Cyanide 1 Cyanide Cyanide Properties Molecular formula CN− Molar mass 26.007 g mol -1 Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references A cyanide is any chemical compound that contains monovalent combining group CN. This group, known as the cyano group, consists of a carbon atom triple-bonded to a nitrogen atom. [1] In inorganic cyanides, such as sodium cyanide, NaCN, this group is present as the negatively-charged polyatomic cyanide ion (CN - ); these compounds, which are regarded as salts of hydrocyanic acid, are highly toxic. The cyanide ion is isoelectronic with carbon monoxide and with molecular nitrogen. [2][3] Most cyanides are highly toxic. Organic cyanides are usually called nitriles; in these, the CN group is linked by a covalent bond to a carbon-containing group, such as methyl (CH 3 ) in methyl cyanide (acetonitrile). Hydrocyanic acid, also known as hydrogen cyanide, or HCN, is a highly volatile liquid used to prepare acrylonitrile, which is used in the production of acrylic fibers, synthetic rubber, and plastics. Cyanides are employed in a number of chemical processes, including fumigation, case hardening of iron and steel, electroplating, and the concentration of ores. In nature, substances yielding cyanide are present in certain seeds, such as the pit of the wild cherry.
Transcript
Cyanide 1
Cyanide
Cyanide
Properties
Molecular formula CN−
Molar mass 26.007 g mol-1
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references
A cyanide is any chemical compound that contains monovalent combining group CN. This group, known as thecyano group, consists of a carbon atom triple-bonded to a nitrogen atom.[1]
In inorganic cyanides, such as sodium cyanide, NaCN, this group is present as the negatively-charged polyatomiccyanide ion (CN-); these compounds, which are regarded as salts of hydrocyanic acid, are highly toxic. The cyanideion is isoelectronic with carbon monoxide and with molecular nitrogen.[2][3] Most cyanides are highly toxic.Organic cyanides are usually called nitriles; in these, the CN group is linked by a covalent bond to acarbon-containing group, such as methyl (CH3) in methyl cyanide (acetonitrile).Hydrocyanic acid, also known as hydrogen cyanide, or HCN, is a highly volatile liquid used to prepare acrylonitrile,which is used in the production of acrylic fibers, synthetic rubber, and plastics. Cyanides are employed in a numberof chemical processes, including fumigation, case hardening of iron and steel, electroplating, and the concentrationof ores. In nature, substances yielding cyanide are present in certain seeds, such as the pit of the wild cherry.
In IUPAC nomenclature, organic compounds that have a –C≡N functional groupare called nitriles. Thus, nitriles are organic compounds.[4][5] An example of anitrile is CH3CN, acetonitrile, also known as methyl cyanide. Nitriles usually donot release cyanide ions. A functional group with a hydroxyl and cyanide bondedto the same carbon is called cyanohydrin. Unlike nitriles, cyanohydridins dorelease hydrogen cyanide. In inorganic chemistry, salts containing the C≡N− ionare referred to as cyanides.
Occurrence and reactions
In nature
Cyanides are produced by certain bacteria, fungi, and algae and are found in anumber of plants. Cyanides are found in substantial amounts in certain seeds andfruit stones, e.g., those of apricots, apples, and peaches. In plants, cyanides areusually bound to sugar molecules in the form of cyanogenic glycosides anddefend the plant against herbivores. Cassava roots (also called manioc), animportant potato-like food grown in tropical countries (and the base from whichtapioca is made), also contain cyanogenic glycosides.
The Madagascar bamboo Cathariostachys madagascariensis produces cyanideas a deterrent to grazing. In response, the golden bamboo lemur, which eats thebamboo, has developed a high tolerance to cyanide.
Interstellar medium
The cyanide radical CN· has been identified in interstellar space. The cyanideradical (called 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-deficientconditions. For example, it can be detected in the exhaust of internal combustion engines and tobacco smoke. Certainplastics, especially those derived from acrylonitrile, release hydrogen cyanide when heated or burnt.
Coordination chemistryThe cyanide anion is a ligand for many transition metals.[6] The high affinities of metals for this anion can beattributed to its negative charge, compactness, and ability to engage in π-bonding. Well-known complexes include:• hexacyanides [M(CN)6]3− (M = Ti, V, Cr, Mn, Fe, Co), which are octahedral in shape;• the tetracyanides, [M(CN)4]2− (M = Ni, Pd, Pt), which are square planar in their geometry;• the dicyanides [M(CN)2]− (M = Cu, Ag, Au), which are linear in geometry.The dye Prussian blue was first accidentally made around 1706, by heating substances containing iron and carbonand nitrogen. Prussian blue consists of an iron-containing compound called "ferrocyanide" ({Fe(CN)6]4-) meaning"blue substance with iron", from Latin ferrum = "iron" and Greek kyanos = "(dark) blue". Prussian blue is thedeep-blue pigment used in the making of blueprints.
The enzymes called hydrogenases contain cyanide ligands attached to iron in their active sites. The biosynthesis ofcyanide in the [NiFe]-hydrogenases proceeds from carbamoylphosphate, which converts to cysteinyl thiocyanate, theCN− donor.
Organic derivativesBecause of the cyanide anion's high nucleophilicity, cyano groups are readily introduced into organic molecules bydisplacement of a halide group (e.g., the chloride on methyl chloride). In general, organic cyanides are called nitriles.Thus, CH3CN can be called methyl cyanide but more commonly is referred to as acetonitrile. 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 befunctionalized.
RX + CN− → RCN + X− (nucleophilic substitution) followed by1. RCN + 2 H2O → RCOOH + NH3 (hydrolysis under reflux with mineral acid catalyst), or2. 2 RCN + LiAlH4 + (second step) 4 H2O → 2 RCH2NH2 + LiAl(OH)4 (under reflux in dry ether, followed by
addition of H2O)
ManufactureThe principal process used to manufacture cyanides is the Andrussow process in which gaseous hydrogen cyanide isproduced from methane and ammonia in the presence of oxygen and a platinum catalyst.
2 CH4 + 2 NH3 + 3 O2 → 2 HCN + 6 H2OGaseous hydrogen cyanide may be dissolved in aqueous sodium hydroxide solution to produce sodium cyanide.
ToxicityMany cyanides are highly toxic. The cyanide anion is an inhibitor of the enzyme cytochrome c oxidase (also knownas aa3) in the fourth complex of the electron transport chain (found in the membrane of the mitochondria ofeukaryotic cells). It attaches to the iron within this protein. The binding of cyanide to this enzyme prevents transportof electrons from cytochrome c to oxygen. As a result, the electron transport chain is disrupted, meaning that the cellcan no longer aerobically produce ATP for energy. Tissues that depend highly on aerobic respiration, such as thecentral nervous system and the heart, are particularly affected. This is an example of histotoxic hypoxia.[7]
The most hazardous compound is hydrogen cyanide, which is a gas at ambient temperatures and pressure and cantherefore be inhaled. For this reason, an air respirator supplied by an external oxygen source must be worn whenworking with hydrogen cyanide. Hydrogen cyanide is produced when a solution containing a labile cyanide is madeacidic, because HCN is a weak acid. Alkaline solutions are safer to use because they do not evolve hydrogen cyanidegas. Hydrogen cyanide may be produced in the combustion of polyurethanes; for this reason, polyurethanes are notrecommended for use in domestic and aircraft furniture. Oral ingestion of a small quantity of solid cyanide or acyanide solution as little as 200 mg, or 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. By contrast, compounds such astrimethylsilyl cyanide (CH3)3SiCN readily release HCN or the cyanide ion upon contact with water.[citation needed]
AntidoteHydroxocobalamin reacts with cyanide to form cyanocobalamin, which can be safely eliminated by the kidneys. Thismethod has the advantage of avoiding the formation of methemoglobin (see below). This antidote kit is sold underthe brand name Cyanokit and was approved by the FDA in 2006.An older cyanide antidote kit included administration of three substances: amyl nitrite pearls (administered byinhalation), sodium nitrite, and sodium thiosulfate. The goal of the antidote was to generate a large pool of ferric iron(Fe3+) to compete for cyanide with cytochrome a3 (so that cyanide will bind to the antidote rather that the enzyme).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 thecyanide 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.
SensitivityMinimum risk levels (MRLs) may not protect for delayed health effects or health effects acquired following repeatedsublethal exposure, such as hypersensitivity, asthma, or bronchitis. MRLs may be revised after sufficient dataaccumulates (Toxicological Profile for Cyanide, U.S. Department of Health and Human Services, 2006).
Applications
MiningCyanide is mainly produced for the mining of gold and silver: It helps dissolve these metals and their ores. In thecyanide process, finely ground high-grade ore is mixed with the cyanide (concentration of about two kilogram NaCNper tonne); low-grade ores are stacked into heaps and sprayed with a cyanide solution (concentration of about onekilogram NaCN per ton). The precious metals are complexed by the cyanide anions to form soluble derivatives, e.g.,[Au(CN)2]− and [Ag(CN)2]−.[8]
4 Au + 8 NaCN + O2 + 2 H2O → 4 Na[Au(CN)2] + 4 NaOHSilver is less "noble" than gold and often occurs as the sulfide, in which case redox is not invoked (no O2 isrequired). Instead, a displacement reaction occurs:
Ag2S + 4 NaCN + H2O → 2 Na[Ag(CN)2] + NaSH + NaOHThe "pregnant liquor" containing these ions is separated from the solids, which are discarded to a tailing pond orspent heap, the recoverable gold having been removed. The metal is recovered from the "pregnant solution" byreduction with zinc dust or by adsorption onto activated carbon. This process can result in environmental and healthproblems. Aqueous cyanide is hydrolyzed rapidly, especially in sunlight. It can mobilize some heavy metals such asmercury if present. Gold can also be associated with arsenopyrite (FeAsS), which is similar to iron pyrite (fool'sgold), wherein half of the sulfur atoms are replaced by arsenic. Gold-containing arsenopyrite ores are similarlyreactive toward inorganic cyanide.Cyanide is also used in electroplating, where it stabilizes metal ions in the electrolyte solution prior to theirdeposition.
Industrial organic chemistrySome nitriles are produced on a large scale, e.g., adiponitrile is a precursor to nylon. Such compounds are oftengenerated by combining hydrogen cyanide and alkenes, i.e., hydrocyanation: RCH=CH2 + HCN → RCH(CN)CH3.Metal catalysts are required for such reactions.
Medical usesThe cyanide compound sodium nitroprusside is used mainly in clinical chemistry to measure urine ketone bodiesmainly as a follow-up to diabetic patients. On occasion, it is used in emergency medical situations to produce a rapiddecrease in blood pressure in humans; it is also used as a vasodilator in vascular research. The cobalt in artificialvitamin B12 contains a cyanide ligand as an artifact of the purification process; this must be removed by the bodybefore the vitamin molecule can be activated for biochemical use. During World War I, a copper cyanide compoundwas briefly used by Japanese physicians for the treatment of tuberculosis and leprosy.
FishingCyanides are illegally used to capture live fish near coral reefs for the aquarium and seafood markets. The practice iscontroversial, dangerous, and damaging but is driven by the lucrative exotic fish market.[citation needed]
Pest controlCyanide is used for pest control in New Zealand particularly for possums, an introduced marsupial that threatens theconservation of native species and spreads tuberculosis amongst cattle. Possums can become bait shy but the use ofpellets containing the cyanide reduces bait shyness. Cyanide has been known to kill native birds, including theendangered kiwi. Cyanide is also effective for controlling the Dama Wallaby, another introduced marsupial pest inNew Zealand. A licence is required to store, handle and use cyanide in New Zealand.
Niche usesPotassium ferrocyanide is used to achieve a blue color on cast bronze sculptures during the final finishing stage ofthe sculpture. On its own, it will produce a very dark shade of blue and is often mixed with other chemicals toachieve the desired tint and hue. It is applied using a torch and paint brush while wearing the standard safetyequipment used for any patina application: rubber gloves, safety glasses, and a respirator. The actual amount ofcyanide in the mixture varies according to the recipes used by each foundry.Cyanide is also used in jewelry-making and certain kinds of photography such as sepia toning.Cyanides are used as insecticides for fumigating ships. Cyanide salts are used for killing ants, and have in someplaces been used as rat poison (the less toxic poison arsenic is more common[citation needed]).Although usually thought to be toxic, cyanide and cyanohydrins have been demonstrated to increase germination invarious plant species.
Human poisoning
Deliberate cyanide poisoning of humans has occurred many times throughout history.[9] For notable cyanide deaths,see Cyanide poisoning: Historical cases.Most significantly, hydrogen cyanide released from pellets of Zyklon-B was used extensively in the systematic massmurders of the Holocaust, especially in extermination camps. Poisoning by hydrogen cyanide gas within a gaschamber (as a salt of hydrocyanic acid is dropped into a strong acid, usually sulfuric acid) is one method ofexecuting a condemned prisoner as the condemned prisoner eventually breathes the lethal fumes.
Due to the high stability of their complexation with iron, ferrocyanides (Sodium ferrocyanide E535, Potassiumferrocyanide E536, and Calcium ferrocyanide E538[10]) do not decompose to lethal levels in the human body and areused in the food industry as, e.g., an anticaking agent in table salt.[11]
Chemical tests for cyanide
Prussian blueIron(II) sulfate is added to a solution suspected of containing cyanide, such as the filtrate from the sodium fusiontest. The resulting mixture is acidified with mineral acid. The formation of Prussian blue is a positive result forcyanide.
para-Benzoquinone in DMSOA solution of para-benzoquinone in DMSO reacts with inorganic cyanide to form a cyanophenol, which isfluorescent. Illumination with a UV light gives a green/blue glow if the test is positive.
Copper and an aromatic amineAs used by fumigators to detect hydrogen cyanide, copper(II) salt and an aromatic amine such as benzidine is addedto the sample; as an alternative to benzidine an alternative amine di-(4,4-bis-dimethylaminophenyl) methane can beused. A positive test gives a blue color. Copper(I) cyanide is poorly soluble. By sequestering the copper(I) thecopper(II) is rendered a stronger oxidant. The copper, in a cyanide facilitated oxidation, converts the amine into acolored compound. The Nernst equation explains this process. Another good example of such chemistry is the wayin which the saturated calomel reference electrode (SCE) works. The copper, in a cyanide-facilitated oxidation,converts the amine into a colored compound.
Pyridine-barbituric acid colorimetryA sample containing inorganic cyanide is purged with air from a boiling acid solution into a basic absorber solution.The cyanide salt absorbed in the basic solution is buffered at pH 4.5 and then reacted with chlorine to form cyanogenchloride. The cyanogen chloride formed couples pyridine with barbituric acid to form a strongly colored red dye thatis proportional to the cyanide concentration. This colorimetric method following distillation is the basis for mostregulatory methods (for instance EPA 335.4) used to analyze cyanide in water, wastewater, and contaminated soils.Distillation followed by colorimetric methods, however, have been found to be prone to interferences fromthiocyanate, nitrate, thiosulfate, sulfite, and sulfide that can result in both positive and negative bias. It has beenrecommended by the USEPA (MUR March 12, 2007) that samples containing these compounds be analyzed byGas-Diffusion Flow Injection Analysis — Amperometry.[citation needed]
Gas diffusion flow injection analysis — amperometryInstead of distilling, the sample is injected into an acidic stream where the HCN formed is passed under ahydrophobic gas diffusion membrane that selectively allows only HCN to pass through. The HCN that passesthrough the membrane is absorbed into a basic carrier solution that transports the CN to an amperometric detectorthat accurately measures cyanide concentration with high sensitivity. Sample pretreatment determined by acidreagents, ligands, or preliminary UV irradiation allow cyanide speciation of free cyanide, available cyanide, and totalcyanide respectively. The relative simplicity of these flow injection analysis methods limit the interferenceexperienced by the high heat of distillation and also prove to be cost effective since time consuming distillations arenot required.
References[1] IUPAC Gold Book cyanides (http:/ / goldbook. iupac. org/ C01486. html)[2] Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.[3][3] G. L. Miessler and D. A. Tarr "Inorganic Chemistry" 3rd Ed, Pearson/Prentice Hall publisher, ISBN 0-13-035471-6.[4] IUPAC Gold Book nitriles (http:/ / goldbook. iupac. org/ N04151. html)[5] NCBI-MeSH Nitriles (http:/ / www. ncbi. nlm. nih. gov/ mesh/ 68009570)[6][6] Sharpe, A. G. The Chemistry of Cyano Complexes of the Transition Metals; Academic Press: London, 1976[7] , Chapter 163, page 939 (http:/ / books. google. com/ books?id=SRIvmTVcYBwC& pg=PA939)[8][8] Andreas Rubo, Raf Kellens, Jay Reddy, Norbert Steier, Wolfgang Hasenpusch "Alkali Metal Cyanides" in Ullmann's Encyclopedia of
substance-info/ profiles/ 29. html)• Eating apple seeds is safe despite the small amount of cyanide (http:/ / www. snopes. com/ food/ warnings/
apples. asp#add)• Toxicological Profile for Cyanide, U.S. Department of Health and Human Services, July 2006 (http:/ / www.
atsdr. cdc. gov/ toxprofiles/ tp8. pdf)Safety data (French):• Institut national de recherche et de sécurité (1997). " Cyanure d'hydrogène et solutions aqueuses (http:/ / www.
