Cyanide

In chemistry, cyanide is an inorganic chemical compound that contains a functional group. This group, known as the cyano group, consists of a carbon atom triple-bonded to a nitrogen atom.
Ionic cyanides contain the cyanide anion. This anion is extremely toxic and causes cyanide poisoning. Soluble cyanide salts such as sodium cyanide, potassium cyanide and tetraethylammonium cyanide are also highly toxic.
Covalent cyanides contain the group, and are usually called nitriles if the group is linked by a single covalent bond to carbon atom. For example, in acetonitrile, the cyanide group is bonded to methyl. In tetracyanomethane, four cyano groups are bonded to carbon. Although nitriles generally do not release cyanide ions, the cyanohydrins do and are thus toxic. The cyano group may be covalently bonded to atoms other than carbon, e.g., in cyanogen azide, phosphorus tricyanide and trimethylsilyl cyanide.
Hydrogen cyanide, or, is a highly volatile toxic liquid that is produced on a large scale industrially. It is obtained by acidification of cyanide salts.

Bonding

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

Occurrence

In nature

Cyanides are produced by certain insects, 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. 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, an important potato-like food grown in tropical countries, also contain cyanogenic glycosides.
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.
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 donor.

Interstellar medium

The cyanide radical ^•CN has been identified in interstellar space. Cyanogen,, is used to measure the temperature of interstellar gas clouds.

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.

Organic derivatives

In IUPAC nomenclature, organic compounds that have a functional group are called nitriles. An example of a nitrile is acetonitrile,. Nitriles usually do not release cyanide ions. A functional group with a hydroxyl and cyanide bonded to the same carbon atom is called cyanohydrin. Unlike nitriles, cyanohydrins do release poisonous hydrogen cyanide.

Reactions

Protonation

Cyanide is basic. The pK_a 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:
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. 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.

Redox

The cyanide ion is a reductant and is oxidized by strong oxidizing agents such as molecular chlorine, hypochlorite, and hydrogen peroxide. These oxidizers are used to destroy cyanides in effluents from gold mining.

Metal complexation

The cyanide anion reacts with transition metals to form M-CN bonds. This reaction is the basis of cyanide's toxicity. 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.
Prussian blue was first accidentally made around 1706, by heating substances containing iron and carbon and nitrogen, and other cyanides made subsequently. Among its many uses, Prussian blue gives the blue color to blueprints, bluing, and cyanotypes.

Production

The principal process used to make 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.
Sodium cyanide, the precursor to most cyanides, is produced by treating hydrogen cyanide with sodium hydroxide:

Toxicity

Among the most toxic cyanides are hydrogen cyanide, sodium cyanide, potassium cyanide, and calcium cyanide. The cyanide anion is an inhibitor of the enzyme cytochrome c oxidase, 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. 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.
Hydrogen cyanide, which is a gas, kills by inhalation. For this reason, working with hydrogen cyanide requires wearing an air respirator supplied by an external oxygen source. Hydrogen cyanide can be 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. 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.
Organic nitriles do not readily release cyanide ions, and so have low toxicities.

Disposal

Due to toxicity considerations, the disposal of cyanide is subject to stringent regulations. Industrial cyanide effluent is typically destroyed by oxidation using peroxysulfuric acid, hydrogen peroxide, sulfur dioxide/copper salts or all three. Use of sodium hypochlorite, traditional for laboratory-scale wastes, is impractical on a commercial scale. Hydrolysis at higher temperatures is highly effective, but requires specialized equipment. Lastly, cyanide wastes can be acidified for recovery of hydrogen cyanide.

Antidote

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. This antidote kit is sold under the brand name Cyanokit and was approved by the U.S. FDA in 2006.
An older cyanide antidote kit included administration of three substances: amyl nitrite pearls, sodium nitrite, and sodium thiosulfate. The goal of the antidote was to generate a large pool of ferric iron to compete for cyanide with cytochrome a₃. 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.

Sensitivity

Minimum risk levels 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.

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 ; low-grade ores are stacked into heaps and sprayed with a cyanide solution. The precious metals are complexed by the cyanide anions to form soluble derivatives, e.g., and . Silver is less "noble" than gold and often occurs as the sulfide, in which case redox is not invoked. 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 resulted in numerous cases of human and aquatic species mortality.
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, which is similar to iron pyrite, wherein half of the sulfur atoms are replaced by arsenic. Gold-containing arsenopyrite ores are similarly reactive toward inorganic cyanide.