Cyanate

The cyanate ion is the anion with the chemical formula written as [OCN] or [NCO]. In aqueous solution it acts as a base, forming isocyanic acid, HNCO. The cyanate ion is an ambidentate ligand, forming complexes with a metal ion in which either the nitrogen or oxygen atom may be the electron-pair donor. It can also act as a bridging ligand. Organic cyanates are called isocyanates when there is a C−NCO bond and cyanates when there is a C−OCN bond.

Cyanate-ion-3D-vdW
Space-filling model of the cyanate anion.

Cyanate ion

The three atoms in a cyanate ion lie on a straight line, giving the ion a linear structure. The electronic structure is described most simply as

:Ö̤−C≡N:

with a single C−O bond and a triple C≡N bond. The infrared spectrum of a cyanate salt has a band at ca. 2096 cm−1; such a high frequency is characteristic of a triple bond.[1] The cyanate ion is a Lewis base. Both the oxygen and nitrogen atoms carry a lone pair of electrons and either one or the other, or both can be donated to Lewis acid acceptors. It can be described as an ambidentate ligand.

Acid-base properties

Isocyansäure
Isocyanic acid

Isocyanic acid, HNCO, is produced when a cyanate salt is acidified. Although the electronic structure according to valence bond theory can be written as HN=C=O, as illustrated, the vibrational spectrum has a band at 2268.8 cm−1 in the gas phase, which clearly requires the C≡N bond to be a triple bond.[2][3] In valence bond theory the canonical form H≡C−O is the major contributor to the resonance hybrid. The pure compound has a melting point of −86.8 °C and a boiling point of 23.5 °C, so it is volatile at ambient temperatures.[4][5] In aqueous solution it is a weak acid.

HNCO ⇌ H+ + NCO;      pKa ≈ 3.7.[6]

On heating isocyanic acid is converted to the trimer cyanuric acid, which itself decomposes on further heating back to isocyanic acid.[4] In the reverse of the famous synthesis of urea by Friedrich Wöhler,

OC(NH2)2 → HNCO + NH3

isocyanic acid is produced and rapidly trimerizes to cyanuric acid.

The tautomer, known as cyanic acid, HOCN, in which the oxygen atom is protonated, is unstable to decomposition, but in solution it is present in equilibrium with isocyanic acid to the extent of about 3%. The vibrational spectrum is indicative of the presence of a triple bond between the nitrogen and carbon atoms.[7]

Cyanate salts

Sodium cyanate is isostructural with sodium fulminate, confirming the linear structure of the cyanate ion.[8] It is made industrially by heating a mixture of sodium carbonate and urea.[9]

Na2CO3 + 2 OC(NH2)2 → 2 NaNCO + CO2 + 2 NH3 + H2O

A similar reaction is used to make potassium cyanate. Cyanates are produced when cyanides are oxidized. Use of this fact is made in cyanide decontamination processes where oxidants such as permanganate and hydrogen peroxide are used to convert toxic cyanide to safer cyanate.

Complexes with the cyanate ion

Cyanate is an ambidentate ligand which can donate the pair of electrons on the nitrogen atom or the oxygen atom, or both. Structurally the isomers can be distinguished by the geometry of the complex. In N-bonded cyanate complexes the M−NCO unit has a Guzman structure, but with O-bonded cyanate the M−O−C unit is bent. Thus, the silver cyanato complex, [Ag(NCO)2], has a linear structure, so is N-bonded.

Infrared spectroscopy has been used extensively to distinguish between isomers. Many complexes of divalent metals are N-bonded. O-bonding has been suggested for complexes of the type [M(OCN)6]n, M = Mo(III), Re(IV) and Re(V). The yellow complex Rh(PPh3)3(NCO) and orange complex Rh(PPh3)3(OCN) are linkage isomers and show differences in their infrared spectra which can be used for diagnosis (PPh3 stands for triphenylphosphine).[10]

The cyanate ion can bridge between two metal atoms by using both its donor atoms. For example, this structure is found in the compound [Ni2(NCO)2(en)2][BPh4]2. In this compound both the Ni−N−C unit and Ni−O−C unit are bent, even though in the first case donation is through the nitrogen atom.[11]

Cyanate in organic compounds

Organic compounds that contain the functional group −N=C=O are known as isocyanates. It is conventional in organic chemistry to write isocyanates with two double bonds, which accords with a simplistic valence bond theory of the bonding. In nucleophilic substitution reactions cyanate usually forms an isocyanate. Isocyanates are widely used in the manufacture of polyurethane[12] products and pesticides; Methyl isocyanate, used to make pesticides, was a major factor in the Bhopal disaster.

Compounds that contain the group −O−C≡N, are known as a cyanates, or cyanate esters. Aryl cyanates such are phenyl cyanate, C6H5OCN, can be formed by a reaction of phenol with cyanogen chloride, ClCN, in the presence of a base.

References

  1. ^ Nakamoto, Part A, p171
  2. ^ Nakamoto, part A, p 190
  3. ^ "The CHNO Isomers". Chemische Berichte. 122: 753–766. doi:10.1002/cber.19891220425.
  4. ^ a b Greenwood, p323
  5. ^ Wells, p 722
  6. ^ IUPAC SC-Database A comprehensive database of published data on equilibrium constants of metal complexes and ligands
  7. ^ Teles, Joaquim Henrique; Maier, Günther; Andes Hess, B.; Schaad, Lawrence J.; Winnewisser, Manfred; Winnewisser, Brenda P. (1989). "The CHNO Isomers". Chem. Ber. 122 (4): 1099–0682. doi:10.1002/cber.19891220425.
  8. ^ Wells, p722.
  9. ^ Greenwood, p324
  10. ^ Nakamoto, Part B, pp 121–123.
  11. ^ Greenwood, Table 8.9
  12. ^ Seymour, Raymond B.; Kauffman, George B. (1992). "Polyurethanes: A Class of Modern Versatile Materials". J. Chem. Educ. 69: 909. Bibcode:1992JChEd..69..909S. doi:10.1021/ed069p909.

External links

Bibliography

  • Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  • Nakamoto, K. (1997). Infrared and Raman spectra of Inorganic and Coordination compounds. Part A (5th ed.). Wiley. ISBN 0-471-16394-5.
  • Nakamoto, K. (1997). Infrared and Raman spectra of Inorganic and Coordination compounds. Part B (5th ed.). Wiley. ISBN 0-471-16392-9.
  • Wells, A.F (1962). Structural Inorganic Chemistry (3rd. ed.). Oxford: Clarendon Press. ISBN 0-19-855125-8.
Ammonium cyanate

Ammonium cyanate is an inorganic compound with the formula NH4OCN. It is a colorless solid.

Cyanase

In molecular biology, cyanase (EC 4.2.1.104, also known as cyanate hydratase or cyanate lyase) is an enzyme which catalyses the bicarbonate dependent metabolism of cyanate to produce ammonia and carbon dioxide. The systematic name of this enzyme is carbamate hydrolyase. In E. coli, cyanase is an inducible enzyme and is encoded for by the cynS gene. Cyanate is a toxic anion, and cyanase catalyzes the metabolism into the benign products of carbon dioxide and ammonia.

Cyanate ester

Cyanate esters are chemical substances in which the hydrogen atom of the phenolic OH group is substituted by a cyanide group. The resulting product with an -OCN group is named a cyanate ester. Cyanate esters based on a bisphenol or novolac derivative are used in the production of resins.

Estrone cyanate

Estrone cyanate, or estrone 3-O-cyanate, also known as estrocyanate, is an estrogen and an estrogen ester – specifically, the 3-O-cyanate ester of estrone – which was investigated for potential use in birth control pills but was found to be of relatively low potency and ultimately was never marketed.

Hydroxycarbamide

Hydroxycarbamide, also known as hydroxyurea, is a medication used in sickle-cell disease, chronic myelogenous leukemia, cervical cancer, and polycythemia vera. In sickle-cell disease it increases hemoglobin and decreases the number of attacks. It is taken by mouth.Common side effects include bone marrow suppression, fevers, loss of appetite, psychiatric problems, shortness of breath, and headaches. There is also concern that it increases the risk of later cancers. Use during pregnancy is typically harmful to the baby. Hydroxycarbamide is in the antineoplastic family of medications. It is believed to work by blocking the making of DNA.Hydroxycarbamide was approved for medical use in the United States in 1967. It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system. Hydroxycarbamide is available as a generic medication. The wholesale cost in the developing world is about US$0.35–0.47 per day. In the United States it costs less than $25 a month.

Isocyanate

Isocyanate is the functional group with the formula R−N=C=O. Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyanates are manufactured for the production of polyurethanes, a class of polymers.Isocyanates should not be confused with cyanate esters and isocyanides, very different families of compounds. The cyanate (cyanate ester) functional group (R−O−C≡N) is arranged differently from the isocyanate group (R−N=C=O). Isocyanides have the connectivity R−N≡C, lacking the oxygen of the cyanate groups.

Isocyanic acid

Isocyanic acid is a chemical compound with the formula HNCO, discovered in 1830 by Liebig and Wöhler. This colourless substance is volatile and poisonous, with a boiling point of 23.5 °C. Isocyanic acid is the simplest stable chemical compound that contains carbon, hydrogen, nitrogen, and oxygen, the four most commonly found elements in organic chemistry and biology.

Isomer

In chemistry, isomers are ions or molecules with identical formulas but distinct structures. Isomers do not necessarily share similar properties. Two main forms of isomerism are structural isomerism (or constitutional isomerism) and stereoisomerism (or spatial isomerism).

Mercury(II) fulminate

Mercury(II) fulminate, or Hg(CNO)2, is a primary explosive. It is highly sensitive to friction, heat and shock and is mainly used as a trigger for other explosives in percussion caps and blasting caps. Mercury(II) cyanate, though its empirical formula is identical, has a different atomic arrangement; the cyanate and fulminate anions are isomers.

First used as a priming composition in small copper caps beginning in the 1820s, mercury fulminate quickly replaced flints as a means to ignite black powder charges in muzzle-loading firearms. Later, during the late 19th century and most of the 20th century, mercury fulminate or potassium chlorate became widely used in primers for self-contained rifle and pistol ammunition. Mercury fulminate has the distinct advantage over potassium chlorate of being non-corrosive, but it is known to weaken with time, by decomposing into its constituent elements. The reduced mercury which results forms amalgams with cartridge brass, weakening it, as well. Today, mercury fulminate has been replaced in primers by more efficient chemical substances. These are non-corrosive, less toxic and more stable over time; they include lead azide, lead styphnate and tetrazene derivatives. In addition, none of these compounds require mercury for manufacture, supplies of which can be unreliable in wartime.

Oxime

An oxime is a chemical compound belonging to the imines, with the general formula RR'C=NOH, where R is an organic side-chain and R' may be hydrogen, forming an aldoxime, or another organic group, forming a ketoxime. O-substituted oximes form a closely related family of compounds. Amidoximes are oximes of amides with general structure R1C(=NOH)NR2R3.

Oximes are usually generated by the reaction of hydroxylamine with aldehydes or ketones. The term oxime dates back to the 19th century, a combination of the words oxygen and imine.

Potassium cyanate

Potassium cyanate is an inorganic compound with the formula KOCN (sometimes denoted KCNO). It is a colourless solid. It is used to prepare many other compounds including useful herbicide. Worldwide production of the potassium and sodium salts was 20,000 tons in 2006.

Pseudohalogen

The pseudohalogens are polyatomic analogues of halogens, whose chemistry, resembling that of the true halogens, allows them to substitute for halogens in several classes of chemical compounds. Pseudohalogens occur in pseudohalogen molecules, inorganic molecules of the general forms

Ps–Ps or Ps–X (where Ps is a pseudohalogen group), such as cyanogen; pseudohalide anions, such as cyanide ion; inorganic acids, such as hydrogen cyanide; as ligands in coordination complexes, such as ferricyanide; and as functional groups in organic molecules, such as the nitrile group.

Well-known pseudohalogen functional groups include cyanide, cyanate, thiocyanate, and azide.

Silver cyanate

Silver cyanate is a chemical compound; it is the cyanate salt of silver.

It can be prepared by the reaction of potassium cyanate or urea with silver nitrate.

Silver cyanate is a beige to gray powder. It crystallises in the monoclinic crystal system in space group P21/m with parameters a = 547.3 pm, b = 637.2 pm, c = 341.6 pm, and β = 91°.

With nitric acid, silver cyanate reacts to form silver nitrate, carbon dioxide, and ammonium nitrate.

Sodium cyanate

Sodium cyanate (NaOCN) is a white crystalline solid that adopts a body centered rhombohedral crystal lattice structure (trigonal crystal system) at room temperature.

Thermoset polymer matrix

A thermoset polymer matrix is a synthetic polymer reinforcement first developed for structural applications, such as glass-reinforced plastic radar domes on aircraft and graphite-epoxy payload bay doors on the space shuttle. In polymer matrix composites, polymers act as binder or matrix to secure in place incorporated particulates, fibres or other reinforcements.

They were first used after World War II, and continuing research has led to an increased range of thermoset resins, polymers or plastics, as well as engineering grade thermoplastics, all developed for use in the manufacture of polymer composites with enhanced and longer-term service capabilities. Thermoset polymer matrix technologies also find use in a wide diversity of non-structural industrial applications.The foremost types of thermosetting polymers used in structural composites are benzoxazine resins, bis-Maleimide resins (BMI), cyanate ester resins, epoxy (epoxide) resins, phenolic (PF) resins, unsaturated polyester (UP) resins, polyimides, polyurethane (PUR) resins, silicones, and vinyl esters.

Thiocyanate

Thiocyanate (also known as rhodanide) is the anion [SCN]−. It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium thiocyanate and sodium thiocyanate. Organic compounds containing the functional group SCN are also called thiocyanates. Mercury(II) thiocyanate was formerly used in pyrotechnics.

Thiocyanate is analogous to the cyanate ion, [OCN]−, wherein oxygen is replaced by sulfur. [SCN]− is one of the pseudohalides, due to the similarity of its reactions to that of halide ions. Thiocyanate used to be known as rhodanide (from a Greek word for rose) because of the red colour of its complexes with iron. Thiocyanate is produced by the reaction of elemental sulfur or thiosulfate with cyanide:

8 CN− + S8 → 8 SCN−

CN− + S2O2−3 → SCN− + SO2−3The second reaction is catalyzed by thiosulfate sulfurtransferase, a hepatic mitochondrial enzyme, and by other sulfur transferases, which together are responsible for around 80% of cyanide metabolism in the body.

Urea

Urea, also known as carbamide, is an organic compound with chemical formula CO(NH2)2. This amide has two –NH2 groups joined by a carbonyl (C=O) functional group.

Urea serves an important role in the metabolism of nitrogen-containing compounds by animals and is the main nitrogen-containing substance in the urine of mammals. It is a colorless, odorless solid, highly soluble in water, and practically non-toxic (LD50 is 15 g/kg for rats). Dissolved in water, it is neither acidic nor alkaline. The body uses it in many processes, most notably nitrogen excretion. The liver forms it by combining two ammonia molecules (NH3) with a carbon dioxide (CO2) molecule in the urea cycle. Urea is widely used in fertilizers as a source of nitrogen (N) and is an important raw material for the chemical industry.

Friedrich Wöhler's discovery in 1828 that urea can be produced from inorganic starting materials was an important conceptual milestone in chemistry. It showed for the first time that a substance previously known only as a byproduct of life could be synthesized in the laboratory without biological starting materials thereby contradicting the widely held doctrine of vitalism.

Urech hydantoin synthesis

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The Urech hydantoin synthesis is the chemical reaction of amino acids with potassium cyanate and hydrochloric acid to give hydantoins.

Wöhler synthesis

The Wöhler synthesis is the conversion of ammonium cyanate into urea. This chemical reaction was described in 1828 by Friedrich Wöhler. It is cited often the starting point of modern organic chemistry. Although the Wöhler reaction concerns the conversion of ammonium cyanate, this salt appears only as an (unstable) intermediate. Wöhler demonstrated the reaction in his original publication with different sets of reactants: a combination of cyanic acid and ammonia, a combination of silver cyanate and ammonium chloride, a combination of lead cyanate and ammonia and finally from a combination of mercury cyanate and cyanatic ammonia (which is again cyanic acid with ammonia).

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