxidize hemoglobin to methemoglobin, which competes with cytochrome oxidase for the cyanide ion. Cyanmethemoglobin is formed and cytochrome oxidase is restored. The major mechanism to remove the cyanide from the body is by enzymatic conversion by the mitochondrial enzyme rhodanese to convert cyanate to thiocyanate, which is a relatively non-toxic molecule that is excreted in the urine. To accelerate the detoxification, sodium thiosulfate is administered to provide a sulfur donor for rhodanese to produce thiocyanate.

Sensitivity

Minimum risk levels (MRLs) may not protect for delayed health effects or health effects acquired following repeated sublethal exposure, such as hypersensitivity, asthma, or bronchitis. MRLs may be revised after sufficient data accumulates (Toxicological Profile for Cyanide, U.S. Department of Health and Human Services, 2006).

Applications

Mining

Cyanide is mainly produced for the mining of gold and silver: It helps dissolve these metals and their ores. In the cyanide process, finely ground high-grade ore is mixed with the cyanide (concentration of about two kilogram NaCN per tonne); low-grade ores are stacked into heaps and sprayed with a cyanide solution (concentration of about one kilogram NaCN per ton). The precious metals are complexed by the cyanide anions to form soluble derivatives, e.g., [Au(CN)₂]⁻ and [Ag(CN)₂]⁻.^[19]

4 Au + 8 NaCN + O₂ + 2 H₂O → 4 Na[Au(CN)₂] + 4 NaOH

Silver is less "noble" than gold and often occurs as the sulfide, in which case redox is not invoked (no O₂ is required). Instead, a displacement reaction occurs:

Ag₂S + 4 NaCN + H₂O → 2 Na[Ag(CN)₂] + NaSH + NaOH

The "pregnant liquor" containing these ions is separated from the solids, which are discarded to a tailing pond or spent heap, the recoverable gold having been removed. The metal is recovered from the "pregnant solution" by reduction with zinc dust or by adsorption onto activated carbon. This process can result in environmental and health problems. Aqueous cyanide is hydrolyzed rapidly, especially in sunlight. It can mobilize some heavy metals such as mercury if present. Gold can also be associated with arsenopyrite (FeAsS), which is similar to iron pyrite (fool's gold), wherein half of the sulfur atoms are replaced by arsenic. Gold-containing arsenopyrite ores are similarly reactive toward inorganic cyanide.

Cyanide is also used in electroplating, where it stabilizes metal ions in the electrolyte solution prior to their deposition.

Industrial organic chemistry

Some nitriles are produced on a large scale, e.g., adiponitrile is a precursor to nylon. Such compounds are often generated by combining hydrogen cyanide and alkenes, i.e., hydrocyanation: RCH=CH₂ + HCN → RCH(CN)CH₃. Metal catalysts are required for such reactions.

Medical uses

The cyanide compound sodium nitroprusside is used mainly in clinical chemistry to measure urine ketone bodies mainly as a follow-up to diabetic patients. On occasion, it is used in emergency medical situations to produce a rapid decrease in blood pressure in humans; it is also used as a vasodilator in vascular research. The cobalt in artificial vitamin B₁₂ contains a cyanide ligand as an artifact of the purification process; this must be removed by the body before the vitamin molecule can be activated for biochemical use. During World War I, a copper cyanide compound was briefly used by Japanese physicians for the treatment of tuberculosis and leprosy.^[20]

Fishing

Cyanides are illegally used to capture live fish near coral reefs for the aquarium and seafood markets. The practice is controversial, dangerous, and damaging but is driven by the lucrative exotic fish market.^{[citation needed]}

Pest control

Cyanide is used for pest control in New Zealand particularly for possums, an introduced marsupial that threatens the conservation of native species and spreads tuberculosis amongst cattle. Possums can become bait shy but the use of pellets containing the cyanide reduces bait shyness. Cyanide has been known to kill native birds, including the endangered kiwi.^[21] Cyanide is also effective for controlling the Dama Wallaby, another introduced marsupial pest in New Zealand.^[22] A licence is required to store, handle and use cyanide in New Zealand.

Niche uses

Potassium ferrocyanide is used to achieve a blue color on cast bronze sculptures during the final finishing stage of the sculpture. On its own, it will produce a very dark shade of blue and is often mixed with other chemicals to achieve the desired tint and hue. It is applied using a torch and paint brush while wearing the standard safety equipment used for any patina application: rubber gloves, safety glasses, and a respirator. The actual amount of cyanide in the mixture varies according to the recipes used by each foundry.

Cyanide is also used in jewelry-making and certain kinds of photography.

Cyanides are used as insecticides for fumigating ships. Cyanide salts are used for killing ants, and have in some places been used as rat poison (the less toxic poison arsenic is more common^{[citation needed]}).

Although usually thought to be toxic, cyanide and cyanohydrins have been demonstrated to increase germination in various plant species.^[23][24]

Human poisoning

Deliberate cyanide poisoning of humans has occurred many times throughout history.^[25] For notable cyanide deaths, see Cyanide poisoning: Historical cases.

Most significantly, hydrogen cyanide released from pellets of Zyklon-B was used extensively in the systematic mass murders of the Holocaust, especially in extermination camps. Poisoning by hydrogen cyanide gas within a gas chamber (as a salt of hydrocyanic acid is dropped into a strong acid, usually sulfuric acid) is one method of executing a condemned prisoner as the condemned prisoner eventually breathes the lethal fumes.

Food additive

Due to the high stability of their complexation with iron, ferrocyanides (Sodium ferrocyanide E535, Potassium ferrocyanide E536, and Calcium ferrocyanide E538^[26]) do not decompose to lethal levels in the human body and are used in the food industry as, e.g., an anticaking agent in table salt.^[27]

Chemical tests for cyanide

Prussian blue

Iron(II) sulfate is added to a solution suspected of containing cyanide, such as the filtrate from the sodium fusion test. The resulting mixture is acidified with mineral acid. The formation of Prussian blue is a positive result for cyanide.

para-Benzoquinone in DMSO

A solution of para-benzoquinone in DMSO reacts with inorganic cyanide to form a cyanophenol, which is fluorescent. Illumination with a UV light gives a green/blue glow if the test is positive.^[28]

Copper and an aromatic amine

As used by fumigators to detect hydrogen cyanide, copper(II) salt and an aromatic amine such as benzidine is added to the sample; as an alternative to benzidine an alternative amine di-(4,4-bis-dimethylaminophenyl) methane can be used. A positive test gives a blue color. Copper(I) cyanide is poorly soluble. By sequestering^{[disambiguation needed ]} the copper(I) the copper(II) is rendered a stronger oxidant. The copper, in a cyanide facilitated oxidation, converts the amine into a colored compound. The Nernst equation explains this process. Another good example of such chemistry is the way in which the saturated calomel reference electrode (SCE) works. The copper, in a cyanide-facilitated oxidation, converts the amine into a colored compound.

Pyridine-barbituric acid colorimetry

A sample containing inorganic cyanide is purged with air from a boiling acid solution into a basic absorber solution. The cyanide salt absorbed in the basic solution is buffered at pH 4.5 and then reacted with chlorine to form cyanogen chloride. The cyanogen chloride formed couples pyridine with barbituric acid to form a strongly colored red dye that is proportional to the cyanide concentration. This colorimetric method following distillation is the basis for most regulatory methods (for instance EPA 335.4) used to analyze cyanide in water, wastewater, and contaminated soils. Distillation followed by colorimetric methods, however, have been found to be prone to interferences from thiocyanate, nitrate, thiosulfate, sulfite, and sulfide that can result in both positive and negative bias. It has been recommended by the USEPA (MUR March 12, 2007) that samples containing these compounds be analyzed by Gas-Diffusion Flow Injection Analysis — Amperometry.^{[citation needed]}

Gas diffusion flow injection analysis — amperometry

Instead of distilling, the sample is injected into an acidic stream where the HCN formed is passed under a hydrophobic gas diffusion membrane that selectively allows only HCN to pass through. The HCN that passes through the membrane is absorbed into a basic carrier solution that transports the CN to an amperometric detector that accurately measures cyanide concentration with high sensitivity. Sample pretreatment determined by acid reagents, ligands, or preliminary UV irradiation allow cyanide speciation of free cyanide, available cyanide, and total cyanide respectively. These relative simplicity of these flow injection analysis methods limit the interference experienced by the high heat of distillation and also prove to be cost effective since time consuming distillations are not required.

See also

• Cyanogen

References

External links

• ATSDR medical management guidelines for cyanide poisoning (US)
• HSE recommendations for first aid treatment of cyanide poisoning (UK)
• Hydrogen cyanide and cyanides (CICAD 61)
• IPCS/CEC Evaluation of antidotes for poisoning by cyanides
• National Pollutant Inventory – Cyanide compounds fact sheet
• Eating apple seeds is safe despite the small amount of cyanide
• Toxicological Profile for Cyanide, U.S. Department of Health and Human Services, July 2006

Safety data (French):

• 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)
• Institut national de recherche et de sécurité (1997). "Cyanure de sodium. Cyanure de potassium". Fiche toxicologique n° 111, Paris:INRS, 6pp. (PDF file, in French)