Cyanide

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Contents

Cyanide anion
Cyanide-montage.png
Names
Preferred IUPAC name
Cyanide
Systematic IUPAC name
Nitridocarbonate(II)
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
PubChem CID
UNII
  • InChI=1S/CN/c1-2/q-1
    Key: XFXPMWWXUTWYJX-UHFFFAOYSA-N
  • [C-]#N
Properties
CN
Molar mass 26.018 g·mol−1
Conjugate acid Hydrogen cyanide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. [1]

In chemistry, a cyanide (from Greek kyanos ' dark blue ') is a chemical compound that contains a C≡N functional group. This group, known as the cyano group, consists of a carbon atom triple-bonded to a nitrogen atom. [2]

In inorganic cyanides, the cyanide group is present as the anion −C≡N. Soluble salts such as sodium cyanide (NaCN) and potassium cyanide (KCN) are highly toxic. [3] Hydrocyanic acid, also known as hydrogen cyanide, or HCN, is a highly volatile liquid that is produced on a large scale industrially. It is obtained by acidification of cyanide salts.

Organic cyanides are usually called nitriles. In nitriles, the C≡N group is linked by a covalent bond to carbon. For example, in acetonitrile (CH3−C≡N), the cyanide group is bonded to methyl (−CH3). Although nitriles generally do not release cyanide ions, the cyanohydrins do and are thus rather toxic.

Bonding

The cyanide ion C≡N is isoelectronic with carbon monoxide C≡O+ and with molecular nitrogen N≡N. A triple bond exists between C and N. The negative charge is concentrated on carbon C. [4] [5]

Occurrence

In nature

Removal of cyanide from cassava in Nigeria. Removal of cyanide poison from cassava.jpg
Removal of cyanide from cassava in Nigeria.

Cyanides are produced by certain bacteria, fungi, and algae. It is an antifeedant in a number of plants. Cyanides are found in substantial amounts in certain seeds and fruit stones, e.g., those of bitter almonds, apricots, apples, and peaches. [6] Chemical compounds that can release cyanide are known as cyanogenic compounds. 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. [7] [8]

The Madagascar bamboo Cathariostachys madagascariensis produces cyanide as a deterrent to grazing. In response, the golden bamboo lemur, which eats the bamboo, has developed a high tolerance to cyanide.

Interstellar medium

The cyanide radical •CN has been identified in interstellar space. [9] Cyanogen, (CN)2, is used to measure the temperature of interstellar gas clouds. [10]

Pyrolysis and combustion product

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. [11]

Cofactor

The hydrogenase enzymes contain cyanide ligands attached to iron in their active sites. The biosynthesis of cyanide in the [NiFe]-hydrogenases proceeds from carbamoyl phosphate, which converts to cysteinyl thiocyanate, the CN donor. [12]

Organic derivatives

In IUPAC nomenclature, organic compounds that have a –C≡N functional group are called nitriles. [13] [14] An example of a nitrile is acetonitrile, CH3−C≡N. Nitriles usually do not release cyanide ions. A functional group with a hydroxyl –OH and cyanide –CN bonded to the same carbon atom is called cyanohydrin (C(–OH)(–CN)). Unlike nitriles, cyanohydridins do release hydrogen cyanide. In inorganic chemistry, salts containing the C≡N ion are referred to as cyanides.

Reactions

Protonation

Cyanide is basic. The pKa of hydrogen cyanide is 9.21. Thus, addition of acids stronger than hydrogen cyanide to solutions of cyanide salts releases hydrogen cyanide.

Hydrolysis

Cyanide is unstable in water, but the reaction is slow until about 170 °C. It undergoes hydrolysis to give ammonia and formate, which are far less toxic than cyanide: [15]

Cyanide hydrolase is an enzyme that catalyzes this reaction.

Alkylation

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. 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. [16]

Redox

The cyanide ion is a reductant and is oxidized by strong oxidizing agents such as molecular chlorine (Cl2), hypochlorite (ClO), and hydrogen peroxide (H2O2). These oxidizers are used to destroy cyanides in effluents from gold mining. [17] [18] [19]

Metal complexation

The cyanide anion reacts with transition metals to form M-CN bonds. This reaction is the basis of cyanide's toxicity. [20] The high affinities of metals for this anion can be attributed to its negative charge, compactness, and ability to engage in π-bonding.

Among the most important cyanide coordination compounds are the potassium ferrocyanide and the pigment Prussian blue, which are both essentially nontoxic due to the tight binding of the cyanides to a central iron atom. [21] Prussian blue was first accidentally made around 1706, by heating substances containing iron and carbon and nitrogen, and other cyanides made subsequently (and named after it). Among its many uses, Prussian blue gives the blue color to blueprints, bluing, and cyanotypes.

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. [22] [23]

Sodium cyanide, the precursor to most cyanides, is produced by treating hydrogen cyanide with sodium hydroxide: [15]

Toxicity

Many cyanides are highly toxic. The cyanide anion is an inhibitor of the enzyme cytochrome c oxidase (also known as aa3), the fourth complex of the electron transport chain found in the inner membrane of the mitochondria of eukaryotic cells. It attaches to the iron within this protein. The binding of cyanide to this enzyme prevents transport of electrons from cytochrome c to oxygen. As a result, the electron transport chain is disrupted, meaning that the cell can no longer aerobically produce ATP for energy. [24] 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. [25]

The most hazardous compound is hydrogen cyanide, which is a gas and kills by inhalation. For this reason, an air respirator supplied by an external oxygen source must be worn when working with hydrogen cyanide. [11] Hydrogen cyanide is produced by adding acid to a solution containing a cyanide salt. Alkaline solutions of cyanide are safer to use because they do not evolve hydrogen cyanide gas. 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 of solid cyanide or a cyanide solution of as little as 200 mg, or exposure to airborne cyanide of 270 ppm, is sufficient to cause death within minutes. [25]

Organic nitriles do not readily release cyanide ions, and so have low toxicities. By contrast, compounds such as trimethylsilyl cyanide (CH3)3SiCN readily release HCN or the cyanide ion upon contact with water. [26]

Antidote

Hydroxocobalamin reacts with cyanide to form cyanocobalamin, which can be safely 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 U.S. FDA in 2006. [27]

An older cyanide antidote kit included administration of three substances: amyl nitrite pearls (administered by inhalation), sodium nitrite, and sodium thiosulfate. The goal of the antidote was to generate a large pool of ferric iron (Fe3+) to compete for cyanide with cytochrome a3 (so that cyanide will bind to the antidote rather than the enzyme). The nitrites oxidize hemoglobin to methemoglobin, which competes with cytochrome oxidase for the cyanide ion. Cyanmethemoglobin is formed and the cytochrome oxidase enzyme is restored. The major mechanism to remove the cyanide from the body is by enzymatic conversion to thiocyanate by the mitochondrial enzyme rhodanese. Thiocyanate is a relatively non-toxic molecule and is excreted by the kidneys. To accelerate this detoxification, sodium thiosulfate is administered to provide a sulfur donor for rhodanese, needed in order to produce thiocyanate. [28]

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. [29]

Applications

Mining

Cyanide is mainly produced for the mining of silver and gold: It helps dissolve these metals allowing separation from the other solids. In the cyanide process , finely ground high-grade ore is mixed with the cyanide (at a ratio of about 1:500 parts NaCN to ore); low-grade ores are stacked into heaps and sprayed with a cyanide solution (at a ratio of about 1:1000 parts NaCN to ore). The precious metals are complexed by the cyanide anions to form soluble derivatives, e.g., [Ag(CN)2] (dicyanoargentate(I)) and [Au(CN)2] (dicyanoaurate(I)). [15] 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:

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. A number of environmental disasters have followed the overflow of tailing ponds at gold mines. Cyanide contamination of waterways has resulting in numerous cases of human and aquatic species mortality. [30]

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.[ citation needed ]

Industrial organic chemistry

The second major application of alkali metal cyanides (after mining) is in the production of CN-containing compounds, usually nitriles. Acyl cyanides are produced from acyl chlorides and cyanide. Cyanogen, cyanogen chloride, and the trimer cyanuric chloride are derived from alkali metal cyanides.

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. [31]

Illegal fishing and poaching

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. [32]

Poachers in Africa have been known to use cyanide to poison waterholes, to kill elephants for their ivory. [33]

Pest control

M44 cyanide devices are used in the United States to kill coyotes and other canids. [34] Cyanide is also 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. [35] Cyanide is also effective for controlling the dama wallaby, another introduced marsupial pest in New Zealand. [36] A licence is required to store, handle and use cyanide in New Zealand.

Cyanides are used as insecticides for fumigating ships. [37] Cyanide salts are used for killing ants, [38] and have in some places been used as rat poison [39] (the less toxic poison arsenic is more common). [40]

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 such as sepia toning.

Although usually thought to be toxic, cyanide and cyanohydrins increase germination in various plant species. [41] [42]

Human poisoning

Deliberate cyanide poisoning of humans has occurred many times throughout history. [43] Common salts such as sodium cyanide are involatile but water-soluble, so are poisonous by ingestion. Hydrogen cyanide is a gas, making it more indiscriminately dangerous, however it is lighter than air and rapidly disperses up into the atmosphere, which makes it ineffective as a chemical weapon. Poisoning by hydrogen cyanide is more effective in an enclosed space, such as a gas chamber. Most significantly, hydrogen cyanide released from pellets of Zyklon-B was used extensively in the extermination camps of the Holocaust.

Food additive

Because of the high stability of their complexation with iron, ferrocyanides (Sodium ferrocyanide E535, Potassium ferrocyanide E536, and Calcium ferrocyanide E538 [44] ) 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. [45]

Chemical tests for cyanide

Cyanide is quantified by potentiometric titration, a method widely used in gold mining. It can also be determined by titration with silver ion. Some analyses begin with an air-purge of an acidified boiling solution, sweeping the vapors into a basic absorber solution. The cyanide salt absorbed in the basic solution is then analyzed. [46]

Qualitative tests

Because of the notorious toxicity of cyanide, many methods have been investigated. Benzidine gives a blue coloration in the presence of ferricyanide. [47] Iron(II) sulfate added to a solution of cyanide, such as the filtrate from the sodium fusion test, gives prussian blue. 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. [48]

Related Research Articles

Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure H−C≡N. It is a colorless, extremely poisonous, 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.

<span class="mw-page-title-main">Potassium ferrocyanide</span> Chemical compound

Potassium ferrocyanide is the inorganic compound with formula K4[Fe(CN)6]·3H2O. It is the potassium salt of the coordination complex [Fe(CN)6]4−. This salt forms lemon-yellow monoclinic crystals.

<span class="mw-page-title-main">Sodium cyanide</span> Chemical compound

Sodium cyanide is a poisonous compound with the formula NaCN. It is a white, water-soluble solid. Cyanide has a high affinity for metals, which leads to the high toxicity of this salt. Its main application, in gold mining, also exploits its high reactivity toward metals. It is a moderately strong base.

<span class="mw-page-title-main">Prussian blue</span> Synthetic pigment

Prussian blue is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. It has the chemical formula FeIII
4
[FeII
(CN)
6
]
3
. Turnbull's blue is chemically identical, but is made from different reagents, and its slightly different color stems from different impurities and particle sizes.

<span class="mw-page-title-main">Potassium ferricyanide</span> Chemical compound

Potassium ferricyanide is the chemical compound with the formula K3[Fe(CN)6]. This bright red salt contains the octahedrally coordinated [Fe(CN)6]3− ion. It is soluble in water and its solution shows some green-yellow fluorescence. It was discovered in 1822 by Leopold Gmelin.

<span class="mw-page-title-main">Cyanohydrin</span> Functional group in organic chemistry

In organic chemistry, a cyanohydrin or hydroxynitrile is a functional group found in organic compounds in which a cyano and a hydroxy group are attached to the same carbon atom. The general formula is R2C(OH)CN, where R is H, alkyl, or aryl. Cyanohydrins are industrially important precursors to carboxylic acids and some amino acids. Cyanohydrins can be formed by the cyanohydrin reaction, which involves treating a ketone or an aldehyde with hydrogen cyanide (HCN) in the presence of excess amounts of sodium cyanide (NaCN) as a catalyst:

In organic chemistry, a nitrile is any organic compound that has a −C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, including methyl cyanoacrylate, used in super glue, and nitrile rubber, a nitrile-containing polymer used in latex-free laboratory and medical gloves. Nitrile rubber is also widely used as automotive and other seals since it is resistant to fuels and oils. Organic compounds containing multiple nitrile groups are known as cyanocarbons.

<span class="mw-page-title-main">Potassium cyanide</span> Chemical compound

Potassium cyanide is a compound with the formula KCN. This colorless crystalline salt, similar in appearance to sugar, is highly soluble in water. Most KCN is used in gold mining, organic synthesis, and electroplating. Smaller applications include jewellery for chemical gilding and buffing.

<span class="mw-page-title-main">Cyanate</span> Anion with formula OCN and charge –1

Cyanate is an anion with the structural formula [O=C=N], usually written OCN. It also refers to any salt containing it, such as ammonium cyanate.

<span class="mw-page-title-main">Ferricyanide</span> Anion in which a Fe3+ ion is complexed by 6 CN− ions

Ferricyanide is the anion [Fe(CN)6]3−. It is also called hexacyanoferrate(III) and in rare, but systematic nomenclature, hexacyanidoferrate(III). The most common salt of this anion is potassium ferricyanide, a red crystalline material that is used as an oxidant in organic chemistry.

<span class="mw-page-title-main">Ferrocyanide</span> Anion in which a Fe2+ ion is complexed by 6 CN− ions

Ferrocyanide is the name of the anion [Fe(CN)6]4−. Salts of this coordination complex give yellow solutions. It is usually available as the salt potassium ferrocyanide, which has the formula K4Fe(CN)6. [Fe(CN)6]4− is a diamagnetic species, featuring low-spin iron(II) center in an octahedral ligand environment. Although many salts of cyanide are highly toxic, ferro- and ferricyanides are less toxic because they tend not to release free cyanide. It is of commercial interest as a precursor to the pigment Prussian blue and, as its potassium salt, an anticaking agent.

<span class="mw-page-title-main">Copper(I) cyanide</span> Chemical compound

Copper(I) cyanide is an inorganic compound with the formula CuCN. This off-white solid occurs in two polymorphs; impure samples can be green due to the presence of Cu(II) impurities. The compound is useful as a catalyst, in electroplating copper, and as a reagent in the preparation of nitriles.

Cyanogen bromide is the inorganic compound with the formula (CN)Br or BrCN. It is a colorless solid that is widely used to modify biopolymers, fragment proteins and peptides, and synthesize other compounds. The compound is classified as a pseudohalogen.

<span class="mw-page-title-main">Thiosulfate</span> Polyatomic ion (S2O3, charge –2)

Thiosulfate is an oxyanion of sulfur with the chemical formula S2O2−3. Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, e.g. sodium thiosulfate Na2S2O3. Thiosulfate also refers to the esters of thiosulfuric acid, e.g. O,S-dimethyl thiosulfateCH3−O−S(=O)2−S−CH3. The prefix thio- indicates that the thiosulfate is a sulfate with one oxygen replaced by sulfur. Thiosulfate is tetrahedral at the central S atom. Thiosulfate salts occur naturally. Thiosulfate ion has C3v symmetry, and is produced by certain biochemical processes. It rapidly dechlorinates water and is notable for its use to halt bleaching in the paper-making industry. Thiosulfate salts are mainly used in dying in textiles and the bleaching of natural substances.

<span class="mw-page-title-main">Trimethylsilyl cyanide</span> Chemical compound

Trimethylsilyl cyanide is the chemical compound with the formula (CH3)3SiCN. This volatile liquid consists of a cyanide group, that is CN, attached to a trimethylsilyl group. The molecule is used in organic synthesis as the equivalent of hydrogen cyanide. It is prepared by the reaction of lithium cyanide and trimethylsilyl chloride:

<span class="mw-page-title-main">Sodium ferrocyanide</span> Chemical compound

Sodium ferrocyanide is the sodium salt of the coordination compound of formula [Fe(CN)6]4−. In its hydrous form, Na4Fe(CN)6 · 10 H2O (sodium ferrocyanide decahydrate), it is sometimes known as yellow prussiate of soda. It is a yellow crystalline solid that is soluble in water and insoluble in alcohol. The yellow color is the color of ferrocyanide anion. Despite the presence of the cyanide ligands, sodium ferrocyanide has low toxicity (acceptable daily intake 0–0.025 mg/kg body weight). The ferrocyanides are less toxic than many salts of cyanide, because they tend not to release free cyanide. However, like all ferrocyanide salt solutions, addition of an acid can result in the production of hydrogen cyanide gas, which is extremely toxic.

<span class="mw-page-title-main">Ammonium cyanide</span> Chemical compound

Ammonium cyanide is an unstable inorganic compound with the formula NH4CN.

Cyanometallates or cyanometalates are a class of coordination compounds, most often consisting only of cyanide ligands. Most are anions. Cyanide is a highly basic and small ligand, hence it readily saturates the coordination sphere of metal ions. The resulting cyanometallate anions are often used as ligands for building more complex structures called coordination polymers, the best known example of which is Prussian blue, a common dyestuff.

<span class="mw-page-title-main">Calcium cyanide</span> Chemical compound

Calcium cyanide is the inorganic compound with the formula Ca(CN)2. It is the calcium salt derived from hydrocyanic acid. It is a white solid, although the pure material is rarely encountered. It hydrolyses readily (even in moist air) to release hydrogen cyanide and is very toxic.

Potassium dicyanoaurate is an inorganic compound with formula K[Au(CN)2]. It is a colorless to white solid that is soluble in water and slightly soluble in alcohol. The salt itself is often not isolated, but solutions of the dicyanoaurate ion ([Au(CN)2]) are generated on a large scale in the extraction of gold from its ores.

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  37. "Sodium Cyanide". PubChem. National Center for Biotechnology Information. 2016. Retrieved 2 September 2016. Cyanide and hydrogen cyanide are used in electroplating, metallurgy, organic chemicals production, photographic developing, manufacture of plastics, fumigation of ships, and some mining processes.
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