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.
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||43.03 g/mol|
|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)|
|Boiling point||23.5 °C (74.3 °F; 296.6 K)|
|Solubility||Soluble in benzene, toluene, ether|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Preparation and reactionsEdit
- H+ + NCO- → HNCO
HNCO also can be made by the high-temperature thermal decomposition of cyanuric acid, a trimer.
- C3H3N3O3 → 3 HNCO
- HNCO + H2O → CO2 + NH3
At sufficiently high concentrations, isocyanic acid oligomerizes to give cyanuric acid and cyamelide, a polymer. These species usually are easily separated from liquid- or gas-phase reaction products. Dilute solutions of isocyanic acid are stable in inert solvents, e.g. ether and chlorinated hydrocarbons.
- 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 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.
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. The acceptor properties of HNCO are compared with other Lewis acid in the ECW model.
Isomers: Cyanic acid and fulminic acidEdit
Low-temperature photolysis of solids containing HNCO has been shown to make H-O-C≡N, known as cyanic acid or hydrogen cyanate; it is a tautomer of isocyanic acid. Pure cyanic acid has not been isolated, and isocyanic acid is the predominant form in all solvents. Sometimes information presented for cyanic acid in reference books is actually for isocyanic acid.
- 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
- Liebig, J.; Wöhler, F. (1830). "Untersuchungen über die Cyansäuren". Ann. Phys. 20 (11): 394. Bibcode:1830AnP....96..369L. doi:10.1002/andp.18300961102.
- 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.
- 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.
- Preidt, Robert. "Chemical in Smoke May Pose Health Risk". MyOptumHealth. AccuWeather. Retrieved 14 September 2011.
- Nelson, J. (1970) Hydrogen-bonded complexes of isocyanic acid: Infrared spectra and thermodynamic measurements. Spectrochimica Acta Part A: Molecular Spectroscopy 26,109-120.
- 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.
- 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.
- Handbook of Chemistry and Physics, 65th. Edition, CRC Press (1984)
- 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)