Cyanide

• 1. Introduction

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  Cyanide can kill within minutes. Swallowing as little as 200 milligrams of a cyanide solution, or breathing air that carries 270 parts per million, is enough. Yet the name covers a whole family of compounds, and not all of them are dangerous. Some of them sit in common food. Others color sculptures and blueprints. What do all these compounds share at the atomic level? How does the poison stop a living cell? Why do plants and fungi make it, and why do some animals shrug it off? And why is most of it manufactured for gold and silver mining?

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• 2. Anatomy of a Triple Bond

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  Acetonitrile is the textbook example of a nitrile, a carbon compound in which a cyano group is attached to a methyl group. Inside every cyano group, a carbon atom and a nitrogen atom share a triple bond. Ionic cyanides carry that unit as a charged anion, and the anion is extremely toxic. Covalent cyanides behave differently. In tetracyanomethane, four cyano groups hang from a single carbon atom. Nitriles generally do not release cyanide ions, but cyanohydrins do, which makes them poisonous. The cyano group can also attach to atoms other than carbon, as in cyanogen azide, phosphorus tricyanide and trimethylsilyl cyanide.

  The cyanide ion has the same number of electrons as carbon monoxide and as molecular nitrogen, which also holds a triple bond. Its negative charge is concentrated on the carbon atom. That charge makes cyanide a base. The pKa of hydrogen cyanide is 9.21, so pouring an acid stronger than hydrogen cyanide into a cyanide salt solution frees the gas.

  Chemists prize the ion for its strong nucleophilicity. It can replace a halide, such as the chlorine on methyl chloride, and so slip a cyano group into an organic molecule. Because the step lengthens a carbon chain by exactly one atom, cyanide is called a C-1 synthon, and the new group can still be modified afterward.

  Water also attacks cyanide, though the reaction stays slow until about 170 °C. It yields ammonia and formate, both far less toxic than the starting ion, and an enzyme called cyanide hydrolase speeds it along. The ion's hunger for metals is what really matters, because its negative charge, compact size and ability to form pi bonds let it grip transition metals tightly.

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• 3. Cellular Sabotage

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  Inside the mitochondria of eukaryotic cells sits cytochrome c oxidase, also known as aa3. It is the fourth complex of the electron transport chain, embedded in the inner membrane. Cyanide attaches to the iron inside this enzyme. That blocks electrons from moving from cytochrome c to oxygen, so the chain is disrupted. The cell can no longer produce ATP aerobically. Tissues that depend heavily on aerobic respiration suffer most, notably the central nervous system and the heart. This condition is called histotoxic hypoxia, meaning the tissue is starved even though oxygen is present.

  Hydrogen cyanide is a gas, and it kills by inhalation. Anyone handling it needs an air respirator supplied by an external oxygen source. Adding acid to a cyanide salt solution generates the gas, so alkaline solutions are safer to use. Soluble salts such as sodium cyanide, potassium cyanide and tetraethylammonium cyanide are highly toxic as well, and calcium cyanide ranks among the most dangerous.

  Published minimum risk levels for exposure come with a warning. They may not protect against delayed effects, or against effects that build up after repeated sublethal exposure, such as hypersensitivity, asthma or bronchitis. The levels may be revised as more data accumulates. Many living things already exploit this weakness, and one lemur has learned to resist it.

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• 4. Nature's Chemical Defense

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  The golden bamboo lemur eats a plant that is armed against it. The Madagascar bamboo Cathariostachys madagascariensis produces cyanide to discourage grazing. In response, the lemur has developed a high tolerance to the poison.

  Many plants rely on the same trick. Chemicals that can release cyanide are called cyanogenic compounds. In plants they are usually bound to sugar molecules as cyanogenic glycosides, and they defend against herbivores. Substantial amounts occur in certain seeds and fruit stones, including those of bitter almonds, apricots, apples and peaches.

  Cassava roots, also called manioc, carry cyanogenic glycosides too. This potato-like food is important in tropical countries and is the base from which tapioca is made. The same chemistry that protects the plant has to be reckoned with by the people who eat it.

  Plants are not the only producers. Certain insects, bacteria, fungi and algae make cyanide as well. It even sits at the heart of hydrogenase enzymes, where cyanide ligands are attached to iron in the active sites. In NiFe hydrogenases, the cyanide is built from carbamoyl phosphate, which converts to cysteinyl thiocyanate, the donor. Despite its reputation, cyanide and cyanohydrins also increase germination in various plant species.

  Fire produces it too. Hydrogen cyanide forms when certain materials burn or undergo pyrolysis with too little oxygen. It can be detected in the exhaust of internal combustion engines and in tobacco smoke. Plastics derived from acrylonitrile release it when heated or burnt.

  And it exists far beyond Earth. The cyanide radical, written •CN, has been identified in interstellar space. Cyanogen is used to measure the temperature of interstellar gas clouds. Not every cyanide is dangerous, though, and one of the most famous was made by accident around 1706.

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• 5. Blue Pigments, Safe Salts

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  Around 1706, someone heating substances that contained iron, carbon and nitrogen accidentally produced Prussian blue. Other cyanides made later were named after it. The pigment gives blueprints, bluing and cyanotypes their blue color. Prussian blue and potassium ferrocyanide are both essentially nontoxic, because the cyanide is bound tightly to a central iron atom.

  That tight grip explains a surprising entry in the food industry. Sodium ferrocyanide, labeled E535, potassium ferrocyanide, E536, and calcium ferrocyanide, E538, do not break down to lethal levels in the human body. Manufacturers use them as an anticaking agent in table salt.

  In a bronze foundry, potassium ferrocyanide serves as a finishing touch. Applied to cast sculptures at the last stage, it produces a blue color. On its own the shade is very dark, so it is often mixed with other chemicals to reach the desired tint. Workers apply it with a torch and a paint brush, wearing rubber gloves, safety glasses and a respirator. The actual amount of cyanide in the mixture varies with each foundry's recipe.

  Cyanide also appears in jewelry-making and in certain kinds of photography, such as sepia toning.

  The pigment even shows up in the laboratory as a detector. Adding iron(II) sulfate to a cyanide solution gives Prussian blue. Benzidine turns blue in the presence of ferricyanide. A solution of para-benzoquinone in DMSO reacts with inorganic cyanide to form a fluorescent cyanophenol, which glows green or blue under UV light. In gold mining, where the stakes are higher, cyanide is typically quantified by potentiometric titration.

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• 6. Mining and Industry

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  A ratio of about one part sodium cyanide to 500 parts of finely ground high-grade ore is the recipe at the heart of the cyanide process. Cyanide is mainly produced for the mining of silver and gold, because it dissolves these metals and lets them be separated from other solids. Low-grade ores are stacked in heaps and sprayed with a cyanide solution, at about one part sodium cyanide to 1000 parts of ore. The cyanide anions complex the precious metals into soluble dicyanoargentate(I) and dicyanoaurate(I). Silver often occurs as a sulfide, so a displacement reaction takes place instead of a redox step.

  The resulting liquid, called pregnant liquor, is separated from the solids. The solids go to a tailing pond or a spent heap. Zinc dust reduces the metal out of the solution, or activated carbon adsorbs it. Gold can also be associated with arsenopyrite, FeAsS, which resembles iron pyrite, or fool's gold, with half the sulfur atoms replaced by arsenic. Such ores react with cyanide in the same way.

  The process has a cost. A number of environmental disasters have followed overflows of tailing ponds at gold mines. Cyanide contamination of waterways has caused numerous deaths among people and aquatic species. Aqueous cyanide is hydrolyzed rapidly, especially in sunlight, but it can mobilize heavy metals such as mercury if they are present.

  The principal route to the chemical itself is the Andrussow process. Methane and ammonia react with oxygen over a platinum catalyst to give gaseous hydrogen cyanide. Treating that gas with sodium hydroxide gives sodium cyanide, the precursor to most other cyanides. After mining, the second major use of alkali metal cyanides is making nitrile compounds. Acyl cyanides come from acyl chlorides and cyanide. Cyanogen, cyanogen chloride and the trimer cyanuric chloride all derive from these salts.

  Because of the toxicity, disposal is subject to stringent regulations. Industrial effluent is typically destroyed by oxidation with peroxysulfuric acid, hydrogen peroxide, sulfur dioxide with copper salts, known as the Inco process, or all three, known as the Combiox process. Sodium hypochlorite, traditional for laboratory waste, is impractical at commercial scale. Hydrolysis at higher temperatures works very well but needs specialized equipment. Waste can also be acidified to recover hydrogen cyanide. Away from the mine, cyanide has quieter and cruder jobs near coral reefs and in the wild.

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• 7. Poison as a Tool

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  Near coral reefs, fishers illegally use cyanide to capture live fish for the aquarium and seafood markets. The practice is controversial, dangerous and damaging, and it is driven by the lucrative exotic fish market.

  In Africa, poachers have been known to poison waterholes with cyanide in order to kill elephants for their ivory.

  In the United States, M44 cyanide devices are used to kill coyotes and other canids. New Zealand uses cyanide against possums, an introduced marsupial that threatens native species and spreads tuberculosis among cattle. Possums can become bait shy, but pellets containing cyanide reduce that problem. The poison has also killed native birds, including the endangered kiwi. It is effective against the dama wallaby, another introduced marsupial pest, and a licence is required to store, handle and use cyanide there.

  Cyanides are used as insecticides for fumigating ships. Cyanide salts kill ants, and in some places they have served as rat poison, although the less toxic arsenic is more common.

  Deliberate cyanide poisoning of humans has occurred many times throughout history. Sodium cyanide is involatile but water-soluble, so it poisons by ingestion. Hydrogen cyanide is a gas, which might seem more indiscriminately dangerous. But it is lighter than air and rapidly disperses upward into the atmosphere, which makes it ineffective as a chemical weapon. Against all this stands a specific antidote that medicine approved only in 2006.

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• 8. Antidotes and Medicine

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  In 2006, the U.S. FDA approved a kit sold under the brand name Cyanokit. Its active ingredient, hydroxocobalamin, reacts with cyanide to form cyanocobalamin, which the kidneys can safely eliminate. The method avoids forming methemoglobin, a drawback of the older approach.

  That older antidote kit used three substances: amyl nitrite pearls, which are inhaled, then sodium nitrite and sodium thiosulfate. The goal was to create a large pool of ferric iron that would compete with cytochrome a3 for the cyanide. The nitrites oxidize hemoglobin to methemoglobin, which competes with cytochrome oxidase for the ion. Cyanmethemoglobin forms, and the cytochrome oxidase enzyme is restored.

  The body has its own main defense. The mitochondrial enzyme rhodanese converts cyanide to thiocyanate, a relatively non-toxic molecule that the kidneys excrete. Sodium thiosulfate speeds this up by supplying the sulfur donor that rhodanese needs.

  Some cyanide compounds are used as medicines in their own right. Sodium nitroprusside is used mainly in clinical chemistry to measure ketone bodies in urine, mostly as a follow-up for diabetic patients. On occasion it is used in emergencies to lower blood pressure rapidly, and it also serves as a vasodilator in vascular research. Artificial vitamin B12 contains a cobalt atom with a cyanide ligand, left over from the purification process, and the body must remove it before the vitamin can be activated.

  During World War I, Japanese physicians briefly used a copper cyanide compound to treat tuberculosis and leprosy.

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