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A cyanide is any chemical compound that contains
the group C≡N, with the carbon atom triple bonded to the nitrogen atom. Inorganic cyanides contain the highly toxic cyanide
ion CN- and are the
salts of the acid hydrogen cyanide (HCN). Organic cyanides contain the CN group single-bonded to another carbon
atom and are also known as nitriles. Two
cyanide ions can bond to each other via their carbon atoms, forming the gas cyanogen.
Appearance
Hydrogen cyanide is a colorless gas with a faint, bitter,
almond-like odor. Some people are unable to smell cyanide at all, due to a genetic trait. Sodium cyanide (NaCN) and potassium cyanide (KCN) are both white solids with a
bitter, almond-like odor in damp air.
Occurrence and use
Cyanides can be produced by certain bacteria, fungi, and algae, and are found in a number of foods and plants. Cyanide
occurs naturally in cassava roots, which are potato-like tubers of cassava plants
grown in tropical countries; these must be processed prior to consumption. Fruits which
have a pit, such as cherries or apricots,
often contain either cyanides or cyanogenic glycosides in the pit. Bitter almonds, from
which almond oil and flavoring is made, also contain cyanide.
Hydrogen cyanide is contained in vehicle exhaust and in tobacco smoke. The smoke
of burning plastics contains hydrogen cyanide, and house fires often result in cyanide poisonings. A deep blue pigment called Prussian blue, used in the making of
blueprints (hence the name cyanide, from cyan, a shade of blue),
produces hydrogen cyanide when exposed to acids.
Cyanides and hydrogen cyanide are used in electroplating, metallurgy, production of chemicals, photographic development,
making plastics, fumigating ships, and some mining processes.
Effects on the human body
To deal with the cyanides contained in many foods, the body has an enzyme (rhodanide synthetase)
which can convert small amounts of cyanides to the harmless sulfur-containing thiocyanate (SCN−). Cyanides are also essential components of vitamin B12.
In larger amounts, cyanides are harmful. Symptoms of moderate poisoning include vomiting, convulsions, deep breathing, shortness of breath and
anxiety; more serious cases result in convulsions, loss of consciousness, and death after apnea and heart arrest.
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.
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 odor in the breath may alert a doctor
that a person was exposed to cyanide.
Mechanism of toxicity and treatment
Cyanide ion kills all aerobic organisms by shutting down the
respiration in cells. It interrupts the electron
transport chain in the inner membrane of the mitochondrion because it binds more strongly than oxygen to
the Fe+3 in cytochrome a3, preventing this cytochrome from combining electrons with oxygen.
Contrary to popular belief, cyanide does not bind well to ferrous hemoproteins,
such as hemoglobin, the mechanism which makes carbon monoxide toxic. One of the therapies for cyanide poisoning is to convert part of the
hemoglobin of the blood from ferrous hemoglobin to ferric; this creates a pool of binding potential that can divert cyanide from the cytochromes it poisons. This is
done with the compound 4-Dimethylaminophenyl.
Use as a poison
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).
Zyklon B, the poison gas used
in Nazi gas chambers
during the Holocaust, works by delivering hydrogen cyanide gas. Cyanide is also
the compound used in US execution chambers.
The cyanide salts are fast acting "suicide pills". When they reach the stomach
acids, cyanide ions are released; therefore they work faster on an empty stomach. Famous cyanide salt suicides include:
Poisoning by cyanide also figures prominently in crime fiction, for
example Agatha Christie's Sparkling Cyanide (also
titled Remembered Death); cyanide is the instrument of one murder in The Big Sleep by Raymond Chandler.
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 evaporate and allow unprotected
access to the captured zone.
Mining
Cyanide salts are used in silver and gold
mining, called the cyanide process. The ore is finely ground
and mixed or sprayed with cyanide solution. The precious metal cations bind to the
cyanide anions and form a soluble cyanide. The left-over dirt is discarded and
the metal is recovered from the clear solution with zinc. This process can result in
environmental and health problems.
Fishing
Cyanides are used to capture live fish near coral reefs for the aquarium and seafood market. 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. Environmental
organizations decry the practice.
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