Cyanide

A cyanide is any chemical compound that contains the cyano group -C≡N, with the carbon atom triple-bonded to the nitrogen atom.

The CN group can be found in many kinds of compounds. Some are gases, others are solids or liquids. Some are salt-like, some covalent. Some are molecular, some ionic, and many are polymeric. Those that can release the cyanide ion CN⁻ are highly toxic.

Appearance and odour

Hydrogen cyanide is a colourless gas with a faint, bitter, almond-like odour. Nearly 40 percent of the population is unable to smell hydrogen cyanide. This seems to be genetically determined in a complex fashion^*. Sodium cyanide and potassium cyanide are both white powders with a bitter, almond-like odour in damp air, due to the presence of HCN formed by hydrolysis:

NaCN + H₂O → HCN + NaOH

Occurrence and uses

In nature

Cyanides can be produced by certain bacteria, fungi, and algae, and are found in a number of foods and plants. In plants, cyanides are usually bound to sugar molecules in the form of cyanogenic glycosides and serve the plant as defense against herbivores. Cassava roots (aka manioc), an important potato-like food grown in tropical

The Fe-only and href="http://articles.gourt.com/en/hydrogenase">hydrogenase enzymes contain cyanide ligands at their active sites. The biosynthesis of cyanide in the [NiFe-hydrogenases proceeds from carbamoylphosphate, which converts to cysteinyl thiocyanate, the CN^- donor. Reissmann, S.; Hochleitner, E.; Wang, H.; Paschos, A.; Lottspeich, F.; Glass, R. S. and Böck, A., "Taming of a Poison: Biosynthesis of the NiFe-Hydrogenase Cyanide Ligands", Science, 2003, 299, 1067-70.

Chemistry

Cyanide is a versatile and invaluable anion that is widely encountered in all fields of chemistry.

Coordination chemistry

Cyanide is considered, in a broad sense, to be the most potent ligand for many transition metals. The very high affinities of metals for cyanide can be attributed to its negative charge, compactness, and ability to engage in π-bonding. Well known complexes include:

hexacyanides ^*³⁻ (M = Ti, V, Cr, Mn, Fe, Co), which are octahedral in geometry;
the tetracyanides, ^*²⁻ (M = Ni, Pd, Pt), which are square planar in geometry;
the dicyanides ^*⁻ (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 acids.

Organic synthesis (see nitriles)

Because of its high nucleophilicity, cyanide is readily introduced into organic molecules by displacement of the corresponding organic halide. Organic cyanides are generally called nitriles. Thus, CH₃CN can be methyl cyanide but more commonly is referred to as acetonitrile. In organic synthesis, cyanide is used to lengthen a carbon chain, concomitant with the introduction of other functionality:

RX + CN⁻ → RCN + X⁻ (Nucleophilic Substitution) followed by

RCN + 2 H₂O → RCOOH + NH₃ (Hydrolysis), or
RCN + 0.5 LiAlH₄ + (second step) 2 H₂O → RCH₂NH₂ + 0.5 LiAl(OH)₄ (under reflux in dry ether, followed by addition of H₂O)

An alternative method for introducing cyanide is via the process of hydrocyanation, whereby hydrogen cyanide and alkenes combine: RCH=CH₂ + HCN → RCH(CN)CH₃ Metal catalysts are required for such reactions.

Other occurrences and uses

Hydrogen cyanide is a product of combustion, including the exhaust of internal combustion engines, tobacco smoke, and especially some plastics derived from acrylonitrile (because of the latter effect, house fires can result in poisonings of the inhabitants.)

Potassium ferrocyanide is used to achieve a blue colour 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.

Mining

Gold and silver cyanides are among the very few soluble forms of these metals, and cyanides are thus used in mining as well as electroplating, metallurgy, jewelry, and photography. In the so-called cyanide process, finely ground high-grade ore is mixed with the cyanide solution (concentration of about two kilogram NaCN per tonne); low-grade ores are stacked into heaps and sprayed with cyanide solution (concentration of about one kilogram NaCN per ton). The precious-metal cations are complexed by the cyanide anions to form soluble derivatives, e.g. and [Ag(CN)₂⁻.

2Au + 4KCN + ½O₂ + H₂O → 2K^* + 2KOH

2Ag + 4KCN + ½O₂ + H₂O → 2K^* + 2KOH

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 + 4KCN → 2K^* + K₂S

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 absorption 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. Au-containing arsenopyrite ores are similarly reactive toward cyanide.

Fishing

Cyanides are illegally used to capture live fish near coral reefs for the aquarium and seafood markets. This fishing occurs mainly in the Philippines, Indonesia and the Caribbean to supply the 2 million marine aquarium owners in the world. In this method, a diver uses a large, needleless syringe to squirt a cyanide solution into areas where the fish are hiding, stunning them so that they can be easily gathered. Many fish caught in this fashion die immediately, or in shipping. Those that survive to find their way into pet stores often die from shock, or from massive digestive damage. The high concentrations of cyanide on reefs so harvested has also resulted in cases of cyanide poisoning among local fishermen and their families.

Environmental organizations decry the practice, as do responsible aquarists and aquarium dealers.

To prevent the trade of illegally-caught aquarium fish, the Marine Aquarium Council (Headquarters: Honolulu, Hawaii) has created a certification in which the tropical fish are caught legally with nets only. To ensure authenticity, "MAC-Certified marine organisms bear the MAC-Certified label on the tanks and boxes in which they are kept and shipped." MAC Certification.

Miscellany

Cyanides are used as insecticides for fumigating ships. In the past cyanide salts have also been used as rat poison.

Toxicity

"Cyanide" is a staple of crime fiction and publicly regarded as meaning deadly poison. Many cyanide-containing compounds are indeed highly toxic, but many are not. Prussian blue, nominally Fe₇(CN)₁₈, a common pigment, is administered orally to counteract the effects of poisoning by Thallium and ¹³⁷Cs.

The most dangerous cyanides are hydrogen cyanide (HCN) and salts derived from it, such as potassium cyanide (KCN) and sodium cyanide (NaCN), but including others. Also some compounds readily release HCN or the cyanide ion, such as trimethylsilyl cyanide (CH₃)₃SiCN upon contact with water and cyanoacrylates upon pyrolysis.

Many thousands of organic compounds contain the CN group. These compounds are called nitriles. Generally, nitriles do not display the toxicity of HCN, NaCN, and KCN. In fact, the nitrile functional group is an integral component of numerous pharmaceutical drugs including cimetidine (Tagamet), verapamil (Isoptin), and citalopram (celexa). The reason for their diminished toxicity is that nitriles do not release the CN⁻ ion, which permanently binds to and inhibits cytochrome c oxidase, the specific basis of the lethality of cyanide (see below).

Absorption

The most usual route of absorption is by inhalation of hydrogen cyanide gas, which can be formed from alkaline cyanides and certain complex cyanides by the action of acid. Hydrogen cyanide poisoning is also common as a result of smoke inhalation after house fires.

Ingestion is equally dangerous, although this route of absorption is usually deliberate (suicidal or criminal). Absorption through the skin is also possible, though rare.

Mechanism of toxicity

Cyanide ions bind to the iron atom of the enzyme cytochrome c oxidase in the mitochondria of cells. This deactivates the enzyme and breaks the electron transport chain, meaning that the cell can no longer use the oxygen that is available to it.

Tissues that mainly depend on aerobic respiration, such as the central nervous system and the heart, are particularly affected.

Plants contain a cyanide-insensitive pathway (photosynthesis instead of the redox reaction) for respiration in their mitochondria, and as a result are insensitive to concentrations of cyanide that are lethal to animals.

Clinical symptoms

It is difficult to give dose figures in this section due to the rapid metabolism of cyanide in the human body. Animal studies are of little help, as different species have widely different sensitivities to cyanide: it is quite possible that there is also a considerable range of sensitivity among human individuals. The Regulatory information section below may give some guidance.

Acute poisoning

Inhalation of high concentrations of hydrogen cyanide causes a coma with seizures, apnea and cardiac arrest, with death following in a matter of minutes.

At lower doses, loss of consciousness may be preceded by general weakness, giddiness, headaches, vertigo, confusion, and perceived difficulty in breathing. At the first stages of unconsciousness, breathing is often sufficient or even rapid, although the state of the victim progresses towards a deep coma, sometimes accompanied by pulmonary edema, and finally cardiac arrest. Skin colour goes pink from high blood oxygen saturation.

Subacute poisoning

At doses insufficient to cause loss of consciousness, the symptoms can also include faintness, drowsiness, anxiety and excitement. Dizziness, nausea, vomiting and sweating are common.

The situation is complicated by the non-specific nature of the symptoms and by notoriety of the product. In some cases, such symptoms are psychosomatic, caused by anxiety at working with cyanides, and this is accentuated by the characteristic odour of hydrogen cyanide, detectable by healthy, undesensitised subjects at levels far below those which are believed to be toxic (odour threshold < 1 ppm). This is not to say that such symptoms should be taken lightly: if the patient is truly a victim of cyanide poisoning, their clinical state may deteriorate rapidly; while if the symptoms are psychosomatic, they will surely recur unless the anxieties about the safety procedures are addressed.

Chronic exposure

Exposure to lower levels of cyanide over a long period (e.g., after use of cassava roots as a primary food source in tropical Africa) results in increased blood cyanide levels. These may result in weakness of the fingers and toes, difficulty walking, dimness of vision, deafness, and decreased thyroid gland function, but chemicals other than cyanide may contribute to these effects. Skin contact with cyanide can produce irritation and sores.

It is not known whether cyanides can directly cause birth defects in people. Birth defects were seen in rats that ate diets of cassava roots. Effects on the reproductive system were seen in rats and mice that drank water containing sodium cyanide.

Diagnosis of poisoning

There are medical tests to measure blood and urine levels of cyanide; however, small amounts of cyanide are not always detectable in blood and urine. Tissue levels of cyanide can be measured if cyanide poisoning is suspected, but cyanide is rapidly cleared from the body, so the tests must be done soon after the exposure. An almond-like odour in the breath may alert a doctor that a person was exposed to cyanide but not all people are able to smell HCN.

Treatment of poisoning and antidotes

The United States standard cyanide antidote kit first uses a small inhaled dose of amyl nitrite followed by intravenous sodium nitrite. Nitrite oxidizes a portion of the hemoglobin's iron from the ferrous state to the ferric state, converting the hemoglobin into methemoglobin. Cyanide preferentially bonds to methemoglobin rather than the cytochrome oxidase. Cyanide converts methemoglobin into cyanmethemoglobin. Therapy with nitrites is not, however, innocuous. The doses given to an adult can cause a fatal methemoglobinemia in children or may cause profound hypotension. Treatment of children affected with cyanide intoxication must be individualized and is based upon their body weight and hemoglobin concentration. The next part of the cyanide antidote kit is sodium thiosulfate, which is administered intravenously. Thiosulfate reacts with the cyanmethemoglobin to give thiocyanate (and sulfite) and hemoglobin. The thiocyanate is excreted.

Alternative methods of treating cyanide intoxication are used in other countries. For example, in France hydroxycobalamin (a form of vitamin B₁₂) is used to bind cyanide to form the harmless vitamin B_12a cyanocobalamin. Cyanocobalamin is eliminated through the urine. Hydroxycobalamin works both within the intravascular space and within the cells to combat cyanide intoxication. This versatility contrasts with methemoglobin, which acts only within the vascular space as an antidote. Administration of sodium thiosulfate improves the ability of the hydroxycobalamin to detoxify cyanide poisoning. This treatment is considered so effective and innocuous that it is administered routinely in Paris to victims of smoke inhalation to detoxify any associated cyanide intoxication. However it is relatively expensive and not universally available.

4-Dimethylaminophenol (4-DMAP) has been proposed in Germany as a more rapid antidote than nitrites with (reportedly) lower toxicity. 4-DMAP is used currently by the German military and by the civilian population. In humans, intravenous injection of 3 mg/kg of 4-DMAP produces 35 percent methemoglobin levels within 1 minute. Reportedly (4-Dimethylaminophenol), 4-DMAP is part of the US Cyanokit, while it is not part of the GERM Cyanokit due to side effects (e. g. hemolysis).

Cobalt salts have also been demonstrated as effective in binding cyanide. One current cobalt-based antidote available in Europe is dicobalt-EDTA, sold as Kelocyanor®. This agent chelates cyanide as the cobalticyanide. This drug provides an antidote effect more quickly than formation of methemoglobin, but a clear superiority to methemoglobin formation has not been demonstrated. Cobalt complexes are quite toxic, and there have been accidents reported in the UK where patients have been given dicobalt-EDTA by mistake based on a false diagnoses of cyanide poisoning.

The International Programme on Chemical Safety issued a survey (IPCS/CEC Evaluation of Antidotes Series) that lists the following antidotal agents and their effects: Oxygen, sodium thiosulfate, amyl nitrite, sodium nitrite, 4-dimethylaminophenol, hydroxocobalamin, and dicobalt edetate ('Kelocyanor'), as well as several others^*. Other commonly-recommended antidotes are 'solutions A and B' (a solution of ferrous sulphate in aqueous citric acid, and aqueous sodium carbonate) and amyl nitrite.

Britain's Health and Safety Executive(HSE) has recommended against the use of solutions A and B because of their limited shelf life, potential to cause iron poisoning, and limited applicability (effective only in cases of cyanide ingestion, whereas the main modes of poisoning are inhalation and skin contact). The HSE has also questioned the usefulness of amyl nitrate due to storage/availability problems, risk of abuse, and lack of evidence of significant benefits, instead recommending Kelocyanor^*.

Glucose

Evidence from animal experiments suggests that coadministration of glucose protects against cobalt toxicity associated with the antidote agent dicobalt edetate. For this reason, glucose is often administered alongside this agent (e.g. in the formulation 'Kelocyanor').

It has also been anecdotally suggested that glucose is itself an effective counteragent to cyanide, reacting with it to form less toxic compounds that can be eliminated by the body. One theory on the apparent immunity of Grigory Rasputin to cyanide was that his killers put the poison in sweet pastries and madeira wine, both of which are rich in sugar; thus, Rasputin would have been administered the poison together with massive quantities of antidote. One study found a reduction in cyanide toxicity in mice when the cyanide was first mixed with glucose^*. However, as yet glucose on its own is not an officially acknowledged antidote to cyanide poisoning.

Poison use

The cyanide ion, if used as poison, is generally delivered in the form of gaseous hydrogen cyanide or in the form of potassium cyanide (KCN) or sodium cyanide (NaCN).

Gas chambers

Zyklon B, the poison gas used in Nazi gas chambers during the Holocaust, worked by delivering hydrogen cyanide gas.

Cyanide is also the compound used in U.S. gas chambers for execution.

War

Cyanides were stockpiled in both the Soviet and the United States chemical weapons arsenals in the 1950s and 1960s. During the Cold War, the Soviet Union was thought to be planning to use hydrogen cyanide as a "blitzkrieg" weapon to clear a path through the opposing front line, knowing that the harmful gas itself would dissipate and allow unprotected access to the captured zone. However, as a military agent, cyanide was not considered very effective, since cyanide is lighter than air and requires a significant dose in order to incapacitate or kill.

Suicide

Cyanide salts are sometimes used as fast-acting suicide devices. Cyanide is reputed to work faster on an empty stomach, possibly because the anion is protonated by stomach acids to give HCN. Famous cyanide salt suicides include:

Erwin Rommel
Adolf Hitler (likely, see article on Hitler's death)
Eva Braun
Joseph Goebbels
Hermann Göring
Heinrich Himmler
Alan Turing
Odilo Globocnik
Martin Bormann
A North-Korean agent identified as Kim Sung Il, who along with a female accomplice in police custody in Bahrain bit into cyanide tablets hidden in cigarettes after having left a bomb onboard Korean Air Flight 858 which subsequently exploded over the Indian Ocean on November 29, 1987. The woman's life was saved by a quick-thinking police officer who knocked the cigarette away at the last minute.
Ramón Sampedro
Nedeljko Čabrinović attempted suicide, but failed

Some espionage agents also carried spectacles with cyanide in the frames. If they were caught by the enemy they could 'casually' chew the frame, releasing the cyanide, and die before having information extracted from them.

Jonestown

Jonestown, Guyana was the site of the largest mass suicide of all time, where 913 members of the Peoples Temple drank a cyanide-laced cup of Flavor Aid in 1978.

Murder

See:

In fiction

Poisoning by cyanide figures prominently in crime fiction, for example Agatha Christie's Sparkling Cyanide (also entitled Remembered Death). Cyanide is also the instrument of murder in The Big Sleep by Raymond Chandler and Roald Dahl's short story "The Landlady".

In Gabriel García Márquez's Love in the Time of Cholera, one of the characters (a photographer) commits suicide using gold cyanide.

In the James Bond movies and novels, double o agents are issued cyanide capsules for use in the event of capture by the enemy. James Bond is described as having thrown his away.

In The Godfather, Part III, Vincent (Mancini) Corleone claims that the right-hand man of Joe Zasa dips his bullets in cyanide.

The Joseph Kesselring play "Arsenic and Old Lace," two old ladies mix wine with arsenic, cyanide and strycnine to use to kill old men.

In the movie High strung, the main character when complaining about smokers, claims that "smoking is the equivalent to carrying around a salt lick, laced with a little bit of cyanide."

In the game No One Lives Forever 2, some of the guns were capable of firing Cyanide rounds.

In Current Events

According to a Time Magazine^*, terrorists planned on releasing cyanide gas into the New York City Subway System shortly after the September 11th attacks. The attack was reportably called off because there would not be enough casualties.

References

Sources

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)

External links

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