Isocyanic acid
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| Names | |
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| IUPAC name
Isocyanic acid
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| Other names
Carbimide[1]
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| Identifiers | |
3D model (JSmol)
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| ChEBI | |
| ChemSpider | |
| ECHA InfoCard | 100.109.068 |
PubChem CID
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| UNII |
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CompTox Dashboard (EPA)
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| Properties | |
| HNCO | |
| Molar mass | 43.025 g·mol−1 |
| Appearance | Colorless liquid or gas (b.p. near room temperature) |
| Density | 1.14 g/cm3 (20 °C) |
| Melting point | −86 °C (−123 °F; 187 K)[3] |
| Boiling point | 23.5 °C (74.3 °F; 296.6 K) |
| Dissolves | |
| Solubility | Soluble in benzene, toluene, diethyl ether |
| Conjugate acid | Oxomethaniminium[2] |
| Conjugate base | Cyanate |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards
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Poisonous |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Isocyanic acid is a chemical compound with the structural formula HNCO, which is often written as H–N=C=O. It is a colourless, volatile and poisonous substance, with a boiling point of 23.5 °C. It is the predominant tautomer of cyanic acid H–O–C≡N.
The derived anion of isocyanic acid is the same as the derived anion of cyanic acid, and that anion is [N=C=O]−
, which is called cyanate. The related functional group –N=C=O is isocyanate; it is distinct from cyanate –O–C≡N, fulminate –O–N+
≡C−
, and nitrile oxide –C≡N+
–O−
.[4]
Isocyanic acid was discovered in 1830 by Justus von Liebig and Friedrich Wöhler.[5]
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. It is the only fairly stable one of the four linear isomers with molecular formula HOCN that have been synthesized, the others being cyanic acid (cyanol) H–O–C≡N and the elusive fulminic acid H–C≡N+
–O−
[6] and isofulminic acid H–O–N+
≡C−
.[4][7]
Structure
Although the electronic structure according to valence bond theory can be written as H−N=C=O, the vibrational spectrum has a band at 2268.8 cm−1 in the gas phase, which clearly indicates a carbon–nitrogen triple bond.[8][9] Thus the canonical form H−N+
≡C−O−
is the major resonance structure.
Properties
Physical
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.[10][11]
Acidity
In aqueous solution it is a weak acid, having a pKa of 3.7:[12]
- HNCO ⇌ H+ + NCO−
Decomposition
Isocyanic acid hydrolyses to carbon dioxide and ammonia:
- HNCO + H2O → CO2 + NH3
Oligomerization
At sufficiently high concentrations, isocyanic acid oligomerizes to give the trimer cyanuric acid and cyamelide, a polymer. These species usually are easily separated from liquid- or gas-phase reaction products. Cyanuric acid itself decomposes on further heating back to isocyanic acid.[10]
Stability in solution
Dilute solutions of isocyanic acid are stable in inert solvents, e.g. ether and chlorinated hydrocarbons.[13]
Reactions
Isocyanic acid reacts with amines to give ureas (carbamides):
- HNCO + RNH2 → RNHC(O)NH2
This reaction is called carbamylation.
HNCO adds across electron-rich double bonds, such as vinylethers, to give the corresponding isocyanates.
Isocyanic acid, HNCO, is a Lewis acid whose free energy, enthalpy and entropy changes for its 1:1 association with a number of bases in carbon tetrachloride solution at 25°C have been reported.[14] The acceptor properties of HNCO are compared with other Lewis acid in the ECW model.
Tautomerism
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.[15]
Low-temperature photolysis of solids containing HNCO creates the tautomer cyanic acid H-O-C≡N, also called hydrogen cyanate.[16] Pure cyanic acid has not been isolated, and isocyanic acid is the predominant form in all solvents.[13] Sometimes information presented for cyanic acid in reference books is actually for isocyanic acid.[citation needed]
Preparation
Isocyanic acid can be made by protonation of the cyanate anion, such as from salts like potassium cyanate, by either gaseous hydrogen chloride or acids such as oxalic acid.[17]
- H+ + NCO− → HNCO
HNCO also can be made by the high-temperature thermal decomposition of the trimer cyanuric acid:
- C3H3N3O3 → 3 HNCO
In the reverse of the famous synthesis of urea by Friedrich Wöhler,
- CO(NH2)2 → HNCO + NH3
isocyanic acid is produced and rapidly trimerizes to cyanuric acid.
Occurrence
Isocyanic acid has been detected in many kinds of interstellar environments.[7]
Isocyanic acid is also present in various forms of smoke, including smog and cigarette smoke. It was detected using mass spectrometry, and easily dissolves in water, posing a health risk to the lungs.[18]
See also
References
- ^ Cyanamide also has this name, and for which it is more systematically correct
- ^ "Oxomethaniminium | CH2NO | ChemSpider". www.chemspider.com. Retrieved 27 January 2019.
- ^ Pradyot Patnaik. Handbook of Inorganic Chemicals. McGraw-Hill, 2002, ISBN 0-07-049439-8
- ^ a b William R. Martin and David W. Ball (2019): "Small organic fulminates as high energy materials. Fulminates of acetylene, ethylene, and allene". Journal of Energetic Materials, volume 31, issue 7, pages 70-79. doi:10.1080/07370652.2018.1531089
- ^ Liebig, J.; Wöhler, F. (1830). "Untersuchungen über die Cyansäuren". Ann. Phys. 20 (11): 394. Bibcode:1830AnP....96..369L. doi:10.1002/andp.18300961102.
- ^ Kurzer, Frederick (2000). "Fulminic Acid in the History of Organic Chemistry". Journal of Chemical Education. 77 (7): 851–857. Bibcode:2000JChEd..77..851K. doi:10.1021/ed077p851.
- ^ a b Donghui Quan, Eric Herbst, Yoshihiro Osamura, and Evelyne Roueff (2010): "Gas-grain modeling of isocyanic acid (HNCO), cyanic acid (HOCN), fulminic acid (HCNO), and isofulminic acid (HONC) in assorted interstellar environments" The Astrophysical Journal, volume 725, issue 2, pages doi:10.1088/0004-637X/725/2/2101
- ^ Nakamoto, part A, p 190
- ^ Teles, Joaquim Henrique; Maier, Günther; Andes Hess, B.; Schaad, Lawrence J.; Winnewisser, Manfred; Winnewisser, Brenda P. (1989). "The CHNO Isomers". Chemische Berichte. 122 (4): 753–766. doi:10.1002/cber.19891220425.
- ^ a b Greenwood, p323
- ^ Wells, p 722
- ^ IUPAC SC-Database A comprehensive database of published data on equilibrium constants of metal complexes and ligands
- ^ a b A. S. Narula, K. Ramachandran “Isocyanic Acid” in Encyclopedia of Reagents for Organic Synthesis, 2001, John Wiley & Sons, New York. doi:10.1002/047084289X.ri072m Article Online Posting Date: April 15, 2001.
- ^ Nelson, J. (1970) Hydrogen-bonded complexes of isocyanic acid: Infrared spectra and thermodynamic measurements. Spectrochimica Acta Part A: Molecular Spectroscopy 26,109-120.
- ^ 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.
- ^ Jacox, M.E.; Milligan, D.E. (1964). "Low-Temperature Infrared Study of Intermediates in the Photolysis of HNCO and DNCO". Journal of Chemical Physics. 40 (9): 2457–2460. Bibcode:1964JChPh..40.2457J. doi:10.1063/1.1725546.
- ^ Fischer, G.; Geith, J.; Klapötke, T. M.; Krumm B. (2002). "Synthesis, Properties and Dimerization Study of Isocyanic Acid" (PDF). Z. Naturforsch. 57b (1): 19–25. doi:10.1515/znb-2002-0103. S2CID 37461221.
- ^ Preidt, Robert. "Chemical in Smoke May Pose Health Risk". MyOptumHealth. AccuWeather. Retrieved 14 September 2011.
External links
- Walter, Wolfgang (1997). Organic Chemistry: A Comprehensive Degree Text and Source Book. Chichester: Albion Publishing. p. 364. ISBN 978-1-898563-37-2. Retrieved 2008-06-21.
- Cyanic acid from NIST Chemistry WebBook (accessed 2006-09-09)
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