The cyanide ion consists of a carbon triple bonded to a nitrogen. It readily reacts with hydrogen to form hydrogen cyanide gas, which has a faint almond-like smell. Most people can smell hydrogen cyanide; however, due to an apparent genetic trait, some individuals cannot. Cyanide gas (HCN) can be generated via 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). Cyanides are also produced by certain bacteria, fungi, and algae and are found in a number of foods and plants. Small amounts of cyanide can be found in apple seeds, mangoes and bitter almonds. Hydrocyanic acid (a solution of hydrogen cyanide in water) is present in freshly distilled bitter almond oil (2-4%) prior to its removal by precipitation as calcium ferrocyanide to give food quality oil. Hydrogen cyanide and most cyanide salts readily dissolve in water (or other biofluids) and exists in solution as the cyanide ion. Cyanide ions bind to the iron atom of the enzyme cytochrome c oxidase (also known as aa3) in the fourth complex in the mitochondrial membrane in the mitochondria of cells. 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 mainly depend on aerobic respiration, such as the central nervous system and the heart, are particularly affected. Because of its respiratory chain toxicity cyanide has been used as a poison many times throughout history. Its most infamous application was the use of hydrogen cyanide by the Nazi regime in Germany for mass murder in some gas chambers during the Holocaust. Hydrogen cyanide (with the historical common name of Prussic acid) is a colorless and highly volatile liquid that boils slightly above room temperature at 26 °C (78.8 °F). Hydrogen cyanide is weakly acidic and partly ionizes in solution to give the cyanide anion, CN-. The salts of hydrogen cyanide are known as cyanides. HCN is a highly valuable precursor to many chemical compounds ranging from polymers to pharmaceuticals. Hydrogen cyanide is a linear molecule, with a triple bond between carbon and nitrogen. It is a weak acid with a pKa of 9.2. A minor tautomer of HCN is HNC, hydrogen isocyanide.
belongs to the class of organic compounds known as nitriles. Nitriles are compounds having the structure RC#N; thus C-substituted derivatives of hydrocyanic acid, HC#N.
Organic nitriles decompose into cyanide ions both in vivo and in vitro. Consequently the primary mechanism of toxicity for organic nitriles is their production of toxic cyanide ions or hydrogen cyanide. Cyanide is an inhibitor of cytochrome c oxidase in the fourth complex of the electron transport chain (found in the membrane of the mitochondria of eukaryotic cells). It complexes with the ferric iron atom in this enzyme. 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 and the cell can no longer aerobically produce ATP for energy. Tissues that mainly depend on aerobic respiration, such as the central nervous system and the heart, are particularly affected. Cyanide is also known produce some of its toxic effects by binding to catalase, glutathione peroxidase, methemoglobin, hydroxocobalamin, phosphatase, tyrosinase, ascorbic acid oxidase, xanthine oxidase, succinic dehydrogenase, and Cu/Zn superoxide dismutase. Cyanide binds to the ferric ion of methemoglobin to form inactive cyanmethemoglobin. (2)
Metabolism
Organic nitriles are converted into cyanide ions through the action of cytochrome P450 enzymes in the liver. Cyanide is rapidly absorbed and distributed throughout the body. Cyanide is mainly metabolized into thiocyanate by either rhodanese or 3-mercaptopyruvate sulfur transferase. Cyanide metabolites are excreted in the urine. (1)
Toxicity Values
Not Available
Lethal Dose
200 to 300 mg for an adult human (cyanide salts). (3)
No indication of carcinogenicity to humans (not listed by IARC).
Uses/Sources
Cyanide compounds are used in electroplating, metallurgy, organic chemicals production, photographic developing, manufacture of plastics, fumigation of ships, and some mining processes. (1)
Minimum Risk Level
Not Available
Health Effects
Exposure to high levels of cyanide for a short time harms the brain and heart and can even cause coma, seizures, apnea, cardiac arrest and death. Chronic inhalation of cyanide causes breathing difficulties, chest pain, vomiting, blood changes, headaches, and enlargement of the thyroid gland. Skin contact with cyanide salts can irritate and produce sores. (1, 2)
Symptoms
Cyanide poisoning is identified by rapid, deep breathing and shortness of breath, general weakness, giddiness, headaches, vertigo, confusion, convulsions/seizures and eventually loss of consciousness. (1, 2)
Treatment
Antidotes to cyanide poisoning include hydroxocobalamin and sodium nitrite, which release the cyanide from the cytochrome system, and rhodanase, which is an enzyme occurring naturally in mammals that combines serum cyanide with thiosulfate, producing comparatively harmless thiocyanate. Oxygen therapy can also be administered. (2)
